U.S. patent application number 10/855708 was filed with the patent office on 2005-10-06 for semiconductor flip-chip package and method for the fabrication thereof.
This patent application is currently assigned to M.A. Capote. Invention is credited to Capote, Miguel A., Zhou, Ligui, Zhou, Zhiming, Zhu, Xiaoqi.
Application Number | 20050218517 10/855708 |
Document ID | / |
Family ID | 26731833 |
Filed Date | 2005-10-06 |
United States Patent
Application |
20050218517 |
Kind Code |
A1 |
Capote, Miguel A. ; et
al. |
October 6, 2005 |
Semiconductor flip-chip package and method for the fabrication
thereof
Abstract
A simplified process for flip-chip attachment of a chip to a
substrate is provided by pre-coating the chip with an encapsulant
underfill material having separate discrete solder columns therein
to eliminate the conventional capillary flow underfill process.
Such a structure permits incorporation of remeltable layers for
rework, test, or repair. It also allows incorporation of electrical
redistribution layers. In one aspect, the chip and pre-coated
encapsulant are placed at an angle to the substrate and brought
into contact with the pre-coated substrate, then the chip and
precoated encapsulant are pivoted about the first point of contact,
expelling any gas therebetween until the solder bumps on the chip
are fully in contact with the substrate. There is also provided a
flip-chip configuration having a complaint solder/flexible
encapsulant understructure that deforms generally laterally with
the substrate as the substrate undergoes expansion or contraction.
With this configuration, the complaint solder/flexible encapsulant
understructure absorbs the strain caused by the difference in the
thermal coefficients of expansion between the chip and the
substrate without bending the chip and substrate.
Inventors: |
Capote, Miguel A.;
(Carlsbad, CA) ; Zhou, Zhiming; (Woodbury, MN)
; Zhu, Xiaoqi; (Vista, CA) ; Zhou, Ligui;
(Pleasanton, CA) |
Correspondence
Address: |
NATH & ASSOCIATES
1030 15th STREET, NW
6TH FLOOR
WASHINGTON
DC
20005
US
|
Assignee: |
M.A. Capote
310 Via Vera Cruz Suite 107
San Marcos
CA
92069
|
Family ID: |
26731833 |
Appl. No.: |
10/855708 |
Filed: |
May 28, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10855708 |
May 28, 2004 |
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10390603 |
Mar 19, 2003 |
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6774493 |
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10390603 |
Mar 19, 2003 |
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09662642 |
Sep 15, 2000 |
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6566234 |
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09662642 |
Sep 15, 2000 |
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09120172 |
Jul 21, 1998 |
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6121689 |
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60053407 |
Jul 21, 1997 |
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60056043 |
Sep 2, 1997 |
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Current U.S.
Class: |
257/738 ;
257/778; 257/787; 257/E21.503; 257/E21.508; 257/E21.511;
257/E23.119; 438/108; 438/127 |
Current CPC
Class: |
H01L 2224/73104
20130101; H01L 2924/01322 20130101; H01L 2224/13111 20130101; H01L
2924/0102 20130101; H01L 2924/01033 20130101; B23K 35/025 20130101;
H01L 2224/1182 20130101; H01L 2224/2929 20130101; H01L 2924/014
20130101; B23K 35/3618 20130101; H01L 2224/8121 20130101; H01L
2224/83856 20130101; H01L 2224/83191 20130101; H01L 2924/3511
20130101; H01L 2924/15311 20130101; H01L 2224/32225 20130101; H01L
2924/01039 20130101; H01L 24/83 20130101; H01L 2924/01018 20130101;
H01L 2924/0665 20130101; H01L 2224/27618 20130101; H01L 2924/01049
20130101; B23K 35/3613 20130101; H01L 2224/06102 20130101; H01L
2224/83101 20130101; H01L 2924/1579 20130101; H05K 3/321 20130101;
H01L 2924/00013 20130101; H01L 2224/29562 20130101; H01L 2924/14
20130101; H01L 23/293 20130101; H01L 2224/16105 20130101; H01L
2924/01004 20130101; H01L 21/563 20130101; H01L 2224/81024
20130101; H01L 2224/81815 20130101; H01L 2924/01015 20130101; H01L
23/49827 20130101; H01L 24/81 20130101; H01L 2224/2919 20130101;
H01L 2224/29299 20130101; H01L 2924/01051 20130101; H01L 2924/01079
20130101; H01L 2924/0781 20130101; C09J 4/00 20130101; H01L
2224/83192 20130101; H01L 2924/01082 20130101; H01L 24/11 20130101;
H01L 2224/83193 20130101; H01L 2924/0132 20130101; H01L 2224/27614
20130101; H01L 2224/16225 20130101; H01L 2224/27436 20130101; H01L
2224/27602 20130101; H01L 2924/01005 20130101; H01L 2924/01047
20130101; H01L 2924/01078 20130101; H01L 2224/13099 20130101; H01L
2224/1403 20130101; H01L 2924/01042 20130101; H01L 2924/01045
20130101; H05K 1/095 20130101; H01L 2224/81011 20130101; H01L
2924/01023 20130101; B32B 7/12 20130101; H01L 2224/1148 20130101;
H01L 2924/01006 20130101; H01L 2924/01046 20130101; H01L 2924/01327
20130101; H01L 2924/10253 20130101; H01L 2924/01027 20130101; H01L
2224/0401 20130101; H01L 2924/0103 20130101; H01L 2224/274
20130101; H01L 2924/01025 20130101; H01L 2924/01029 20130101; H01L
24/29 20130101; H01L 2224/16108 20130101; H01L 2224/29082 20130101;
H01L 2224/29111 20130101; H01L 2924/12042 20130101; H01L 2224/1147
20130101; H01L 23/4334 20130101; H01L 23/49883 20130101; H01L
2224/2969 20130101; H01L 2224/29 20130101; H01L 2224/73204
20130101; H01L 2224/02379 20130101; H01L 2224/29101 20130101; H01L
2924/0105 20130101; C09J 4/00 20130101; C08F 222/1006 20130101;
C09J 4/00 20130101; C08F 222/20 20130101; H01L 2924/0665 20130101;
H01L 2924/00 20130101; H01L 2224/29101 20130101; H01L 2924/014
20130101; H01L 2924/00 20130101; H01L 2224/73204 20130101; H01L
2224/16225 20130101; H01L 2224/32225 20130101; H01L 2924/00
20130101; H01L 2224/83192 20130101; H01L 2224/32225 20130101; H01L
2924/0132 20130101; H01L 2924/01029 20130101; H01L 2924/0105
20130101; H01L 2924/0132 20130101; H01L 2924/01028 20130101; H01L
2924/0105 20130101; H01L 2924/0132 20130101; H01L 2924/0105
20130101; H01L 2924/01082 20130101; H01L 2224/16225 20130101; H01L
2224/13111 20130101; H01L 2924/00 20130101; H01L 2224/83191
20130101; H01L 2224/83101 20130101; H01L 2924/00 20130101; H01L
2224/32225 20130101; H01L 2924/00 20130101; H01L 2224/83192
20130101; H01L 2224/83101 20130101; H01L 2924/00 20130101; H01L
2224/32225 20130101; H01L 2924/00 20130101; H01L 2224/13111
20130101; H01L 2924/01082 20130101; H01L 2924/00012 20130101; H01L
2224/32225 20130101; H01L 2924/00 20130101; H01L 2924/3512
20130101; H01L 2924/00 20130101; H01L 2924/00014 20130101; H01L
2224/32225 20130101; H01L 2924/00 20130101; H01L 2224/13111
20130101; H01L 2924/01028 20130101; H01L 2924/00014 20130101; H01L
2224/32225 20130101; H01L 2924/00 20130101; H01L 2224/13111
20130101; H01L 2924/01029 20130101; H01L 2924/00014 20130101; H01L
2224/32225 20130101; H01L 2924/00 20130101; H01L 2924/15311
20130101; H01L 2224/73204 20130101; H01L 2224/16225 20130101; H01L
2224/32225 20130101; H01L 2924/00 20130101; H01L 2224/2919
20130101; H01L 2924/0665 20130101; H01L 2924/00014 20130101; H01L
2224/29299 20130101; H01L 2924/00014 20130101; H01L 2924/00013
20130101; H01L 2224/29099 20130101; H01L 2924/00013 20130101; H01L
2224/29199 20130101; H01L 2924/00013 20130101; H01L 2224/29299
20130101; H01L 2924/00013 20130101; H01L 2224/2929 20130101; H01L
2924/10253 20130101; H01L 2924/00 20130101; H01L 2224/83192
20130101; H01L 2224/73204 20130101; H01L 2224/16225 20130101; H01L
2224/32225 20130101; H01L 2924/00 20130101; H01L 2924/12042
20130101; H01L 2924/00 20130101; H01L 2224/27614 20130101; H01L
2924/00014 20130101; H01L 2224/27618 20130101; H01L 2924/00014
20130101; H01L 2224/27602 20130101; H01L 2924/00014 20130101 |
Class at
Publication: |
257/738 ;
257/778; 257/787; 438/108; 438/127 |
International
Class: |
H01L 023/48; H01L
021/48 |
Goverment Interests
[0002] The U.S. Government has a paid-up license in this invention
and the right in limited circumstances to require the patent owner
to license others on reasonable terms as provided for by the terms
of contract no. N00164-96-C-0089 awarded by Defense Advanced
Research Projects Agency.
Claims
1.-52. (canceled)
53. An encapsulated semiconductor chip comprising an active surface
with an encapsulant composition deposited on said active surface,
said encapsulant composition comprising at least two distinct
portions: a) a distinct first portion having a thermally curable
composition comprising an inorganic filler; and b) a distinct
second portion having a thermally curable composition without an
inorganic filler.
54. The encapsulated semiconductor chip of claim 53 wherein the
thermally curable composition of the distinct first portion further
comprises an epoxy resin.
55. The encapsulated semiconductor chip of claim 53 wherein the
inorganic filler material comprises silica.
56. The encapsulated semiconductor chip of claim 53 wherein said
encapsulant composition is a solid.
57. The encapsulated semiconductor chip of claim 53 wherein said
encapsulant composition is a viscous liquid.
58. The encapsulated semiconductor chip of claim 53 wherein the
semiconductor chip further comprises solder bumps.
59. The encapsulated semiconductor chip of claim 53 wherein the
thermally curable composition of the distinct second portion has
fluxing properties.
60. The encapsulated semiconductor chip of claim 53 wherein the
thermally curable composition of the distinct first portion is
uncured.
61. The encapsulated semiconductor chip of claim 53 wherein the
thermally curable composition of the distinct second portion is
uncured.
62. A method for manufacturing an encapsulated semiconductor chip
comprising the steps of: a) providing a chip with solder bumps on
an active surface thereof; b) depositing a distinct first portion
of an encapsulant composition on said active surface, said distinct
first portion having a having a thermally curable composition
comprising an inorganic filler; c) providing a substrate having a
pattern of separate discrete solderable metal pads thereon, at
least one of said pads corresponding to at least one of said solder
bumps; d) depositing a distinct second portion of said encapsulant
composition on said substrate surface, said distinct second portion
having a thermally curable composition without an inorganic filler
and wherein said first distinct portion has a large amount of
inorganic filler with respect to the amount of inorganic filler in
said second distinct portion; and e) attaching the chip to the
substrate to form said encapsulated semiconductor chip wherein the
solder bumps are facing the substrate and aligned with the solder
pads.
63. The method according to claim 62 further comprising the step of
heating the encapsulated semiconductor chip to cure said
encapsulant composition and reflow the solder, said heating being
done after the step of attachment.
64. The method according to claim 63 wherein the step of heating is
accomplished using solder reflow technology.
65. The method of claim 62 wherein the thermally curable
composition of the distinct first portion further comprises an
epoxy resin.
66. The method of claim 62 wherein the inorganic filler material
comprises silica.
67. The method of claim 62 wherein said encapsulant composition is
a solid.
68. The method of claim 62 wherein said encapsulant composition is
a viscous liquid.
69. The method of claim 62 wherein the thermally curable
composition of the distinct second portion has fluxing
properties.
70. The method of claim 62 wherein the thermally curable
composition of the distinct first portion is uncured.
71. The method of claim 62 wherein the thermally curable
composition of the distinct second portion is uncured.
72. A method for manufacturing an encapsulated semiconductor chip
comprising the steps of: a) providing a chip with solder bumps on
an active surface thereof; b) depositing a distinct first portion
of an encapsulant composition on said active surface, said distinct
first portion having a having a thermally curable composition
comprising an inorganic filler; c) depositing a distinct second
portion of said encapsulant composition on said distinct first
portion, said distinct second portion having a thermally curable
composition without an inorganic filler and wherein said first
distinct portion has a large amount of inorganic filler with
respect to the amount of inorganic filler in said second distinct
portion; d) providing a substrate having a pattern of separate
discrete solderable metal pads thereon, at least one of said pads
corresponding to at least one of said solder bumps; and e)
attaching the chip to the substrate to form said encapsulated
semiconductor chip wherein the solder bumps are facing the
substrate and aligned with the solder pads.
73. The method according to claim 72 further comprising the step of
heating the encapsulated semiconductor chip to cure said
encapsulant composition and reflow the solder, said heating being
done after the step of attachment.
74. The method according to claim 73 wherein the step of heating is
accomplished using solder reflow technology.
75. The method of claim 72 wherein the thermally curable
composition of the distinct first portion further comprises an
epoxy resin.
76. The method of claim 72 wherein the inorganic filler material
comprises silica.
77. The method of claim 72 wherein said encapsulant composition is
a solid.
78. The method of claim 72 wherein said encapsulant composition is
a viscous liquid.
79. The method of claim 72 wherein the semiconductor chip further
comprises solder bumps.
80. The method of claim 72 wherein the thermally curable
composition of the distinct second portion has fluxing
properties.
81. The method of claim 72 wherein the thermally curable
composition of the distinct first portion is uncured.
82. The method of claim 72 wherein the thermally curable
composition of the distinct second portion is uncured.
83. An encapsulated semiconductor chip comprising an active surface
with an encapsulant composition deposited on said active surface,
said encapsulant composition comprising at least two distinct
portions: a) a distinct first portion having a thermally curable
composition comprising an inorganic filler; and b) a distinct
second portion having a thermally curable composition comprising an
inorganic filler; wherein said first distinct portion has a large
concentration of inorganic filler with respect to the concentration
of inorganic filler in said second distinct portion.
84. The encapsulated semiconductor chip of claim 83 wherein the
thermally curable composition of the distinct first portion further
comprises an epoxy resin.
85. The encapsulated semiconductor chip of claim 83 wherein the
inorganic filler material comprises silica.
86. The encapsulated semiconductor chip of claim 83 wherein said
encapsulant composition is a solid.
87. The encapsulated semiconductor chip of claim 83 wherein said
encapsulant composition is a viscous liquid.
88. The encapsulated semiconductor chip of claim 83 wherein the
semiconductor chip further comprises solder bumps.
89. The encapsulated semiconductor chip of claim 83 wherein the
thermally curable composition of the distinct second portion has
fluxing properties.
90. The encapsulated semiconductor chip of claim 83 wherein the
thermally curable composition of the distinct first portion is
uncured.
91. The encapsulated semiconductor chip of claim 83 wherein the
thermally curable composition of the distinct second portion is
uncured.
92. A method for manufacturing an encapsulated semiconductor chip
comprising the steps of: a) providing a chip with solder bumps on
an active surface thereof; b) depositing an encapsulant on said
active surface, said encapsulant comprising: (1) a distinct first
portion thermally curable composition comprising an inorganic
filler; (2) a distinct second portion thermally curable composition
without an inorganic filler; d) providing a substrate having a
pattern of separate discrete solderable metal pads thereon, at
least one of said pads corresponding to at least one of said solder
bumps; and e) attaching the chip to the substrate to form said
encapsulated semiconductor chip wherein the solder bumps are facing
the substrate and aligned with the solder pads.
93. The method according to claim 92 further comprising the step of
heating the encapsulated semiconductor chip to cure said
encapsulant composition and reflow the solder, said heating being
done after the step of attachment.
94. The method according to claim 93 wherein the step of heating is
accomplished using solder reflow technology.
95. The method of claim 92 wherein the thermally curable
composition of the distinct first portion further comprises an
epoxy resin.
96. The method of claim 92 wherein the inorganic filler material
comprises silica.
97. The method of claim 92 wherein said encapsulant composition is
a solid.
98. The method of claim 92 wherein said encapsulant composition is
a viscous liquid.
99. The method of claim 92 wherein the semiconductor chip further
comprises solder bumps.
100. The method of claim 92 wherein the thermally curable
composition of the distinct second portion has fluxing
properties.
101. The method of claim 92 wherein the thermally curable
composition of the distinct first portion is uncured.
102. The method of claim 92 wherein the thermally curable
composition of the distinct second portion is uncured.
103. The method of claim 92 wherein said first distinct portion has
a large amount of inorganic filler with respect to the amount of
inorganic filler in said second distinct portion.
104. A method for manufacturing an encapsulated semiconductor chip
comprising the steps of: a) providing a chip with solder bumps on
an active surface thereof; b) depositing an encapsulant on said
active surface, said encapsulant comprising: (1) a distinct first
portion having a thermally curable composition comprising an
inorganic filler; and; (2) a distinct second portion having a
thermally curable composition comprising an inorganic filler,
wherein said first distinct portion has a large concentration of
inorganic filler with respect to the concentration of inorganic
filler in said second distinct portion. d) providing a substrate
having a pattern of separate discrete solderable metal pads
thereon, at least one of said pads corresponding to at least one of
said solder bumps; and e) attaching the chip to the substrate to
form said encapsulated semiconductor chip wherein the solder bumps
are facing the substrate and aligned with the solder pads.
105. The method according to claim 104 further comprising the step
of heating the encapsulated semiconductor chip to cure said
encapsulant composition and reflow the solder, said heating being
done after the step of attachment.
106. The method according to claim 105 wherein the step of heating
is accomplished using solder reflow technology.
107. The method of claim 104 wherein the thermally curable
composition of the distinct first portion further comprises an
epoxy resin.
108. The method of claim 104 wherein the inorganic filler material
comprises silica.
109. The method of claim 104 wherein said encapsulant composition
is a solid.
110. The method of claim 104 wherein said encapsulant composition
is a viscous liquid.
111. The method of claim 104 wherein the semiconductor chip further
comprises solder bumps.
112. The method of claim 104 wherein the thermally curable
composition of the distinct second portion has fluxing
properties.
113. The method of claim 104 wherein the thermally curable
composition of the distinct first portion is uncured.
114. The method of claim 104 wherein the thermally curable
composition of the distinct second portion is uncured.
Description
[0001] This application claims the benefit of U.S. Provisional
Application Nos. 60/053,407, filed Jul. 21, 1997, and 60/056,043,
filed Sep. 2, 1997, and incorporates herein the disclosures of
those applications in their entirety.
FIELD OF THE INVENTION
[0003] This invention relates generally to semiconductor chips
electrically and mechanically connected to a substrate,
particularly to flip-chip configurations.
BACKGROUND OF THE INVENTION
[0004] Flip-chip technology is well known in the art. A
semiconductor chip having solder bumps formed on the active side of
the semi-conductor chip is inverted and bonded to a substrate
through the solder bumps by reflowing the solder. Structural solder
joints are formed between the semi-conductor chip and the substrate
to form the mechanical and electrical connections between the chip
and substrate. A narrow gap is left between the semi-conductor chip
and the substrate.
[0005] One obstacle to flip-chip technology when applied to polymer
printed circuits is the unacceptably poor reliability of the solder
joints due to the mismatch of the coefficients of thermal expansion
between the chip, having a coefficient of thermal expansion of
about 3 ppm/.degree. C., and the polymer substrate, e.g.
epoxy-glass having a coefficient of thermal expansion of about 16
to 26 ppm/.degree. C., which causes stress build up in the solder
joints. Because the structural solder joints are small, they are
thus subject to failures. In the past, the solder joint integrity
of flip-chip interconnects to a substrate has been enhanced by
underfilling the volume between the chip and the substrate with an
underfill encapsulant material comprised of a suitable polymer. The
underfill material is typically dispensed around two adjacent sides
of the semiconductor chip, then the underfill material slowly flows
by capillary action to fill the gap between the chip and the
substrate. The underfill material is then hardened by baking for an
extended period. For the underfill encapsulant to be effective, it
is important that it adhere well to the chip and the substrate to
improve the solder joint integrity. Underfilling the chip with a
subsequently cured encapsulant has been shown to reduce solder
joint-cracking caused by thermal expansion mismatch between the
chip and the substrate. The cured encapsulant reduces the stresses,
induced by differential expansion and contraction, on the solder
joints.
[0006] The underfill process, however, makes the assembly of
encapsulated flip-chip printed wire boards (PWB) a time consuming,
labor intensive and expensive process with a number of
uncertainties. To join the integrated circuit to the substrate, a
flux, generally a no-clean, low residue flux, is placed on the chip
or substrate. Then the integrated circuit is placed on the
substrate. The assembly is subjected to a solder reflowing thermal
cycle, soldering the chip to the substrate. The surface tension of
the solder aids to self align the chip to the substrate terminals.
After reflow, due to the close proximity of the chip to the
substrate, removing flux residues from under the chip is such a
difficult operation that it is generally not done. Therefore the
flux residues are generally left in the space between the chip and
the substrate. These residues are known to reduce the reliability
and integrity of the encapsulant.
[0007] After reflow, underfill encapsulation of the chip generally
follows. In the prior art, the polymers of choice for the underfill
encapsulation have been epoxies, the coefficient of thermal
expansion and moduli of the epoxies being adjusted with the
addition of inorganic fillers. To achieve optimum reliability, a
coefficient of thermal expansion in the vicinity of 25 ppm/.degree.
C. is preferred and a modulus of 4 GPa or more. Since the preferred
epoxies have coefficient of thermal expansions exceeding 80
ppm/.degree. C. and moduli of less than 4 GPa, the inorganic
fillers selected generally have much lower coefficient of thermal
expansions and much higher moduli so that in the aggregate, the
epoxy-inorganic mixture is within the desired range.
[0008] The underfill encapsulation technique of the prior art has
four principal disadvantages:
[0009] 1. The reflowing of the solder bump and then underfilling
and curing the encapsulant is a multi-step process that results in
reduced production efficiency;
[0010] 2. To underfill a flip-chip assembly takes too long because
the material must flow through the tiny gap between the chip and
the substrate;
[0011] 3. The flux residues remaining in the gap reduce the
adhesive and cohesive strengths of the underfill encapsulating
adhesive, affecting the reliability of the assembly; and
[0012] 4. As the size of chips increase, the limiting effect of
capillary action becomes more critical and makes the encapsulation
procedure more time consuming, more susceptible to void formation
and to the separation of the polymer from the fillers during
application.
[0013] Clearly, many improvements to this process are feasible to
increase reliability, reduce the time required and decrease the
likelihood of producing a void in the encapsulant while providing
the required low coefficient of thermal expansion and high
modulus.
[0014] Other prior art methods of encapsulating the chip have
attempted to overcome the above limitations by applying the
encapsulating resin through a hole in the substrate located near
the center of the chip. After the soldering and cleaning
operations, the. encapsulating resin is forced through the hole and
around the periphery of the chip to ensure complete coverage of the
chip surface. This method suffers from the need to reserve an area
in the center of the substrate that is free of circuitry in order
to provide an unused space for the hole. It also does not eliminate
the problems of entrapped air bubbles.
[0015] Another prior art method in U.S. Pat. No. 5,128,746
(Pennisi) teaches a method wherein an adhesive material including a
fluxing agent is applied to the chip or substrate. The chip is
positioned on the substrate and the solder bumps are reflowed.
During the reflow step, the fluxing agent promotes wetting of the
solder to the substrate metallization pattern and the adhesive
material is cured, mechanically interconnecting and encapsulating
the substrate to the component. The limitation of this technique is
that in order for the molten solder to readily wet the substrate
metallization and also to allow the solder, through surface
tension, to self-align the chip bumps to the substrate
metallization pattern, the material must maintain very low
viscosity during the reflow step. But the viscosity of these
materials is severely increased by the presence of the required
inorganic fillers. As a result, this approach has failed to produce
a material that can serve as both the flux and the encapsulant with
the required low coefficient of thermal expansion and high modulus
for optimum reliability.
[0016] Referring to FIGS. 1 and 2, underfiring the chip 100 with a
subsequently hardened encapsulant 102 has been shown to reduce
solder joint cracking caused by thermal expansion mismatch between
the chip and the substrate 104. The hardened encapsulant 102
transfers the stresses, induced by differential expansion and
contraction, from the solder joints 106 to deformation of the chip
100 and substrate 104 as shown in FIG. 1 for expansion-induced
strain at elevated temperatures and FIG. 2 for contraction-induced
strain at reduced temperatures. In other words, the main effect of
the hardened encapsulant during thermal expansion or contraction is
to effectively force the chip and the substrate to take up the
stress caused by the coefficients of thermal expansion mismatch by
bending and bulging the chip and substrate. This bending and
bulging reduces the stress on the solder joints and virtually
eliminates solder fatigue failure.
[0017] Unfortunately, the effect of the encapsulant bending the
substrate and the chip causes its own new set of problems. One such
problem is that the bending makes the chips susceptible to
cracking. Another such problem is that the degree of stress relief
is highly dependent on the flexibility of the under-lying substrate
and is thus an unpredictable function of the design of the printed
circuit. Another limitation is that relying on such bending for
stress relief on the solder joints prevents the placement of flip
chips directly opposite one another on a double-sided printed
circuit.
[0018] Another limitation of prior art flip-chip attachment is the
difficulty of performing rework. Chip removal, once underfill has
been performed, is very destructive to both the printed circuit
board and the chip. Rework is almost impossible with prior art
materials and processes. For example, the prior art procedure for
removing an encapsulated die from a printed wire board is to grind
it off manually.
[0019] Another limitation of the prior art is the expense of
applying solder bumps to a chip. The solder bumps have been applied
to chips by one of several methods. Coating the solder on the chip
bumps by evaporation of solder metals through a mask is one such
method. This method suffers from 1) long deposition times, 2)
limitations on the compositions of solder that can be applied to
those metals that can be readily evaporated, and 3) evaporating the
metals over large areas where the solder is ultimately not wanted.
Also, since most solders contain lead, a toxic metal, evaporation
involves removal and disposal of excess coated lead from equipment
and masks. Another common method in the prior art is electroplating
of the solder onto the chip pads through a temporary sacrificial
mask. Electroplating is a slow and expensive process that also
deposits the solder over large areas where the solder is ultimately
not wanted. Another method is to screen print solder paste on the
chips pad through a stencil, then reflowing the solder to form a
ball or bump on the pad. This technique is limited to bump
dimensions that can be readily stencil printed, so it is not
practical in bump pitches of 25 microns or less.
[0020] Another limitation of the prior art is the difficulty in
distributing electrical signals from the small dimension of the
chip to the large dimensions of the substrates. Most chips are
manufactured with the electrical interconnection pads around their
periphery with a pad pitch of 0.25 mm or less. On the other hand,
printed circuits are manufactured with pad pitches of 0.25 mm and
larger. This discrepancy in dimensions requires that the
chip-to-substrate interconnection provide some method of
redistributing the chip pad locations over a larger area so that
they can match the dimensions of the printed circuit. Today, this
discrepancy is bridged by creating expensive redistribution layers
on the printed circuit. Few manufacturers are able to produce
printed circuits at the tight dimensional tolerances required for
redistribution, but those who are capable of doing so achieve this
with significant production yield penalties. Another method to
bridge the dimension discrepancy involves complete redesign of the
chip to redistribute the electrical pads over the entire area of
the chip, an expensive procedure that chip manufacturers generally
want to avoid.
SUMMARY OF THE INVENTION
[0021] In one aspect of the present invention there is provided a
chip with underfilling encapsulant and separate discrete solder
bumps pre-coated and pre-assembled on the chip for assembly to a
substrate. This configuration provides a simple, cost-effective
assembly procedure wherein the chip/encapsulant/discrete solder
bump combination is placed on the substrate and subsequently heat
is applied so that the solder is reflowed while simultaneously the
encapsulant hardens, without the labor intensive underfill steps of
the prior art.
[0022] In another aspect of the present invention there is provided
a chip precoated with underfilling encapsulant having holes therein
which expose metallized contact pads on the active surface of the
chip. The holes are subsequently filled with molten solder which is
then cooled and hardened to create the chip/encapsulant/discrete
solder bump assembly. The assembly can be placed on a substrate and
subsequently, the solder is reflowed while simultaneously the
encapsulant hardens, eliminating the labor intensive underfill
steps of the prior art. Alternatively, the
chip/encapsulant/discrete solder bump assembly is coated with a
thin layer of a flux adhesive and, subsequently, the solder is
reflowed while simultaneously the flux adhesive and encapsulant
harden.
[0023] In another aspect of the present invention there is provided
a chip precoated with underfilling encapsulant having holes therein
which expose metallized contact pads on the active surface of the
chip. The holes are subsequently filled with an electrically
conductive adhesive to create a chip/encapsulant/conductive
adhesive bump assembly. The assembly can be placed on the substrate
and subsequently the encapsulant and conductive adhesive are
simultaneously hardened, without the labor intensive underfill
steps of the prior art.
[0024] The present invention also provides a substrate precoated
with the encapsulant having holes therein which expose the
metallized solder pads on the substrate. The holes are subsequently
filled with molten solder or electrically conductive adhesive which
is then cooled and hardened prior to attachment of the chip to the
substrate by reflow. In another embodiment, the substrate has
encapsulant and separate discrete solder columns pre-assembled
thereon.
[0025] In one aspect of the present invention, there is provided a
first portion of an underfilling encapsulant and separate discrete
solder bumps pre-coated and pre-assembled on a chip for assembly to
a substrate. The first portion of encapsulant can be either a solid
or a thick liquid, partially or fully uncured. A second portion of
the encapsulant is applied to the substrate. The first portion of
the encapsulant is filled, preferably highly filled, with a filler
material to produce a reduced coefficient of thermal expansion and
increased modulus. The second portion of the encapsulant is either
lightly filled or completely devoid of filler material. At least
the second portion of the encapsulant comprises an adhesive
material with solder fluxing properties, for example, an adhesive
flux. The first portion of the encapsulant can comprise a similar
material or a conventional epoxy. The first portion is filled with
a filler having a lower coefficient of thermal expansion and higher
modulus than the encapsulant material without filler to increase
the encapsulant's modulus and reduce its coefficient of thermal
expansion. The invention provides a simple, cost-effective assembly
procedure wherein the chip/first portion of encapsulant/discrete
solder bump combination is placed on the substrate/second portion
of encapsulant combination and subsequently heat is applied so that
the solder is reflowed while simultaneously the encapsulant cures,
without the labor intensive, time-consuming underfill steps of the
prior art. Preferably, the second portion constitutes a relatively
thin layer in the overall encapsulant structure which somewhat
intermixes with the first portion during cure and has minimal
effect on the reliability of the flip-chip structure, despite the
second portion having generally a lower modulus and higher
coefficient of thermal expansion than the first portion. An
advantage of the present invention is that the lower viscosity of
the unfilled or lightly filled second portion during the reflow
process allows the solder to flow without impediment from the thick
viscosity of the first portion of the encapsulant. The present
invention provides a low coefficient of thermal expansion and high
modulus in the first portion of the encapsulant while at the same
time achieving good solder wetting and chip self aligning in the
second portion of the encapsulant.
[0026] In another aspect of the present invention, the chip/first
portion of encapsulant/discrete solder bump assembly described
above is coated with a thin layer of the second portion of the
encapsulant which is either lightly filled or completely devoid of
filler material. Placement of the chip, solder reflow and adhesive
cure follows as described above.
[0027] In another aspect of the present invention, there is
provided a method for placing a flip-chip onto a substrate that
avoids entrapment of gas bubbles or creation of voids. The chip,
having the first portion of encapsulant thereon, is oriented at an
angle to the substrate having the second portion thereon, then
pivoted about the first point of contact until the solder bumps on
the chip are in contact with the solder pads on the substrate,
creating an underfill of encapsulant material as the chip is
pivoted while expelling the gas from between the chip and
substrate.
[0028] Another aspect of the present invention provides a chip with
underfilling encapsulant pre-coated and pre-assembled on the chip
for assembly to a substrate, wherein the encapsulant consists of
more than one layer, each layer performing one or more distinct
functions such as attachment, stress distribution, electrical
redistribution, reworkability, adhesion, or other functions. The
bulk of the encapsulant, consisting of one or more layers, is
applied and partially or fully hardened prior to assembly of the
chip on the substrate. Holes therein which expose metallized
contact pads on the active surface of the chip are subsequently
filled with solder or an electrically conductive adhesive as
previously described to create an encapsulated subassembly. Then a
flux adhesive is applied between the chip/encapsulant/solder bump
combination and the substrate which can be fully hardened after or
when the chip/encapsulant/solder bump combination is placed on the
substrate and the solder is reflowed.
[0029] Removal of the chip from the substrate is made possible by
incorporating in the pre-coated multi-layer encapsulant a polymer
layer that can be remelted even after the chip has been assembled
to the substrate. Remelting the solder and the polymer encapsulant
layer allows removal of the chip for repair or replacement after
assembly or for test and burn-in of the chip prior to final
assembly. Thus the chip can be disassembled from the substrate
without damage to either chip or substrate.
[0030] In another aspect of the present invention there is provided
a redistribution of the chip's electrical interconnection pads by
incorporating in the precoated multilayer encapsulant an electrical
redistribution layer comprising a thin printed circuit layer with
electrical circuitry thereon. The interconnect pads on the chip are
attached by solder bumps, conductive adhesive or wire bonds to the
redistribution layer. The redistribution layer is subsequently
encapsulated. Holes in the encapsulant expose metallized contact
pads on the active surface of the redistribution layer. The holes
are subsequently filled with solder as previously described. Then a
flux adhesive layer is applied between the
chip/encapsulant/redistribution layer subassembly and the
substrate. The flux adhesive is applied remaining unhardened until
the subassembly is placed on the substrate and the solder is
reflowed.
[0031] Another aspect of the present invention also provides within
the precoated encapsulant a novel compliant flexible structure
wherein the solder and encapsulant expand or contract laterally
without cracking or delaminating upon heating or cooling of the
chip and substrate. The novel encapsulant mainly provides the
adhesive mechanical bond required to hold the chip on the substrate
while the solder mainly provides the electrical interconnection
required between the chip and the substrate.
[0032] The compliant solder and flexible encapsulant of the present
invention absorb the stress caused by the mismatched coefficients
of thermal expansion without relying on bending of the chip and
substrate. Since the mechanical adhesion of the chip to the
substrate relies primarily on the encapsulant, a relatively soft,
fatigue-less, highly pliable solder is used for the solder bumps to
provide the electrical interconnection of the chip with the
substrate. The compliant solder may have relatively weak mechanical
properties on its own, therefore the encapsulant provides the
mechanical strength. Relieving the solder of its mechanical tasks
allows the use of soft, ductile and fluid-like solders that deform
laterally with the expansion and contraction of the structure
without the fatigue cracking normally experienced by conventional
solders.
[0033] Another embodiment of the present invention also provides
within the novel compliant encapsulant previously described a
compliant conductive adhesive which expands or contracts laterally
upon heating or cooling to absorb the stresses created by the
mismatch in the coefficients of thermal expansion and prevent
bending of the chip and substrate. Independent of each other, the
structural properties of the novel encapsulant provides the
mechanical connection required in the structure while the
electrical properties of the compliant conductive adhesive provides
the required electrical connection between the chip and the
substrate.
[0034] The semiconductor chip package structures of the present
invention provide, among other advantages, simple chip placement
followed by reflow without labor intensive underfill steps; a
solder bumped or conductive adhesive bumped chip or substrate with
an encapsulant pre-attached, with the encapsulant performing a
mechanical function and the solder or conductive adhesive
performing an electrical function; a pre-coated chip encapsulant of
two or more layers, each layer performing a distinct function of
attachment or reworkability; a reworkable flip chip assembly by
means of a remeltable polymer in the encapsulant; an electrical
redistribution layer within the encapsulant; a low-cost method for
applying the solder bumps to a flip chip or flip chip substrate by
creating holes in a pre-coated encapsulant; and a low-cost method
for applying the conductive adhesive bumps to a flip chip or
substrate by creating holes in a pre-coated encapsulant; and a
compliant chip understructure that includes a fatigue-less solder
or conductive adhesive.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] FIG. 1 is a diagrammatic representation of a prior art
underfilled flip-chip structure under expansion-induced strain at
elevated temperatures.
[0036] FIG. 2 is a diagrammatic representation of a prior art
underfilled flip-chip structure under contraction-induced strain at
reduced temperatures.
[0037] FIG. 3 is an assembled flip-chip structure in accordance
with one embodiment of the present invention.
[0038] FIG. 4 is a diagrammatic representation of one embodiment
for forming a flip-chip structure.
[0039] FIGS. 5-7 are diagrammatic representations of another
embodiment for forming a flip-chip structure.
[0040] FIGS. 8 and 9 are diagrammatic representations of another
embodiment for forming a flip-chip structure.
[0041] FIG. 10 is a diagrammatic representation of a flip-chip
structure wherein the first portion of the encapsulant material is
applied to the bumped chip and the second portion is applied to the
substrate.
[0042] FIG. 11 is a diagrammatic representation of the flip-chip
structure of FIG. 10 after assembly.
[0043] FIG. 12 is a diagrammatic representation of a flip-chip
structure wherein the first portion of the encapsulant material is
applied to the bumped chip and the second portion is applied over
the first portion.
[0044] FIGS. 13 and 14 illustrate a method for placing a flip-chip
onto a printed circuit board that avoids entrapment of gas bubbles
or formation of voids in the encapsulant.
[0045] FIG. 15 is a diagrammatic representation of a compliant
flip-chip structure in accordance with the present invention.
[0046] FIG. 16 is a diagrammatic representation of the compliant
flip-chip structure of FIG. 15 under expansion-induced strain at
elevated temperatures.
[0047] FIG. 17 is a diagrammatic representation of the compliant
flip-chip structure of FIG. 15 under contraction-induced strain at
reduced temperatures.
[0048] FIGS. 18 and 19 are diagrammatic representations of yet
another embodiment for forming a flip-chip structure.
[0049] FIG. 20 is a diagrammatic representation of the
reworkability of the flip-chip structure of FIGS. 18 and 19.
[0050] FIGS. 21 and 22 are diagrammatic representations of still
another embodiment of a flip-chip structure.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0051] Referring to FIG. 3, an integrated circuit chip 10 is shown
mounted on a substrate 20. A plurality of solder pads 12 on top
surface 26 of the substrate 20 are arranged to receive
corresponding solder bumps 14 connected to the contact pads 24 of
the chip 10. Each of the solder pads 12 is metallized so as to
become solderable and electrically conductive to provide an
electrical interconnection between the chip 10 and the substrate
20. With this flip-chip mounting arrangement, a gap 18 is formed
around the solder bumps 14 between the top surface 26 of the
substrate 20 and the bottom surface 16 of the chip 10. The gap 18
typically varies from 2 to 6 mils. The gap 18 is completely filled
with an encapsulant material 22. In one embodiment of the
invention, the encapsulant material 22 is a compliant polymer
composition. One preferred compliant composition, which is not
meant to limit the invention but only by way of example, is a
compliant polyimide-siloxane co-polymer such as SumiOxy.RTM.
2421-A6-SP available from Oxychem, Grand Island, N.Y.
Alternatively, in another embodiment of the instant invention, the
encapsulant material 22 is a rigid polymer composition. One
preferred composition, which is not meant to limit the invention
but only by way of example, is an anhydride-cured epoxy resin.
Other types of encapsulants known to those skilled in the art are
possible. The encapsulant material 22 is applied to the chip in
either liquid or adhesive tape form, then hardened.
[0052] In another embodiment, the chip 10 having separate discrete
solder bumps 14 pre-assembled thereon is precoated with the
encapsulant material 22 prior to assembly to the substrate 20 (FIG.
4) to alleviate the underfill problems of the prior art processes
and to overcome the performance limitations of substrates which are
pre-coated with a homogeneous combination of adhesive material,
fluxing agent and curing agent or chips which are pre-coated with a
homogeneous combination of adhesive material, fluxing agent, curing
agent and metal particles. The separate discrete solder bumps with
encapsulant material therearound provide superior electrical
performance compared to a distribution of metal particles spread
throughout an encapsulating material. The encapsulating material 22
is uniformly spread across the surface 16 of the chip 10 between
the solder bumps 14 covering the remainder of the chip 10. The chip
10 is then positioned so that the solder bumps 14 are facing the
substrate 20 and aligned with the solder pads 12 of the substrate.
In one embodiment, the solder bumps 14 protrude beyond the
encapsulant after the encapsulant coating step. In an alternate
embodiment, the solder bumps 14 are covered by the encapsulant 22
wherein the encapsulant is ground, melted away, shaved off or
otherwise removed to expose the solder bumps prior to attachment to
the substrate. The encapsulant 22 and solder bumps 14 are moved
into intimate contact with the substrate 20 and solder pads 12,
respectively. The assembly is heated to cure the encapsulant 22 and
reflow the solder using infrared reflow technology, preferably in a
nitrogen blanket to attach the solder bumps 14 to the contact pads
12 of the substrate 20. Other heating and reflow techniques, known
to those skilled in the art, are possible. The encapsulant 22
provides a continuous seal between the chip 10 and the substrate
20.
[0053] In yet another embodiment (FIG. 5), the circuitry on the
bottom surface 16 of the chip 10 is coated with the encapsulant 22
then the contact pads 24 are exposed by making vias 28 through the
encapsulant 22 (e.g., either with a laser, plasma, chemical
etching, a drill or by photo-imaging and development or any other
method known to one skilled in the art) (FIG. 6). The vias 28
within the encapsulant 22 are then filled with solder 30 (FIG. 7)
or a conductive adhesive as described in U.S. Pat. No. 5,376,403
which is forced into the holes by solder injection molding, solder
jetting, screen printing, or other methods known to those skilled
in the art. With any of these embodiments, the solder 30 is
reflowed to form the electrical connection between the chip and the
substrate while the encapsulant 22 bonds to the substrate 20 and
the chip 10 to form the structural connection. As can be easily
appreciated by one of ordinary skill in the art, any of the
above-described embodiments can be modified by precoating the
substrate 20 (rather than the chip) with the encapsulant 22 or
encapsulant 22 and solder 30 combination as shown in FIGS. 8 and 9,
respectively.
[0054] Referring to FIG. 10, an integrated circuit chip 10 is shown
mounted on a substrate 20 in accordance with another embodiment of
the present invention. A plurality of solder pads 12 on top surface
26 of the substrate 20 are arranged to receive corresponding solder
bumps 14 connected to the contact pads (not shown in this
embodiment) of the chip 10. Each of the solder pads 12 is
metallized so as to become solderable and electrically, conductive
to provide an electrical interconnection between the chip 10 and
the substrate 20. With this flip-chip mounting arrangement, a gap
18 is formed around the solder bumps 14 between the top surface 26
of the substrate 20 and the bottom surface 16 of the chip 10. The
gap 18 is completely filled with an encapsulant material 22. The
gap 18 typically varies from 2 to 6 mils.
[0055] FIG. 11 illustrates one embodiment for forming the flip-chip
package illustrated in FIG. 10 using two pre-coated portions of
encapsulant. The chip 10 having separate discrete solder bumps 14
pre-assembled thereon is pre-coated with the first portion 37 of an
encapsulant material 22 (FIG. 10) prior to assembly to the
substrate 20. The first portion 37 contains a filler material,
preferably highly filled, to reduce its coefficient of thermal
expansion and increase its modulus relative to the encapsulant
material not having any filler. The substrate 20, having a pattern
of separate discrete solderable metal pads 12 thereon, is
pre-coated with the second portion 39 of the encapsulant material
prior to assembly with the chip 10. The second portion 39 of the
encapsulant material contains little or no filler material. The
two-layer configuration alleviates the underfill problems of the
prior art processes and overcomes the performance limitations of
substrates which are pre-coated with a homogeneous combination of
adhesive material, fluxing agent and curing agent or chips which
are pre-coated with a homogeneous combination of adhesive material,
fluxing agent, curing agent and metal filler particles.
[0056] The first portion 37 (FIG. 11) which makes up part of the
encapsulating material 22 (FIG. 10) is uniformly spread across the
surface 16 of the chip 10 between the solder bumps 14 to cover the
remainder of the chip surface. The second portion 39 (FIG. 11)
which makes up part of the encapsulating material 22 (FIG. 10) is
uniformly spread across the surface 26 of the substrate 20 over the
solderable metal pads 12 covering the chip region of the substrate
20. The chip 10 is then positioned so that the solder bumps 14 are
facing the substrate 20 and aligned with the solder pads 12 of the
substrate 20. The solder bumps 14 can protrude beyond the first
portion 37 (as shown in FIG. 11) of the encapsulant after the
encapsulant coating step of the chip 10. The encapsulant portion 37
and solder bumps 14 are moved into intimate contact with the
encapsulant portion 39 and solder pads 12, respectively. The
combination of portions 37 and 39 forms the encapsulant 22 (FIG.
10). The assembly is heated to cure the encapsulant 22 and reflow
the solder using infrared reflow technology, preferably in a
nitrogen blanket to attach the solder bumps 14 to the contact pads
12 of the substrate 20. Other heating and reflow techniques, known
to those skilled in the art, can be used in the present invention.
The encapsulant 22 provides a continuous seal between the chip 10
and the substrate 20.
[0057] FIG. 12 illustrates another embodiment for forming the
flip-chip package illustrated in FIG. 10 using two pre-coated
discrete portions of encapsulant. The chip 10 having separate
discrete solder bumps 14 pre-assembled thereon is pre-coated with
the first portion 37 of an encapsulant material 22 (FIG. 10) prior
to assembly to the substrate 20. The first portion 37 contains a
filler material, preferably highly filled, to reduce its
coefficient of thermal expansion and increase its modulus. The
first portion 37 is then pre-coated with the second portion 39 of
the encapsulant material prior to assembly with the substrate 20.
The second portion 39 of the encapsulant material contains little
or no filler material. The first portion 37 (FIG. 12) which makes
up part of the encapsulating material 22 (FIG. 10) is uniformly
spread across the surface 16 of the chip 10 between the solder
bumps 14 to cover the remainder of the chip surface. The second
portion. 39 (FIG. 12) which makes up part of the encapsulating
material 22 (FIG. 10) is uniformly spread over the prior applied
first portion 37. The chip 10 is then positioned so that the solder
bumps 14 are facing the substrate 20 and aligned with the solder
pads 12 of the substrate 20 as described before. The encapsulant
portions 37 and 39 and solder bumps 14 are moved into intimate
contact with the substrate 20 and solder pads 12. The combination
of portions 37 and 39 forms the encapsulant 22 (FIG. 10). The.
assembly is heated to cure the encapsulant 22 and reflow the solder
as described before to attach the solder bumps 14 to the contact
pads 12 of the substrate 20.
[0058] FIGS. 13 and 14 illustrate an alternate embodiment for
attachment of portion 37 and solder bumps 14 with the portion 39
and solder pads 12, respectively. this method is described with
respect to the embodiment shown in FIG. 11, but is equally
applicable to the embodiment shown in FIG. 12. The chip 10 is
initially oriented at an angle to the substrate 20. As the
encapsulant portion 37 and the solder bump on the end of the chip
10 are moved into intimate contact with the portion 39 and solder
pad 12, the chip is pivoted about the first point of contact until
all of the solder bumps 14 are in contact with the solder pads 12.
In this manner, any gas that could possibly be entrapped between
the first portion 37 and the second portion 39 is expelled as
indicated by arrow 41 in FIG. 14 to prevent formation of voids in
the encapsulant.
[0059] Generally, the chip 10 is passivated with a thin layer of
either silicon nitride, polyimide, or benzocyclobutene. To adhere
well to the passivation layer (not shown) on the chip 10, a chip
bonding layer (not shown in this embodiment) may incorporate a
coupling agent (not shown) such as a silane. To adhere well to the
encapsulant 22, the coupling agent (not shown) provides a
chemically compatible moiety for bonding. For example, the
preferred moieties can be epoxides, anhydrides, hydroxyls, or other
moiety that readily bonds to the encapsulant 22.
[0060] The adhesive of the first portion 37 can be either an
adhesive flux or a compatible non-fluxing adhesive. The significant
properties of the first portion 37 are:
[0061] 1. After cure, a coefficient of thermal expansion in the
vicinity of 25 ppm/.degree. C.;
[0062] 2. After cure, a Tg above 120.degree. C.;
[0063] 3. After cure, a modulus greater than 0.1 GPa, preferably
greater than 4 GPa;
[0064] 4. After cure, high adhesion to the chips passivation layer
that usually consists of silicon nitride, polyimide, or
benzocyclobutene;
[0065] 5. Solventless;
[0066] 6. A chemical composition such that it does not interfere or
adversely affect the properties of the second portion 39 of the
encapsulant to which it will be mated; and
[0067] 7. After cure, high adhesion to the second portion 39 of the
encapsulant.
[0068] Since the intrinsic coefficient of thermal expansion and
moduli of most polymeric adhesives do not satisfy the first or
third properties above, the most distinguishing feature of the
first portion 37 of the present invention is that it is filled with
a high concentration of a powdered filler having a lower
coefficient of thermal expansion and higher modulus, generally an
inorganic material, and most preferably silica. The filler having a
higher modulus and lower coefficient of thermal expansion than the
adhesive alone produces an adhesive-filler aggregate having desired
properties. Examples of such adhesives are Araldite CW1195US with
cureer HW1196US available from Ciba Geigy Corporation and
Hysol.RTM. FP4527 and Hysol.RTM. FP4511 available from the Dexter
Corporation of Industry, California.
[0069] Preferably, the adhesive of the first portion 37 is
preferably an adhesive flux. The significant property of the
preferred first portion is that, in addition to the properties
listed above, the preferred first portion adhesive does not
diminish the flow of the solder during the solder reflowing
operation. Many non-fluxing adhesives tend to either cure too
quickly or react with the second portion adhesive in ways that
decrease the wetting and spread of the solder during reflow. Using
an adhesive flux highly filled with a powder filler that imparts
the required coefficient of thermal expansion and modulus to the
adhesive flux provides a first portion 37 that has little or no
effect on the spread of the solder.
[0070] The embodiment of FIG. 15 is a multi-layer compliant
understructure configuration having two discrete bonding layers 32
and 34 in combination with any of the above-described embodiments
for the encapsulant and solder. Chip bonding layer 32 is a thin
polymer, or coupling agent, with high adhesion to the chip
passivation layer (not shown) on the face of the chip 10. The chip
bonding layer 32 is a thin interfacial layer adhering the
encapsulant material 22 to the chip 10. The substrate bonding layer
34 is a thin, adhesive flux layer adhering the encapsulant 22 to
the substrate 30.
[0071] The chip bonding layer 32 has the following properties:
[0072] 1) chemically bonds to the encapsulant 22 to provide high
adhesive strength to the encapsulant; and
[0073] 2) chemically bonds to the passivation layer on the chip 10
to provide high adhesive strength to the chip.
[0074] Generally, the chip 10 is passivated with a thin layer of
either silicon nitride, polyimide, or benzocyclobutene. To adhere
well to the passivation layer (not shown) on the chip 10, the chip
bonding layer 32 may incorporate a coupling agent (not shown) such
as a silane. To adhere well to the encapsulant 22, the coupling
agent (not shown) provides a chemically compatible moiety for
bonding. For example, the preferred moieties can be epoxides,
anhydrides, hydroxyls, or other moiety that readily bonds to the
encapsulant 22.
[0075] The substrate bonding layer or adhesive flux 34 is a
composition with the following properties:
[0076] 1) a strong fluxing agent that removes oxides from the metal
surfaces to be soldered and promotes wetting of the metal pads to
be soldered;
[0077] 2) crosslinks into an adhesive polymer during the soldering
operation, chemically immobilizing the fluxing agent and the flux
reaction byproducts;
[0078] 3) has a sufficiently low viscosity during the soldering
operation that it does not impede the flow of the molten
solder;
[0079] 4) after curing, no cleaning or washing for flux removal is
required;
[0080] 5) high adhesive strength after cure;
[0081] 6) corrosion resistance and resistance to degradation at
soldering temperatures; and
[0082] 7) does not evolve any gases that can cause voids or bubbles
in the adhesive during curing.
[0083] In general terms, the substrate bonding layer or adhesive
flux 34 comprises a liquid or solid composition which acts as both
a primary fluxing agent and a crosslinking monomer or polymer. More
specifically, the adhesive fluxes comprise the following:
[0084] 1) chemical components with carboxylic acid moieties for
fluxing;
[0085] 2) chemical components with polymerizable moieties for
crosslinking the composition;
[0086] 3) a chemical or mechanical mechanism for impeding or
preventing the onset of polymerization of the composition until the
solder has melted and wetted all the surfaces to be soldered;
and
[0087] 4) optional solvents, fillers, moderating agents,
surfactants, modifiers, resins and other additives performing
desirable functions and generally known to those skilled in the
art.
[0088] A number of compositions are known in the prior art
comprising these features, such as described in U.S. Pat. Nos.
5,376,403, 5,088,189, 5,136,365 and 5,128,746. A preferred
composition is directed to fluxing adhesive compositions that
include a fluxing agent comprising a single active component which
is capable of functioning as both a primary fluxing agent and a
crosslinking monomer. Generally, depending upon the intended end
use, the inventive thermally curable adhesive composition comprises
(a) a fluxing agent having a carboxylic acid group and one or more
carbon-carbon double bonds, (b) optionally, a crosslinkable
diluent, (c) optionally, a free-radical initiator, and (d)
optionally, a resin.
[0089] In addition the thermally curable adhesive composition may
include a solvent for adjusting the viscosity. Other viscosity
modifiers, thickeners and thixotropic agents may also be added.
Fillers, such as silica powder, can be employed for increased
modulus and lower thermal coefficient of expansion.
[0090] 1. Fluxing Agents. The preferred fluxing agent has the
structure RCOOH, wherein R comprises a moiety which include two or
more carbon-carbon double bonds.
[0091] For high flux activity due to the presence of multiple
carboxylic acids, the preferred fluxing agent is a carboxylic acid
that is selected from the group consisting of compounds represented
by Formulae I, II, III, and IV and mixtures thereof,
HOOCCH.dbd.CH(O)COR.sup.18OC(O)CH.dbd.CHCOOH (I)
R.sup.2H.sub.2C(HCOR.sup.n).sub.nCH.sub.2OR.sup.3 (II)
[X.sup.1X.sup.2X.sup.3X.sup.4]C (III)
R.sup.17COOH (IV)
[0092] wherein R.sup.18 is an alkyl having 1 to 16 carbons,
preferably 1 to 9 carbons, and more preferably 1 to 3 carbons,
wherein n is an integer from 1 to 16 preferably an integer from 1
to 9, and more preferably an integer from 1 to 3, wherein each of
R.sup.1, R.sup.2, . . . R.sup.n, is independently selected from
--C(O)CH.dbd.CHCOOH, and H, wherein X.sup.1, X.sup.2, X.sup.3, and
X.sup.4, are each independently selected from --CH.sub.2OH and
--CH.sub.2OC(O)CH.dbd.CHCOOH, and wherein R.sup.17 is a moiety
having two or more carbon-carbon double bonds and an amine moiety.
The fluxingagent typically comprises about 0.01%-100%, preferably
about 5%-80%, and more preferably about 10%-70% by volume of the
thermally curable adhesive composition. A particularly preferred
fluxing agent which has low-viscosity and high flux activity is
tris (maleic acid) glycerol monoester which is described in Example
1.
[0093] The fluxing agents of the preferred flux exhibit flux
activities that are superior to that of prior art polymer-fluxing
agent mixtures. Since the inventive fluxing agents are
intrinsically self-crosslinking, the preferred thermally curable
adhesive composition does not require the use of epoxy resins for
crosslinking. As a corollary, the shelf life or pot life of the
preferred composition is long and its flux activity high relative
to conventional polymer-fluxing mixtures that include epoxy
resins.
[0094] Further, the adhesion properties, mechanical integrity, and
corrosion resistance achieved with the fluxing agents are superior
to those achieved with prior art polymer fluxing agents because
there is no need to add aggressive fluxing activators. The
inventive fluxing agents are fully cross-linked and all components
thereof are chemically immobilized upon curing. Even the reaction
by-products of flux deoxidization of the metals may be chemically
bound in the polymer matrix.
[0095] Carboxylic acids function well as fluxing agents to remove
oxides from metals. In addition, carboxylic acids are also very
effective crosslinking moieties when present in their reactive form
in a fluxing composition containing a suitable thermosetting resin,
such as an epoxy. For this reason, in the prior art, chemical
protection of the carboxylic acid was essential to achieving
stability and preventing premature reactions. Protection was
achieved by binding the fluxing agent with a chemically- or
thermally-triggered species so that it becomes reactive only at or
near the time that the solder melts. However, with the present
invention, no such protection is necessary because the compositions
can be formulated without any components that can crosslink with
the carboxylic acid moiety. This results in a fluxing agent that
can function at its full strength with the metal oxides to produce
fluxing that is superior to any heretofore polymerizable fluxing
agent. The flux activity of the inventive fluxing agent in some
applications may be too high thereby requiring dilution of the
fluxing agent to prevent formation of undesirable gaseous
by-products.
[0096] With the inventive fluxing agent, the principal crosslinking
mechanism occurs at the carbon-carbon double bonds existing in the
fluxing agent molecule and not at the carboxylic acid groups. The
carboxylic acids do not react with the double bonds, therefore on
its own, in the absence of other molecules that can react with the
carboxylic acid, the fluxing agent does not polymerize at ambient
temperatures. It is at elevated temperatures that the double bonds
begin to open and react with other opened double bonds to
crosslink. Since each fluxing agent molecule contains at least two
double bonds, the molecules crosslink into polymeric networks.
[0097] By eliminating the need for a separate thermosetting resin
in the flux composition, as is required in the prior art, the flux
activity can be kept very high without concern about pre-maturely
cross-linking the thermosetting resin. By crosslinking the fluxing
agent itself, an adhesive having a higher glass transition
temperature and lower coefficient of thermal expansion can be
created without sacrificing fluxing activity.
[0098] A preferred embodiment of the fluxing agent has an amine
moiety that is incorporated into the fluxing agent molecule itself.
The generalized structure for carboxylic acids containing two or
more carbon-carbon double bonds and also containing an amine is:
1
[0099] in which R.sup.7 comprises at least one amine group and two
carbon-carbon double bonds. For high flux activity due to the
presence of multiple carboxylic acids, the presently preferred
carboxylic acids containing double carbon-carbon bonds has the
general structure: 2
[0100] where R.sup.3, R.sup.4, and R.sup.5 are either --H or
--OCCH.dbd.CHCOOH. For its low viscosity and high flux activity, a
particularly preferred amine containing fluxing agent is tris
(maleic acid) triethanolamine monoester which is described in
Example 2.
[0101] The fluxing agent molecules having an amine moiety can
moderate each other without the addition of a separate component,
as illustrated here:
[0102] The net result of this moderating mechanism is to cause the
fluxing agent to gel at room temperature. Yet, as the temperature
is elevated above approximately 50-100.degree. C., these materials
will liquefy readily to a low viscosity liquid, indicating the
3
[0103] thermal disassociation of these ionic bonds. Thus the
carboxylic acid moiety is then fully discharged to flux the
oxidized metal surfaces at temperatures above 50-100.degree. C.
[0104] Fluxing agents that do not contain nitrogen (e.g., amine) as
represented, for example, by Formulae I, II, III, and IV, typically
are liquid at ambient temperatures (.about.23.degree. C.).
Therefore, no solvent is required. In contrast, amine containing
fluxing agents are solid or semi-solid at ambient temperatures and
form gels. with the addition of water or other solvent. Thus, by
employing both amine and non-nitrogen containing fluxing agents and
optionally including a solvent, a thermally curable adhesive
composition having the consistency of a tacky gel mixture can be
formulated. With the present invention, thermally curable adhesive
compositions can be formulated to be in the liquid, gel, or solid
state.
[0105] 2. Diluents. The presence of carbon-carbon double bond(s) in
the preferred fluxing agent molecule allows much flexibility in the
formulation of a flux composition with exceptional thermomechanical
properties. This is achieved by the addition of double bond
containing diluents that can also crosslink with the preferred flux
to create a superior adhesive. This technique permits the design of
fluxing adhesive compositions that can attain high crosslink
densities, which are desirable for good thermomechanical properties
and good adhesion. Moreover, this is accomplished without the
concern of premature crosslinking and reduced pot life associated
with the prior art. Preferred diluents include, for example, (a)
penta erythritol tetraacrylate,
C(CH.sub.2OOCCH.dbd.CH.sub.2).sub.4, (b)
triallyl-1,3,5-triazine-2,4,6 (1H,3H,5H)-trione, (c)
tris[2-(acryloxy)ethyl]isocyanurate, and mixtures thereof. Diluents
(b) and (c) have the following structures: 4
[0106] Other double bond compounds, many of which are commercially
available, including, for example, diallyl phthalate and divinyl
benzene can also be used. Hydrophobic diluents as described are
preferred but hydrophilic diluents can also be employed when
appropriate. The diluent when employed typically can comprise up to
about 90%, preferably between about 5%-80%, and more preferably
between about 50%-80% by volume of the thermally curable adhesive
composition.
[0107] One benefit of employing hydrophobic diluents is that their
presence tends to reduce the amount of water which the cured
adhesive composition will absorb. The reason is that the fluxing
agent, when crosslinked, will have active carboxylic groups that
can attract water, even though these carboxylic groups, being part
of a network, are immobile. Water acts as a plasticizer which
softens the cured adhesive composition. The use of hydrophobic
diluents which are crosslinked to the fluxing agent will counteract
the hydrophilic effects of the carboxylic acid groups. Indeed, the
cured adhesive compositions containing hydrophobic diluents can
have less than 2% (wt) moisture when exposed to ambient
conditions.
[0108] 3. Free Radical Initiators. While the preferred thermally
curable adhesive flux composition can be cured using heat alone,
the cross linking reaction can be initiated and facilitated by the
presence of free-radicals, including, for example, those generated
by benzoyl peroxide, butyl hydroperoxide,
2,2'-azobisisobutyronitrile, and mixtures thereof. These free
radical initiators or sources are commercially available.
[0109] Free-radicals can be created in-situ by exposure of the
free-radical initiator to heat, radiation, or other conventional
energizing sources. Introduction of an appropriate free-radical
initiator can accelerate the onset of crosslinking to the desired
moment in a solder reflow operation. The presence of a small amount
of free-radical crosslinking initiator in the fluxing agent can be
used to control the rate and the temperature of crosslinking of the
fluxing agent, ensuring effective fluxing action and strong
adhesion of the fluxing agent to the substrates upon curing.
[0110] The free radical initiator when employed typically comprises
up to about 5%, preferably between about 0%-3%, and more preferably
about 0.3%-1% by weight of the thermally curable adhesive
composition.
[0111] 4. Resins. The preferred thermally curable adhesive flux
composition does not require resins; further, compositions that do
not include resins tend to have longer pot lives and lower
viscosities during solder reflow. However, as an option, a resin
can be employed and it functions to increase the adhesion of the
cured composition to the substrate and to increase the cohesive
strength and glass transition temperature of the cured composition.
The resin may be any suitable resin that is compatible (i.e.,
blendable) with the preferred fluxing agent. By blendable is meant
that the resins do not have to be chemically bonded to the fluxing
agent and/or diluent, however, preferred resins can crosslink with
the carboxylic acid groups in the fluxing agent or by other
reactive moieties, such as optional --OH groups, in the diluent.
Resins which meet these requirements include, but are not limited
to, epoxies, phenolics, novalacs (both phenolic and cresolic),
polyurethanes, polyimides, bismaleimides, maleimides, cyanate
esters, polyvinyl alcohols, polyesters, and polyureas. Preferred
resins 1,4-cyclohexanedimethanol diglycidyl ether,
3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxyla- te,
N,N-diglycidyl-4-glycidyl-oxyanilline, bisphenol A based epoxy
resins, and mixtures thereof. These are commercially available.
[0112] Suitable compounds (including polymers) can also be modified
to form resins that are blendable with the diluent and/or the
carboxylic acid fluxing agent. Examples of such compounds are
acrylics, rubbers (butyl, nitrile, etc.), polyamides,
polyacrylates, polyethers, polysulfones, polyethylenes,
polypropylenes, polysiloxanes, polyvinyl acetates/polyvinyl esters,
polyolefins, cyanoacrylates, and polystyrenes. Generally, any
compound can function as a resin if it can be modified to contain
at least one of the following illustrative functional groups that
act as reactive sites for polymerization: anhydrides, carboxylic
acids, amides, amines, alcohols/phenols, nitrites, carbamates,
isocyanates, sulfonamides, semicarbazones, oximes, hydrazones,
cyanohydrins, ureas, phosphoric esters/acids, thiophosphoric
esters/acids, phosphonic esters/acids, phosphites, phosphonamides,
and sulfonic esters/acids. For example, a polyolefin which has no
reactive sites for binding and has poor adhesive properties is
typically not a suitable resin, however, a carboxylated polyolefin
functions well when matched with a suitable cross-linking agent. A
combination of these and other resins, such as non-cross-linkable
thermoplastic resins, may also be used as resins. Resins when
employed can comprise up to about 80%, preferably between about
10%-80%, and more preferably about 60%-70% by volume of the
thermally curable adhesive composition.
[0113] In preparing the preferred fluxing composition, the
proportions of the four components may be varied over a
considerable range and still yield acceptable fluxing activity as
well as good post cured material properties. Preferably, the
fluxing composition employed does not produce gaseous byproducts
that can result in the formation of bubbles in the final cured
composition. This can be achieved with thermally curable adhesive
compositions preferably formulated as follows:
[0114] a) Fluxing agent comprising about 5%-80% (vol) of the
composition;
[0115] b) Diluent comprising about 5%-80% (vol) of the
composition;
[0116] c) Free radical initiator comprising about 0%-3% (wt) of the
composition; and
[0117] d) Resin comprising about 0%-80% (vol) of the
composition.
[0118] Some of the thermally curable adhesive compositions within
these ranges may exhibit undesirably high moisture absorption, low
glass transition temperatures, or high coefficients of thermal
expansions after cured, but they remain useful as fluxing
compositions in applications where these characteristics are not
critical.
[0119] Most preferably, the thermally curable adhesive composition
after being cured has a coefficient of thermal expansion of about
25 ppm/.degree. C., a glass transition temperature in excess of
150.degree. C. and moisture content of less than 2%. These
characteristics can be achieved with thermally curable adhesive
compositions preferably formulated without any free radical
initiator or resin but comprising about 10%-70% (vol) fluxing agent
and about 20%-80% (vol) diluent.
[0120] While, again, some of the fluxing agents within these ranges
may exhibit high coefficient of thermal expansion or low glass
transition temperature when cured, they remain useful as fluxes in
applications where these characteristics are not critical.
[0121] In order for the thermally curable adhesive composition to
achieve the largest spreading and wetting by the solder, it must
achieve and maintain low viscosity up to the temperature at which
the solder melts and wets the metallizations. If the composition
becomes too thick before the solder has melted, it will impede the
flow of the solder melt and reduce the degree of metal soldering.
For this reason, the curing of the composition must occur slowly
relative to the time required to reach the melting point of the
solder. This can be achieved by selection of the components with
appropriate crosslinking temperatures and formulating the
appropriate proportions by use of a differential scanning
calorimeter to control reaction rates and times.
[0122] The thermally curable adhesive composition can also be used
as a fluxing adhesive for use in sinterable conductive ink
compositions that comprises:
[0123] a) 1% to 65% (wt) of a high melting point metal or metal
alloy powder, typically comprising Cu powder, however, other metals
such as, for example, Ag, Au, Pt, Pd Be, Rh, Ni, Co, Fe, Mo, and
high-melting point alloys thereof;
[0124] b) 6% to 65% (wt) of a low melting point metal or metal
alloy powder (solder), typically comprising Sn, Bi, Pb, Cd, Zn, Ga,
In, Hg, Sb, or an alloy thereof or other metal having a melting
point that is lower than that of the high melting metal powder in
part (a); and
[0125] c) 5% to 50% (wt) of the thermally curable adhesive flux
composition that also serves a flux composition and as an
adhesive.
[0126] Preferably the conductive ink composition comprises 13% to
65% (wt) of the high melting point metal, 6% to 29% (wt) of the low
melting point metal, and/or 5% to 35% (wt) of the thermally curable
adhesive flux composition.
[0127] Techniques for employing electrically conductive ink
compositions are described in U.S. Pat. Nos. 5,376,403, 5,538,789,
and 5,565,267 which are incorporated herein. During the curing
process of the sinterable conductive ink compositions, in order for
the solder alloy to readily wet the other powder and sinter, the
principal requirement of the thermally curable adhesive composition
is that the polymers not harden before melting of the solder powder
is achieved. Additionally, after curing, the composition must act
as an adhesive that strongly binds the cured ink composition to the
printed circuit board substrate. The flux compositions of the
instant invention are particularly suited for these
applications.
[0128] The inventive thermally curable composition exhibit the
following features:
[0129] a) provides sufficient flux activity to promote the solder
bump to readily wet the metallization on the substrate during
solder reflow, without the presence of corrosive flux activators
that can contaminate the silicon chip;
[0130] b) promotes solder wetting and self-alignment of the chip to
the pads on the substrate by action of the wetting force of the
molten solder, during the solder reflow cycle, no curing of the
flux composition occurs until the solder bump has been melted;
[0131] c) reduces or eliminates gaseous evolution during the reflow
cycle that would otherwise create voids;
[0132] d) cures quickly and soon after solder bump melts;
[0133] e) demonstrates little shrinkage of the composition during
curing to maximize the stress resulting from the curing process and
subsequent cooling; and
[0134] f) forms strong adhesion of the cured composition to the
chip, substrate and solder joints.
Synthesis of Fluxing Agents
EXAMPLE 1
Preparation of tris (maleic acid) glycerol monoester, a non-amine
fluxing agent with the structure
[0135] 5
[0136] Three moles of maleic anhydride (294 grams) were heated in a
flask at 80.degree. C. until fully melted at which time one mole of
glycerol (92 grams) was slowly added thereto. The composition was
constantly stirred and maintained at 80.degree. C. for three hours.
The temperature was then raised to 110.degree. C. for one hour to
complete the reaction. Thereafter the product was allowed to cool
to room temperature. The reactants were kept in a nitrogen
atmosphere throughout. Monitoring the reaction on a
Fourier-transform infrared spectrometer, the OH vibrational band at
3,400-3,500 cm.sup.-1 of the glycerol was observed to become
minimized while an ester vibration band at 1,710-1,740 cm.sup.-1
appeared and maximized, indicating complete reaction of the
glycerol and the anhydride. This fluxing agent is characterized by
its low viscosity and high flux activity.
EXAMPLE 2
Preparation of tris (maleic acid) triethanolamine monoester: an
amine fluxing agent, with the structure
[0137] 6
[0138] Three moles of maleic anhydride (294 grams) were heated in a
flask at 80.degree. C. until fully melted at which time one mole of
triethanolamine (149 grams) was slowly added thereto over the
course of one hour, so that gelation did not occur. The composition
was constantly stirred and maintained at 80.degree. C. To ensure
that the reaction went to completion, the product was maintained at
80.degree. C. with constant stirring for an additional hour. The
reactants were kept in a nitrogen atmosphere throughout. Then the
product was allowed to cool to room temperature. Monitoring the
reaction on a Fourier-transform infrared spectrometer, the OH
vibrational band at 3,400-3,500 cm.sup.-1 of the triethanolamine
was observed to become minimized while an ester vibration band at
1,710-1,740 cm.sup.-1 appeared and maximized, indicating complete
reaction of the triethanolamine and the anhydride.
[0139] This fluxing agent is also characterized by its low
viscosity and high flux activity.
EXAMPLE 3
Preparation of methyl meso-erythritol tetramaleic acid
monoester
[0140] 39 g maleic anhydride was heated to 80.degree. C. until all
the maleic anhydride was melted before 12.2 g of meso-erythritol
was added under mechanical stirring. The temperature was then
raised to 130.degree. C. for 30 minutes followed by cooling down to
80.about.90.degree. C. for 2 hours. The reaction is: 7
EXAMPLE 4
Preparation of pentaerythritol ethoxylate tetramaleic acid
monoester
[0141] 39 g maleic anhydride was heated to 80.degree. C. until all
the maleic anhydride was melted before 27 g pentaerythritol
ethoxylate (average Mn ca 270) was added under mechanical stirring.
The reactants are stirred at 80.degree. C. for 2.about.3 hours to
complete the reaction. The reaction is: 8
EXAMPLE 5
Preparation of adonitol pentamaleic acid monoester
[0142] 49 g maleic anhydride was heated to 80.degree. C. until all
the maleic anhydride was melted before 15.2 g of adonitol was added
under mechanical stirring. The temperature was then increased to
120.degree. C. for 30 minutes followed by cooling down to
80.degree. C. The reactants were stirred at 80.degree. C. for 3
hours to finish the reaction. The reaction is: 9
[0143] The embodiment of FIG. 15 of a compliant multilayer
encapsulating structure comprises a soft solder that is more
compliant than conventional tin-lead eutectic solder or the 95%
lead-5% tin solder used often for flip chip solder. bumps . There
are at least two methods for accomplishing this, and other methods
will be known to those skilled in the art. One method comprises a
tin-lead solder that has been modified with a small concentration,
generally less than 1%, of an additive or additives. One. such
additive, described in U.S. Pat. No. 5,308,578, is a small
concentration of cadmium, indium, antimony, or combination thereof
which is known to increase the fatigue life of the solder by up to
twenty fold. Another method involves incorporating a small
concentration of tin-copper or tin-nickel intermetallic which is
known to decrease fatigue-induced microstructural coarsening
leading to fatigue failure in tin-lead solder.
[0144] Another method for increasing the compliance of solder in
the present invention involves using a low-melting point solder. It
is well known in the art that solders become more compliant as the
temperature is elevated to approach their melting temperatures. In
the instant invention, the preferred solder has a melting point
near the highest operating temperature of the flip chip. Near such
a melting point, the solder provides little mechanical resistance
to the compliance of the chip-to-substrate interconnection and will
readily conform to the stress induced by expansion or contraction
of the interconnect during temperature excursions with little
fatigue.
[0145] Another method for increasing the compliance of solder in
the present invention involves using a non-eutectic solder
operating between the liquids and solidus temperatures. It is well
established that in non-eutectic solders, there exist a temperature
region in which the solder is neither fully solid nor fully liquid,
but instead is a mixture of both phases, i.e., the plastic range.
In the plastic range, the solder does not flow as liquid, yet it
has very little mechanical integrity or structural strength. Under
stress in the plastic range, the solder will flow readily,
conforming to the applied stress without cracks or fatigue,
provided the solder is not allowed to leak out of the solder bump
by the encapsulating polymer. For this reason, a non-eutectic
solder alloy can be selected that will be plastic over most of the
temperature range experienced by the chip interconnect. Such
solders may contain, but are not limited to, alloys of tin, lead,
bismuth, indium, cadmium, gallium, zinc, antimony, and other metals
known to the art of soldering.
[0146] With the present invention illustrated in FIG. 15, as the
flip-chip configuration is heated causing the substrate 20 to
laterally expand greater than the chip 10 because of the mismatch
in the thermal coefficients of expansion between the chip and
substrate, the complaint solder 14 and flexible encapsulant
material 22 deform by expanding with the substrate 20 to absorb the
strain without causing bending of the chip and substrate (FIG. 16).
Likewise, as the flip-chip configuration is cooled below ambient
temperature, the complaint solder 14 and flexible encapsulant 22
contract with the substrate 20 to absorb the strain (FIG. 17).
[0147] In another embodiment of the invention, there is a chip 10
having solder bumps 14 pre-assembled thereon and being pre-coated
with a multi-layer encapsulant material 36 prior to assembly to the
substrate 20 (FIG. 18). The multi-layer encapsulant material 36 is
uniform across the surface of the chip 10 between the solder bumps
14. Each layer of the multi-layer encapsulant material 36 perform
distinct functions. Layers 38 and 40 are attachment and stress
distribution layers. Layer 42 is the reworkability layer. Layers 38
and 40 are generally stiffer than layer 42. Layers 38 and 40 are
generally polymers or polymers filled with inorganic materials so
as to have a high modulus and a low coefficient of thermal
expansion such as polyimide. Layer 42 is generally a meltable
polymer such as a thermoplastic, for example a polyimide-siloxane
co-polymer. The layers can be comprised of coated tape, such as
SumiOxy.RTM. ITA-5120 or ITA-5315 available from Oxychem, Grand
Island, N.Y. A flux adhesive 34 as described previously is applied
between the chip/encapsulant/solder bump combination and the
substrate. The solder is reflowed and the flux adhesive 34 is
hardened. Rework is made possible by the layer 42. The layer 42 and
solder bumps 14 are remelted and the chip 10 is pulled away from
the substrate 20 (FIG. 20). The flux adhesive 34 firmly retains the
layer 40 and part of the solder bumps 14 on the substrate 20 while
the chip bonding layer 38 firmly retains the other part of the
solder bumps 14 on the chip 10 as the reworkable layer 42 separates
without damage to the chip 10 or the substrate 20.
[0148] In another embodiment of the present invention, there is
provided a multi-layer encapsulant material 44 attached to a chip
10 (FIG. 21). Within the multi-layer encapsulant 44 is an
electrical redistribution layer 46 of electrically conductive
traces 48 on an insulating layer 50. The insulating layer 50, such
as a polymer, encapsulate the solder bumps 52 and 54. Solder bump
52 is connected to the chip 10 in a conventional manner. Solder
bumps 54 are attached to the closely spaced contact pads 24 by the
electrically conductive traces 48 of the redistribution layer 46.
Flux adhesive 34 as described previously retains the
chip/multi-layer encapsulant/solder bump combination on the
substrate 20 (FIG. 22).
[0149] It will now be apparent to those skilled in the art that
various modifications, variations, substitutions, and equivalents
exist for various elements of the invention but which do not
materially depart from the spirit and scope of the invention.
Accordingly, it is expressly intended that all such modifications,
variations, substitutions and equivalents which fall within the
spirit and scope of the invention as defined by the appended claims
be embraced thereby.
* * * * *