U.S. patent application number 09/961351 was filed with the patent office on 2003-02-20 for method of forming metal bumps.
This patent application is currently assigned to Fujitsu Limited. Invention is credited to Sukuyama, Seiki.
Application Number | 20030036255 09/961351 |
Document ID | / |
Family ID | 18983607 |
Filed Date | 2003-02-20 |
United States Patent
Application |
20030036255 |
Kind Code |
A1 |
Sukuyama, Seiki |
February 20, 2003 |
Method of forming metal bumps
Abstract
Metal bumps are formed by using a bump material containing a
metal material which melts only partially at a first temperature
and melts entirely at a second temperature higher than the first
temperature. A resin film is first formed on a surface of a
substrate provided with electrodes. Then, openings are formed in
the resin film for exposing the electrodes. Then, the bump material
is loaded into the openings. Then, the bump material is heated to
the first temperature for melting only part of the metal material,
followed by cooling the bump material below the first temperature.
Then, the resin film is removed. Finally, the bump material is
heated to the second temperature for entirely melting the metal
material.
Inventors: |
Sukuyama, Seiki;
(Kawasaki-shi, JP) |
Correspondence
Address: |
ARMSTRONG,WESTERMAN & HATTORI, LLP
1725 K STREET, NW.
SUITE 1000
WASHINGTON
DC
20006
US
|
Assignee: |
Fujitsu Limited
Kawasaki
JP
|
Family ID: |
18983607 |
Appl. No.: |
09/961351 |
Filed: |
September 25, 2001 |
Current U.S.
Class: |
438/613 |
Current CPC
Class: |
H01L 2224/05568
20130101; H05K 2203/043 20130101; H05K 2203/0568 20130101; H05K
3/3494 20130101; H01L 21/4846 20130101; H01L 2224/05573 20130101;
H05K 3/3485 20200801; H01L 21/4853 20130101; H05K 3/3452 20130101;
H05K 2203/1476 20130101; H01L 2224/1147 20130101; H01L 2924/00014
20130101; H01L 2924/00014 20130101; H01L 2224/05599 20130101 |
Class at
Publication: |
438/613 |
International
Class: |
H01L 021/44 |
Foreign Application Data
Date |
Code |
Application Number |
May 27, 2001 |
JP |
2001-136299 |
Claims
1. A method of forming metal bumps comprising the steps of: forming
a resin film on a surface of a substrate provided with electrodes;
forming openings in the resin film for exposing the electrodes;
loading a bump material into the openings, the bump material
containing a metal material which melts only partially at a first
temperature, the metal material melting entirely at a second
temperature higher than the first temperature; heating the bump
material to the first temperature for melting only part of the
metal material; cooling the bump material below the first
temperature; removing the resin film; and heating the bump material
to the second temperature for entirely melting the metal
material.
2. The method according to claim 1, wherein the metal material
comprises a metal alloy of a composition which has a solid-liquid
coexistent temperature range between a solidus temperature and a
liquidus temperature, the first temperature being equal to or
higher than the solidus temperature and lower than the liquidus
temperature, the second temperature being equal to or higher than
the liquidus temperature.
3. The method according to claim 1, wherein the metal material
contains a plurality of different metals, one of the metals having
a lowest melting point, another of the metals having a highest
melting point, the first temperature being equal to or higher than
the lowest melting point and lower than the highest melting point,
the second temperature being equal to or higher than the highest
melting point.
4. The method according to claim 1, wherein the resin film is made
of a photosensitive resin.
5. The method according to claim 1, wherein the metal material is
contained in the bump material as powder, the bump material
comprising a solder paste containing the metal powder mixed with a
resin and a solvent.
6. A method of forming metal bumps comprising the steps of: forming
a resin film on a surface of a substrate provided with electrodes;
forming openings in the resin film for exposing the electrodes;
loading a bump material into the openings, the bump material
containing a metal of a composition which has a solid-liquid
coexistent temperature range between a solidus temperature and a
liquidus temperature; heating the bump material to a first
temperature which is equal to or higher than the solidus
temperature and lower than the liquidus temperature; cooling the
bump material below the solidus temperature; removing the resin
film; and heating the bump material to a second temperature which
is equal to or higher than the liquidus temperature.
7. A method of forming metal bumps comprising the steps of: forming
a resin film on a surface of a substrate provided with electrodes;
forming openings in the resin film for exposing the electrode
portions; loading a bump material into the openings, the bump
material containing a plurality of different metals, one of the
metals having a lowest melting point, another of the metals having
a highest melting point; heating the bump material to a first
temperature which is equal to or higher than the lowest melting
point and lower than the highest melting point; cooling the bump
material below the lowest melting point; removing the resin film;
and heating the bump material to a second temperature which is
equal to or higher than the highest melting point.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a bump forming method. More
particularly, the present invention relates to a method of forming
metal bumps on electrodes provided on a printed circuit board, a
wafer or a ceramic board by utilizing a resin film as a mask.
[0003] 2. Description of the Related Art
[0004] Recently, there is an increasing demand for mounting
electronic components on a substrate (e.g. a printed circuit board)
at high densities. For meeting such a demand, much attention is
focused on bear chip mounting. The bear chip mounting includes the
face-up bonding which utilizes wire-bonding for providing
electrical connection between a chip and a wiring pattern on a
circuit board, and the face-down bonding which utilizes metal bumps
for providing electrical connection. Recently, the face-down
bonding increasingly replaces the face-up bonding.
[0005] The face-down bonding which utilizes metal bumps is capable
of connecting electronic components at a low resistance. In forming
metal bumps, the following requirements should be satisfied.
[0006] When the electrodes of an electronic component are arranged
at a high density, metal bumps need be correspondingly arranged at
a small pitch to be precisely positioned on the electrodes. This is
particularly true in forming metal bumps on the electrodes of a
semiconductor device. Further, all metal bumps need to have an
equal height for ensuring reliable connection between electronic
components. In addition, there is also an inherent demand for a
decrease in the manufacturing cost.
[0007] Conventionally, metal bumps for face-down bonding have been
formed by plating or vapor deposition. However, the formation of
metal bumps by such methods requires much cost for the equipment
while providing difficulties in controlling the height and
composition of the bumps. Recently, therefore, the conventional
methods are increasingly replaced with a metal mask printing method
and a resin mask loading method for realizing a cost reduction
while providing a higher freedom in controlling the composition of
metal bumps.
[0008] FIGS. 2a through 2e illustrate a prior art metal mask
printing process for forming metal bumps. According to the metal
mask method, as shown in FIG. 2a, use is made of a board 20
provided with electrodes 21, and a metal mask 22 formed with
openings 22a corresponding to the electrodes 21. As shown in FIG.
2b, the metal mask 22 is placed on the board 20 for bringing the
openings 22a in conformity with the electrodes 21. Then, as shown
in FIG. 2c, a solder paste 23 containing solder powder is loaded in
each of the openings 22 by printing. Then, as shown in FIG. 2d, the
metal mask 22 is removed from the board 20. Subsequently, as shown
in FIG. 2e, the solder powder contained in the solder paste 23 is
melted by heating, thereby providing generally spherical metal
bumps 24 on the electrodes 21 of the board 20. The metal bump
formation by such a metal mask printing method is disclosed in
JP-A-7-302972 for example.
[0009] FIGS. 3a through 3e illustrate a prior art resin mask
loading process for forming metal bumps. First, as shown in FIG.
3a, a resin film 32 is formed on a board 30 provided with
electrodes 31. Then, as shown in FIG. 3b, the resin film 32 is
partially etched away for forming openings 32a for exposing the
electrodes 31 of the board 30. Then, as shown in FIG. 3c, a solder
paste 33 containing solder powder is loaded in each of the openings
32a. Subsequently, as shown in FIG. 3d, the solder powder contained
in the solder paste 23 is melted by heating, thereby providing
generally spherical metal bumps 34 on the electrodes 31 of the
board 30. Finally, as shown in FIG. 3e, the resin mask 32 is
removed from the board 30.
[0010] The metal mask printing method described above has a
drawback that it has difficulties in controlling the height of the
metal bumps formed at a relatively small pitch. Specifically, when
the openings 22a of the metal mask 22 are arranged at a small
pitch, removal of the metal mask 22 may cause part of the solder
paste 23 filled in the openings 22a to be removed together. As a
result, the metal bumps 24 may vary significantly in height, which
may hinder reliable mounting of electronic components.
[0011] In the resin mask loading method, on the other hand, the
resin film 32 as a printing mask is removed after the solder paste
33 are melted by heating. Therefore, it is possible to reliably
form each of the metal bumps 35 with a predetermined amount of
solder paste even when electrodes are arranged at a small pitch.
Thus, in comparison with the metal mask printing method, the resin
mask loading method is preferable for forming metal bumps at a
small pitch which is necessary for realizing high density mounting
of electronic components.
[0012] However, the prior art resin mask loading method has the
following problems. In melting the solder powder contained in the
solder paste 33 in the step shown in FIG. 3d, the solder powder is
generally heated at a temperature which is 30.about.50.degree. C.
higher than the melting point of the solder metal. In this heating
step, however, the resin film (typically made of a thermosetting
resin) hardens to some extent under heating. Therefore, in removing
the resin film in the subsequent step shown in FIG. 3e, part of the
resin film thus hardened may remain on the surface of the board,
which hinders reliable mounting of electronic components.
SUMMARY OF THE INVENTION
[0013] It is therefore an object of the present invention to
provide a method of forming metal bumps using a resin film, which
is capable of easily removing the resin film for forming good bumps
which allow reliable mounting of electronic components.
[0014] According to a first aspect of the present invention, a
method of forming metal bumps is provided which comprises the steps
of: forming a resin film on a surface of a substrate provided with
electrodes; forming openings in the resin film for exposing the
electrodes; loading a bump material into the openings, the bump
material containing a metal material which melts only partially at
a first temperature, the metal material melting entirely at a
second temperature higher than the first temperature; heating the
bump material to the first temperature for melting only part of the
metal material; cooling the bump material below the first
temperature; removing the resin film; and heating the bump material
to the second temperature for entirely melting the metal
material.
[0015] With the above-described method, it is possible to remove
the resin film from the surface of the substrate more reliably than
in the prior art method. Specifically, when the bump material
loaded in the openings is heated to the first temperature
(hereinafter referred to as "primary heating"), only part of the
metal material changes from solid phase to liquid phase. At this
time, due to surface tension, the liquid phase part tends to cohere
for integration with the remaining solid phase part. When the bump
material is thereafter cooled below the first temperature, the
liquid phase part returns to solid phase for fixing to the
electrodes (hereinafter referred to as "provisionally fixing"). The
cooling herein includes natural cooling wherein no positive cooling
using a coolant is performed. According to the present invention,
the resin film formed on the surface of the board as a mask is
removed after the provisional fixing.
[0016] In the prior art method, since the bump material is heated
immediately to such a high temperature as to completely melt the
metal material contained therein, the resin film hardens
undesirably due to such heating, consequently leading to difficulty
in removing the resin film. According to the present invention, by
contrast, the bump material is provisionally fixed to the
electrodes by the primary heating followed by subsequent cooling,
and the resin film is removed before the solder material is heated
at the second temperature (hereinafter referred as "secondary
heating") which is higher than the first temperature. Therefore,
the resin film can be removed more easily than in the prior art
method. With no resin film left on the substrate, electronic
components can be reliably mounted on the substrate via the
bumps.
[0017] In a first embodiment, the metal material comprises a metal
alloy of a composition which has a solid-liquid coexistent
temperature range between a solidus temperature and a liquidus
temperature. In the first embodiment, the first temperature is
equal to or higher than the solidus temperature and lower than the
liquidus temperature, whereas the second temperature being equal to
or higher than the liquidus temperature.
[0018] Herein, the solidus temperature and the liquidus temperature
are defined as follows. Under a given pressure, the solidus
temperature of an alloy is a temperature below which the alloy
exists only in solid phase, whereas the liquidus temperature of an
alloy is a temperature above which the alloy exists only in liquid
phase. At temperatures including and between the solidus
temperature and the liquidus temperature, the solid phase and
liquid phase of the alloy coexist.
[0019] In a second embodiment, the metal material contains a
plurality of different metals. In the second embodiment, one of the
metals has a lowest melting point, whereas another of the metals
has a highest melting point. Further, the first temperature is
equal to or higher than the lowest melting point and lower than the
highest melting point, whereas the second temperature is equal to
or higher than the highest melting point.
[0020] Herein, the melting point of a metal is the "melting point"
in its normal sense for a mono-elemental metal. For an alloy, on
the other hand, the melting point means the liquidus temperature of
that alloy under a given pressure.
[0021] According to the second embodiment, it is possible to
individually select said one metal for melting under the primary
heating and said another metal for melting under the secondary
heating, so that the composition of the resulting bumps can be
easily controlled.
[0022] According to a second aspect of the present invention, a
method of forming metal bumps is provided which comprises the steps
of: forming a resin film on a surface of a board provided with
electrode; forming openings in the resin film for exposing the
electrodes; loading a bump material into the openings, the bump
material containing a metal of a composition which has a
solid-liquid coexistent temperature range between a solidus
temperature and a liquidus temperature; heating the bump material
to a first temperature which is equal to or higher than the solidus
temperature and lower than the liquidus temperature; cooling the
bump material below the solidus temperature; removing the resin
film; and heating the bump material to a second temperature which
is equal to or higher than the liquidus temperature.
[0023] According to a third aspect of the present invention, a
method of forming metal bumps comprising the steps of: forming a
resin film on a surface of a substrate provided with electrodes;
forming openings in the resin film for exposing the electrode
portions; loading a bump material into the openings, the bump
material containing a plurality of different metals, one of the
metals having a lowest melting point, another of the metals having
a highest melting point; heating the bump material to a first
temperature which is equal to or higher than the lowest melting
point and lower than the highest melting point; cooling the bump
material below the lowest melting point; removing the resin film;
and heating the bump material to a second temperature which is
equal to or higher than the highest melting point.
[0024] The substrate used in the present invention may be a silicon
wafer or a circuit board formed of glass-fiber-reinforced epoxy
resin. The substrate is provided, at predetermined portions
thereof, with a plurality of electrodes made of copper, nickel, or
gold for example.
[0025] The resin film may be made of a photosensitive resin such as
an acryl-based, epoxy-based or imide-based resin or a combination
of these resins. The resin film may be etched by utilizing the
known photolithography including light-exposure and
development.
[0026] Alternatively, use may be made of a non-photosensitive
resin. In such a case, etching may be performed by the application
of a laser beam.
[0027] The resin film may be separately prepared and attached to
the substrate. Instead, the resin film may be formed in situ by
applying a liquid resin on the substrate. For forming high bumps on
the electrodes at a small pitch, it is preferable that the resin
film has a thickness of 30.about.150 .mu.m.
[0028] The resin film may be removed with a stripping agent.
Examples of the stripping agent include strong alkali such as an
aqueous solution of sodium hydroxide, organic alkali such as an
aqueous solution of monoethanolamine or an aqueous solution of
tetramethylammonium hydroxide. In use, the stripping agent may be
mixed with an additive which preferably has a function of breaking
the resin film into small pieces to prevent the resin from
remaining on the board.
[0029] Preferably, the bump material may be in the form of a paste
prepared by kneading metal powder with a flux. The flux may be a
mixture of rosin, a thixotropic agent, an activator, and a solvent
for example.
[0030] The bump material may contain an alloy consisting of at
least two metal elements selected from the group consisting of Sn,
Pb, Ag, Sb, Bi, Cu, In, and Zn for example. Specific examples
include Sb--Sn alloy, Sn--Bi alloy, Sn--In alloy, Sn--Pb alloy,
Sn--Ag alloy, Sn--Cu alloy, Sn--Zn alloy and Sn--Pb--Sb alloy. More
specifically, 5% Sn-95% Pb alloy, 43% Sn-57% Bi alloy or 48% Sn-52%
In alloy may be used. Alternatively, the bump material may contain
a mono-elemental metal such as Sn, Pb or In.
[0031] Rosin has a primary role of increasing tackiness of the
solder paste. Specifically, use may be made of polymerized rosin,
hydrogenated rosin or esterified rosin.
[0032] The thixotropic agent has a primary role of providing the
solder paste with shape-holding ability. Examples of the
thixotropic agent include hardened caster oil and stearic
amide.
[0033] The activator has a role of removing an oxide film or the
like formed on the surfaces of the solder particles and/or on the
surfaces of the electrodes when the solder paste is heated. With
the aid of the activator, the surfaces of the solder particles
and/or the surfaces of the electrodes can be cleaned, which
enhances adhesion of the solder to the electrodes and thereby
enables reliable formation of good metal bumps. Organic acid and/or
organic amine may be used as the activator. Examples include
sebacic acid, succinic acid, adipic acid, glutaric acid,
triethanolamine, and monoethanolamine.
[0034] The solvent has a role of melting soluble components and
making the flux vehicle into a pasty state. Preferably, a solvent
having a boiling point which is close to or higher than the melting
point of the solder may be used alone or in combination with
another such solvent. Examples include higher alcohols and glycol
ethers such as diethylene glycol dimethyl ether, n-buthyl phenyl
ether, 2-methyl-2,4-pentanediol, and diethylene glycol monobutyl
ether.
[0035] Preferably, each of the openings formed in the resin film
for loading the solder paste has an area no more than 25 times the
area of a corresponding electrode. This makes the solder collect
reliably on each of the electrodes upon melting for reliably
forming a spherical bump.
[0036] Other features and advantages of the present invention will
become clearer from the detailed description given below with
reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] FIGS. 1a through 1g illustrate a series of process steps for
forming metal bumps in accordance with the present invention.
[0038] FIGS. 2a through 2e illustrate a series of process steps for
forming metal bumps according to a prior art metal mask printing
method.
[0039] FIGS. 3a through 3e illustrate a series of process steps for
forming metal bumps according to a prior art resin film loading
method.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0040] Preferred embodiments of the present invention will be
described below in detail with reference to FIGS. 1a.about.1g.
[0041] First, as shown in FIG. 1a, a substrate 10 (e.g. a circuit
board) on which bumps are to be formed is prepared. The substrate
10 has a surface which is previously formed with a plurality of
electrodes 11 at a predetermined pitch. The surface of the
substrate 10 is further formed with a wiring pattern (not shown)
electrically connected to the electrodes 11.
[0042] Then, as shown in FIG. 1b, a resin film 12 formed of a
photosensitive resin is disposed and bonded under pressure onto the
board 10 for covering the electrodes 11. The resin film 12 may be
formed by applying a liquid resin to the surface of the board 10 by
spin-coat followed by thermally hardening the resin.
[0043] Subsequently, as shown in FIG. 1c, a plurality of openings
12a are formed in the resin film 12 at locations corresponding to
the electrodes 11 for exposing the electrodes 11. The openings 12a
may be formed by light-exposure using a predetermined photo-mask
(not shown), followed by developing.
[0044] Then, as shown in FIG. 1d, a solder paste 13 is loaded in
each of the openings 12a. At this time, it is desirable that a
large amount of excess solder paste does not remain on the upper
surface of the resin film 12. For this purpose, the excess of
solder paste adhering on the upper surface of the resin film 12 is
scraped off using a squeegee for example. The solder paste 13 in
this embodiment contains a single kind of alloy. The alloy is in
the form of powder and so composed as to include solid-liquid
coexisting temperature range between the liquidus temperature and
the solidus temperature. The solder paste may contain plural kinds
of alloy.
[0045] Subsequently, in a primary heating step shown in FIG. 1e,
the solder paste 13 is heated to a first temperature (primary
heating) between the liquidus temperature and solidus temperature
of the alloy contained in the solder paste, and the first
temperature is kept for a predetermined period of time. Due to
heating at the first temperature, part of the alloy changes from
solid phase to liquid phase while the remaining part of the alloy
remains in solid phase so that the alloy is kept in equilibrium of
liquid phase and solid phase. Further, at the first temperature,
most part of the ingredients of the solder paste other than the
alloy (including rosin for example), vaporizes for dissipation from
the solder paste 13. As a result, as shown in FIG. 1e, due to the
surface tension of the molten part of the alloy and of the rosin
remaining in a small amount, the solder powder in the solder paste
material integrates or collects to have a spherical shape in each
of the opening 12a. Thereafter, the solder paste is cooled below
the solidus temperature so that the molten part of the alloy
returns to solid phase. As a result, the entire solder paste
material is provisionally fixed to each of the electrodes to
provide an incomplete bump 14.
[0046] As shown in FIG. 1f, after the incomplete bumps 14 are
formed in the above-described manner, the resin film 12 is removed
from the surface of the board 10. At this time, a suitable solvent
may be selected for removing the resin film 12. Since the resin
film 12 has not yet undergone higher-temperature secondary heating
for completely melting the alloy and therefore is not hardened to a
higher extent, the resin film 12 can be removed easily under gentle
conditions.
[0047] Then, in the secondary heating step shown in FIG. 1g, the
solder paste is heated to a second temperature which is equal to or
higher than the liquidus temperature of the alloy, and the second
temperature is kept for a predetermined period of time. As a
result, the entire alloy melts. Thus, upon subsequent cooling, the
melted alloy solidifies into a complete bump 14' on each of the
electrodes 11 on the board 10.
[0048] Even when the solder paste 13 contains a plurality of
different metals which differs from each other in melting point, it
is possible to reliably form metal bumps 14' by easily removing the
resin film 12 by the above-described process steps shown in FIGS.
1a.about.1g. The "metals" referred to herein include mono-elemental
metals and alloys. For a mono-elemental metal, the term "melting
point" means the melting point in the normal sense, whereas, for an
alloy, the term "melting point" means the liquidus temperature
under a given pressure.
[0049] Specifically, in the primary heating step shown in FIG. 1e,
the solder paste 13 is heated to a first temperature equal to or
higher than the melting point of a lowest-melting-point metal and
lower than the melting point of a highest-melting-point metal, and
the first temperature is kept for a predetermined period of time.
In the secondary heating step shown in FIG. 1g, the solder paste is
heated to a second temperature equal to or higher than the melting
point of the highest-melting-point metal, and the second
temperature is kept for a predetermined period of time. As a
result, an incomplete bump 14 is formed by removing the resin film
12 after the primary heating (but before the secondary heating),
and a complete bump 14' is formed on each of the electrodes 11 on
the board 10 after the secondary heating.
[0050] When a plurality of alloys are contained in the solder
material, it is possible to reliably form metal bumps 14' by the
process steps shown in FIGS. 1a-1g. In this case, the primary
heating is performed at a first temperature equal to or higher than
a lowest solidus temperature and lower than a highest liquidus
temperature, whereas the secondary heating is performed at a second
temperature equal to or higher than the highest liquidus
temperature.
[0051] Next, description is made as to specific examples of the
present invention.
EXAMPLE 1
[0052] (Preparation of Solder Paste)
[0053] Solder of 50% Sn-50% Pb alloy (solidus temperature:
183.degree. C., liquidus temperature: 238.degree. C.) was powdered
to have an average particle size of 20 .mu.m, and the solder powder
was mixed with a flux at a volume ratio of 1:1 to prepare a solder
paste. The flux used was a mixture of 50 wt % POLY-PALE resin as
rosin, 20 wt % diethylene glycol monobutyl ether and 20 wt %
2-methyl-2,4-pentanediol as solvents, 8 wt % sebacic acid as an
activator, and 2 wt % hydrogenated castor oil as a thixotropic
agent.
[0054] (Formation of Bumps)
[0055] A film made of acrylic photosensitive resin (NIT-250
available Nichigo-Morton Co., Ltd.) having a thickness of 50 .mu.m
was bonded by thermo-compression (at 100.degree. C. under a
pressure of 3.5 kg/mm.sup.2) onto a wafer provided with 300,000
electrodes (Diameter: 70 .mu.m; Pitch: 150 .mu.m). Then, the resin
film was exposed to light at portions corresponding to the
electrodes by the use of a glass mask. Then, the resin film was
etched using 1.0% aqueous solution of sodium carbonate for
development to form a plurality of openings having a diameter of
130 .mu.m at locations corresponding to the electrodes. Then, the
above-described solder paste was loaded into the openings of the
resin film by using a urethane squeegee. Subsequently, primary
heating was performed for one minute at 220.degree. C. which was
higher than the solidus temperature of the 50% Sn-50% Pb solder to
roughly or provisionally integrate the solder. Thereafter, the
solder was cooled and provisionally fixed to the electrodes as
incomplete bumps. Then, by immersing into 10% aqueous solution of
monoethanolamine, the resin film was removed. Thereafter, a flux
(R5003 available from Alpha Metals Japan Ltd.) was applied to the
incomplete bumps provisionally fixed to the electrodes, and
secondary heating was performed for two minutes at 275.degree. C.
which was higher than the liquidus temperature of the 50% Sn-50% Pb
solder to completely melt and integrate the solder. The solder was
then cooled to provide complete bumps on the electrodes.
[0056] (Results)
[0057] In Example 1, the removal of the resin film after the
primary heating was satisfactory. The complete bumps had a height
of 80 .mu.m.+-.3 .mu.m. In this way, highly uniform bumps having
little height variation were formed. The alloy composition, solidus
temperature, liquidus temperature, primary heating temperature,
secondary heating temperature in the above-described Example 1 are
given in Table 1 together with those for Examples 2.about.5 to be
described below.
EXAMPLE 2
[0058] Solder of 20% Sn-80% Pb alloy (solidus temperature:
183.degree. C., liquidus temperature: 277.degree. C.) was used to
prepare a solder paste in a manner similar to Example 1. In Example
2, the primary heating was performed at 240.degree. C., whereas the
secondary heating was performed at 320.degree. C. As a result, the
removal of the resin film after the primary heating was
satisfactory. Further, the complete bumps had a height of 80
.mu.m.+-.3 .mu.m. In this way, highly uniform bumps of little
height variation were formed.
EXAMPLE 3
[0059] Solder of 10% Sn-90% Pb alloy (solidus temperature:
275.degree. C., liquidus temperature: 300.degree. C.) was used to
prepare a solder paste in a manner similar to Example 1. In Example
3, the primary heating was performed at 285.degree. C., whereas the
secondary heating was performed at 340.degree. C. As a result, the
removal of the resin film after the primary heating was
satisfactorily performed. Further, the complete bumps had a height
of 80 .mu.m.+-.3 .mu.m. In this way, highly uniform bumps of little
height variation were formed.
EXAMPLE 4
[0060] Solder of 92% Sn-8% Sb alloy (solidus temperature:
238.degree. C., liquidus temperature: 251.degree. C.) was used to
prepare a solder paste in a manner similar to Example 1. In Example
4, the primary heating was performed at 240.degree. C., whereas the
secondary heating was performed at 280.degree. C. As a result, the
removal of the resin film after the primary heating was
satisfactory. Further, the complete bumps had a height of 80
.mu.m.+-.3 .mu.m. In this way, highly uniform bumps of little
height variation were formed.
EXAMPLE 5
[0061] Solder of 10% Sn-85% Pb-5% Sb alloy (solidus temperature:
239.degree. C., liquidus temperature: 277.degree. C.) was used to
prepare a solder paste in a manner similar to Example 1. In Example
5, the primary heating was performed at 260.degree. C., whereas the
secondary heating was performed at 300.degree. C. As a result, the
removal of the resin film after the primary heating was
satisfactory. The complete bumps had a height of 80 .mu.m.+-.3
.mu.m. In this way, highly uniform bumps of little height variation
were formed.
1 TABLE 1 Solidus Liquidus Primary Secondary Composition
Temp./.degree. C. Temp./.degree. C. Heating/.degree. C.
Heating/.degree. C. Ex.1 50% Sn-50% Pb 183 238 220 275 Ex.2 20%
Sn-80% Pb 183 277 240 320 Ex.3 10% Sn-90% Pb 275 300 285 340 Ex.4
92% Sn-8% Sb 238 251 240 280 Ex.5 10% Sn-85% Pb-5% Sb 239 277 260
300
EXAMPLE 6
[0062] (Preparation of Solder Paste)
[0063] Solder of 63% Sn-37% Pb alloy as Metal I having a lower
liquidus temperature (liquidus temperature: 183.degree. C.) and
solder of 2% Sn-98% Pb alloy as Metal II having a higher liquidus
temperature (liquidus temperature: 320.degree. C.) were powdered to
have an average particle size of 20 .mu.m, and mixed in a weight
ratio of 1:9. The mixed solder was mixed with a flux to prepare a
solder paste. The flux used was a mixture of 50 wt % Polypale as
rosin resin, 20 wt % diethylene glycol monobutyl ether and 20 wt %
2-methyl-2,4-pentanediol as solvents, 8 wt % sebacic acid as an
activator, and 2 wt % hardened castor oil as a thixotropic
agent.
[0064] (Formation of Bump)
[0065] A film made of acrylic photosensitive resin (NIT-250
available from Nichigo Morton) having a thickness of 50 .mu.m was
bonded by thermo-compression (at 100.degree. C. under a pressure of
3.5 kg/mm.sup.2) onto a wafer provided with 300,000 electrodes
(Diameter: 70 .mu.m; Pitch: 150 .mu.m). Then, the resin film was
exposed to light at portions corresponding to the electrodes by the
use of a glass mask and etched using 1.0% aqueous solution of
sodium carbonate for development to form a plurality of openings
having a diameter of 130 .mu.m at locations corresponding to the
electrodes. Then, the above-described solder paste was loaded into
the openings of the resin film by using a urethane squeegee.
Subsequently, primary heating was performed for one minute at
213.degree. C. which was higher than the liquidus temperature of
the 63% Sn-37% Pb solder to roughly or provisionally integrate the
solder. Then, the solder was cooled and provisionally fixed to the
electrodes as incomplete bumps. Then, the resin film was removed by
immersing into 10% aqueous solution of monoethanolamine. Then, a
flux (R5003 available from Alpha Metals) was applied to the
incomplete bumps provisionally fixed to the electrodes, and
secondary heating was performed for two minutes at 350.degree. C.
which was higher than the liquidus temperature of the 2% Sn-98% Pb
solder to completely melt and integrate the solder. The solder was
then cooled to provide complete bumps on the electrodes.
[0066] (Results)
[0067] In Example 6, the removal of the resin film after the
primary heating was satisfactory, and the complete bumps had a
height of 80 .mu.m.+-.3 .mu.m. In this way, highly uniform bumps of
little height variation were formed. The composition of the
complete bumps was controlled to a deviation of up to .+-.0.2% from
the target 8% Sn-92% Pb composition. The metal composition, mixture
proportion, primary heating temperature, secondary heating
temperature, and resulting composition in Example 6 are given in
Table 2 together with those for Examples 7.about.9 to be described
below.
EXAMPLE 7
[0068] Solder of 35% Sn-65% Pb alloy as Metal I (liquidus
temperature: 246.degree. C.) and solder of 2% Sn-98% Pb alloy as
Metal II (liquidus temperature: 320.degree. C.) mixed in a weight
ratio of 1:9 were used to prepare a solder paste in a manner
similar to Example 6. In Example 7, the primary heating was
performed at 265.degree. C. As a result, the removal of the resin
film after the primary heating was satisfactory, and the complete
bumps had a height of 80 .mu.m.+-.3 .mu.m. In this way, highly
uniform bumps of little height variation were formed. Further, the
composition of the complete bumps was controlled to a deviation of
up to .+-.0.2% from the target 5% Sn-95% Pb composition.
EXAMPLE 8
[0069] 100% Sn as Metal I (melting point: 232.degree. C.) and 100%
Pb as Metal II (melting point: 327.degree. C.) mixed in a weight
ratio of 1:9 were used to prepare a solder paste in a manner
similar to Example 6. In Example 8, the primary heating was
performed at 262.degree. C., whereas the secondary heating was
performed at 357.degree. C. As a result, the removal of the resin
film after the primary heating was satisfactory, and the complete
bumps had a height of 80 .mu.m.+-.3 .mu.m. In this way, highly
uniform bumps of little height variation were formed. Further, the
composition of the complete bumps was controlled to a deviation of
up to .+-.0.2% from the target 10% Sn-90% Pb composition.
EXAMPLE 9
[0070] 100% Sn as Metal I (melting point: 232.degree. C.) and 100%
Pb as Metal II (melting point: 327.degree. C.) mixed in a weight
ratio of 1:19 were used to prepare a solder paste in a manner
similar to Example 6. The primary heating was performed at
262.degree. C., whereas the secondary heating was performed at
357.degree. C. As a result, the removal of the resin film after the
primary heating was satisfactory, and the complete bumps had a
height of 80 .mu.m.+-.3 .mu.m. In this way, highly uniform bumps of
little height variation were formed. Further, the composition of
the complete bumps was controlled to a deviation of up to .+-.0.2%
relative to the target 5% Sn-95% Pb composition.
2 TABLE 2 Metal I Metal II Mix Ratio Primary Secondary Final
Composition Composition I:II Heating/.degree. C. Heating/.degree.
C. Composition Ex. 6 63% Sn-37% Pb 2% Sn-98% Pb 1:9 213 350 8%
Sn-92% Pb (Liq.Tem.183.degree. C.) (Liq.Temp.320.degree. C.) Ex. 7
2% Sn-98% Pb 35% Sn-65% Pb 1:9 265 350 5% Sn-95% Pb
(Liq.Tem.246.degree. C.) (Liq.Temp.320.degree. C.) Ex. 8 100% Sn
100% Pb 1:9 262 357 10% Sn-90% Pb (M.P.232) (M.P.327) Ex. 9 100% Sn
100% Pb 1:19 262 357 5% Sn-95% Pb (M.P.232) (M.P.327)
[0071] Thus, with the bump forming method according to the present
invention, it is possible to prevent the resin film from hardening,
which facilitates removal of the resin film from the board for
reliably mounting electronic components on the board via the
bumps.
[0072] The present invention being thus described, it is obvious
that the same may be varied in many ways. Such variations should
not be regarded as a departure from the spirit and scope of the
present invention, and all such variations as would be obvious to
those skilled in the art are intended to be included in the scope
of the following claims.
* * * * *