U.S. patent application number 12/709575 was filed with the patent office on 2010-06-17 for method of manufacturing component-embedded printed wiring board.
Invention is credited to Kazuhiko HONJO, Eiji KAWAMOTO, Junichi KIMURA, Motoyoshi KITAGAWA, Toshihiko MORI.
Application Number | 20100146779 12/709575 |
Document ID | / |
Family ID | 37426056 |
Filed Date | 2010-06-17 |
United States Patent
Application |
20100146779 |
Kind Code |
A1 |
HONJO; Kazuhiko ; et
al. |
June 17, 2010 |
METHOD OF MANUFACTURING COMPONENT-EMBEDDED PRINTED WIRING BOARD
Abstract
A component-embedded printed wiring board (PWB) is disclosed.
This PWB includes (a) a fluid-resin embedding section formed at a
location corresponding to electronic components such that the
embedding section covers the electronic components, (b) a resin
flow-speed accelerator placed in parallel with a top face of a
circuit board and surrounding the embedding section, and (c)
bonding resin placed at least between the accelerator and the
circuit board. The fluid resin embedding section is filled up with
the same resin as the bonding resin. This structure allows the
accelerator to compress the resin with pressure applied to the PWB,
so that the resin tends to flow along the circuit board. As a
result, the fluid-resin embedding section is thoroughly filled up
with the resin without leaving any air, and the reliable PWB is
thus obtainable.
Inventors: |
HONJO; Kazuhiko; (Gifu,
JP) ; MORI; Toshihiko; (Osaka, JP) ; KAWAMOTO;
Eiji; (Osaka, JP) ; KIMURA; Junichi; (Aichi,
JP) ; KITAGAWA; Motoyoshi; (Gifu, JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK L.L.P.
1030 15th Street, N.W., Suite 400 East
Washington
DC
20005-1503
US
|
Family ID: |
37426056 |
Appl. No.: |
12/709575 |
Filed: |
February 22, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11434763 |
May 17, 2006 |
7694415 |
|
|
12709575 |
|
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|
Current U.S.
Class: |
29/832 ; 29/840;
29/841 |
Current CPC
Class: |
H01L 2924/3511 20130101;
Y10T 29/49124 20150115; H05K 3/4652 20130101; Y10T 29/49144
20150115; H01L 2924/00011 20130101; H01L 2924/00014 20130101; H05K
3/284 20130101; H01L 2924/19105 20130101; H01L 2224/16225 20130101;
H01L 2924/00014 20130101; Y10T 29/49146 20150115; H05K 1/186
20130101; H01L 2924/00011 20130101; H01L 2224/0401 20130101; Y10T
29/4913 20150115; H01L 2224/0401 20130101 |
Class at
Publication: |
29/832 ; 29/841;
29/840 |
International
Class: |
H05K 3/30 20060101
H05K003/30 |
Foreign Application Data
Date |
Code |
Application Number |
May 20, 2005 |
JP |
2005-147864 |
Claims
1. A method of manufacturing a component-embedded printed wiring
board in which a plurality of electronic components are embedded,
the method comprising the steps of: mounting the plurality of
electronic components on land patterns on a top face of a circuit
board and fixing electrodes of the electronic components on the
land patterns using a coupling and fixing member; after the
mounting and fixing step, hanging a resin by disposing the resin on
the circuit board such that spaces are formed between the resin and
the circuit board at least at peripheries of the electronic
components; and after the hanging step, unifying the resin and the
circuit board together by heating and pressing, wherein the resin
is a thermosetting resin having no fluidity within a first
temperature range, and has a fluidity in a second temperature range
higher than the first range, and is hardened in a third temperature
range higher than the second range, and wherein the unifying step
includes (a-1) a first heating step for heating the resin to the
second temperature range and softening the resin, and then (a-2)
forcible flow-in step which forcibly charges the resin into a space
formed between the electronic components and the circuit board by
compressing the resin before the resin is heated to the third
temperature range, and then (a-3) a second heating step for heating
the resin to the third temperature range, wherein the temperature
rises slower in the forcible flow-in step than in the first heating
step.
2. The method of manufacturing a component-embedded printed wiring
board of claim 1, wherein the temperature rises slower in the
second heating step than in the first heating step.
3. The method of manufacturing a component-embedded printed wiring
board of claim 1, wherein the resin is in a liquid phase in the
hanging step.
4. The method of manufacturing a component-embedded printed wiring
board of claim 3, wherein a temperature of the resin is set lower
than a temperature at which a viscosity of the resin becomes a
minimum value, and the thermosetting resin is charged into the
space by decompressing the resin and the circuit board at the lower
temperature, in the forcible flow-in step.
5. The method of manufacturing a component-embedded printed wiring
board of claim 3, wherein a temperature of the resin is set lower
than a temperature at which a viscosity of the resin becomes a
minimum value at least by an amount of a temperature rise value
when the resin is charged into the space, in the forcible flow-in
step.
6. The method of manufacturing a component-embedded printed wiring
board of claim 3, wherein the resin temperature when the resin is
charged into the space is set lower than a temperature at which a
viscosity of the resin becomes a minimum value in the forcible
flow-in step.
7. The method of manufacturing a component-embedded printed wiring
board of claim 3, wherein the coupling and fixing member is a
solder, and wherein a temperature of the resin is set lower than a
temperature of a melting temperature of the solder at least by an
amount of a temperature rise value of the resin when the resin is
charged into the space, in the forcible flow-in step.
8. The method of manufacturing a component-embedded printed wiring
board of claim 3, wherein the coupling and fixing member is a
solder, and wherein the resin temperature when the resin is charged
into the space is set lower than a melting temperature of the
solder in the forcible flow-in step.
9. The method of manufacturing a component-embedded printed wiring
board of claim 3, wherein a pressure applied in the forcible
flow-in step is set small enough for a viscosity of the resin not
to rise due to a temperature rise caused by friction between the
resin and the electronic components or the circuit board when the
resin flows into the space.
10. The method of manufacturing a component-embedded printed wiring
board of claim 3, wherein the coupling and fixing member is a
solder, and wherein a pressure applied in the forcible flow-in step
is set small enough for a temperature of the resin not to exceed a
melting temperature of the solder due to a temperature rise caused
by friction between the resin and the electronic components or the
circuit board when the resin flows into the space.
11. The method of manufacturing a component-embedded printed wiring
board of claim 3, wherein the resin is charged into the space in
the forcible flow-in step, and a viscosity of the resin decreases
due to a heating caused by the charging.
12. The method of manufacturing a component-embedded printed wiring
board of claim 3, wherein a compression pressure is maintained in
the forcible flow-in step.
13. The method of manufacturing a component-embedded printed wiring
board of claim 3, wherein a temperature characteristic of the resin
includes an overshooting temperature range in the forcible flow-in
step.
14. The method of manufacturing a component-embedded printed wiring
board of claim 3, wherein a maximum temperature of the overshooting
temperature range is set to be lower than a temperature at which a
viscosity of the resin becomes a minimum value by an amount of a
temperature rise value due to a friction or a pressure decrease
when the resin is charged into the space.
15. The method of manufacturing a component-embedded printed wiring
board of claim 3, wherein a temperature of the resin is maintained
in the forcible flow-in step.
16. The method of manufacturing a component-embedded printed wiring
board of claim 3, wherein the circuit board is hanged in the
hanging step.
17. The method of manufacturing a component-embedded printed wiring
board of claim 3, wherein the resin is prevented from flowing out
to an outside.
18. A method of manufacturing a component-embedded printed wiring
board in which a plurality of electronic components are embedded,
the method comprising the steps of: mounting the plurality of
electronic components on land patterns on a top face of a circuit
board and fixing electrodes of the electronic components on the
land patterns using a coupling and fixing member; after the
mounting and fixing step, hanging a resin by disposing the resin on
the circuit board such that spaces are formed between the resin and
the circuit board at least at peripheries of the electronic
components; after the hanging step, decompressing by sucking an air
in the spaces and making at least the resin to touch a top faces of
the electronic components; and after the decompression step,
unifying the resin and the circuit board together by heating and
pressing, wherein the resin is a thermosetting resin having
fluidity within a first temperature range, and hardens in a second
temperature range higher than the first range, and wherein the
unifying step includes (a-1) forcible flow-in step which forcibly
charges the resin into a space formed between the electronic
components and the circuit board by compressing the resin within
the first temperature range, and then (a-3) a second heating step
for heating the resin to the second temperature range.
19. The method of manufacturing a component-embedded printed wiring
board of claim 18, wherein the resin is in a liquid phase in the
hanging step.
20. The method of manufacturing a component-embedded printed wiring
board of claim 19, wherein a temperature of the resin is set lower
than a temperature at which a viscosity of the resin becomes a
minimum value, and the thermosetting resin is charged into the
space by decompressing the resin and the circuit board at the lower
temperature, in the forcible flow-in step.
21. The method of manufacturing a component-embedded printed wiring
board of claim 19, wherein a temperature of the resin is set lower
than a temperature at which a viscosity of the resin becomes a
minimum value at least by an amount of a temperature rise value
when the resin is charged into the space, in the forcible flow-in
step.
22. The method of manufacturing a component-embedded printed wiring
board of claim 19, wherein the resin temperature when the resin is
charged into the space is set lower than a temperature at which a
viscosity of the resin becomes a minimum value in the forcible
flow-in step.
23. The method of manufacturing a component-embedded printed wiring
board of claim 19, wherein the coupling and fixing member is a
solder, and wherein a temperature of the resin is set lower than a
temperature of a melting temperature of the solder at least by an
amount of a temperature rise value of the resin when the resin is
charged into the space, in the forcible flow-in step.
24. The method of manufacturing a component-embedded printed wiring
board of claim 19, wherein the coupling and fixing member is a
solder, and wherein the resin temperature when the resin is charged
into the space is set lower than a melting temperature of the
solder in the forcible flow-in step.
25. The method of manufacturing a component-embedded printed wiring
board of claim 19, wherein a pressure applied in the forcible
flow-in step is set small enough for a viscosity of the resin not
to rise due to a temperature rise caused by friction between the
resin and the electronic components or the circuit board when the
resin flows into the space.
26. The method of manufacturing a component-embedded printed wiring
board of claim 19, wherein the coupling and fixing member is a
solder, and wherein a pressure applied in the forcible flow-in step
is set small enough for a temperature of the resin not to exceed a
melting temperature of the solder due to a temperature rise caused
by friction between the resin and the electronic components or the
circuit board when the resin flows into the space.
27. The method of manufacturing a component-embedded printed wiring
board of claim 19, wherein the resin is charged into the space in
the forcible flow-in step, and a viscosity of the resin decreases
due to a heating caused by the charging.
28. The method of manufacturing a component-embedded printed wiring
board of claim 19, wherein a compression pressure is maintained in
the forcible flow-in step.
29. The method of manufacturing a component-embedded printed wiring
board of claim 19, wherein a temperature characteristic of the
resin includes an overshooting temperature range in the forcible
flow-in step.
30. The method of manufacturing a component-embedded printed wiring
board of claim 19, wherein a maximum temperature of the
overshooting temperature range is set to be lower than a
temperature at which a viscosity of the resin becomes a minimum
value by an amount of a temperature rise value due to a friction or
a pressure decrease when the resin is charged into the space.
31. The method of manufacturing a component-embedded printed wiring
board of claim 19, wherein a temperature of the resin is maintained
in the forcible flow-in step.
32. The method of manufacturing a component-embedded printed wiring
board of claim 19, wherein the circuit board is hanged in the
hanging step.
33. The method of manufacturing a component-embedded printed wiring
board of claim 19, wherein the resin is prevented from flowing out
to an outside.
Description
[0001] This application is a Divisional of U.S. application Ser.
No. 11/434,763, filed May 17, 2006.
FIELD OF THE INVENTION
[0002] The present invention relates to a component-embedded
printed wiring board which includes electronic components built
therein, and a method of manufacturing the same printed wiring
board.
BACKGROUND OF THE INVENTION
[0003] A conventional component-embedded printed wiring board (PWB)
is described hereinafter. FIG. 24 shows a structure of the
conventional PWB which includes electronic components built
therein. As shown in FIG. 24, the conventional PWB comprises plate
1 made of metallic material, and substrates 2a-2e formed of
thermoplastic resin and layered over metallic plate 1.
[0004] Holes 4 are opened through substrates 2c and 2d for
embedding electronic component 3. Patterns 5 are provided on
substrates 2a-2e, and via-holes 7 opened through substrates 2a-2e
are filled with conductive paste 6. Electrodes 8 placed at both
sides of component 3 are conductive to paste 6.
[0005] Conductive paste 6 is made by mixing tin grains with silver
grains. Between component 3 and holes 4, clearance of 20 .mu.m is
provided surrounding component 3 for accurately positioning
electrodes 8 with respect to via-holes 7 filled with paste 6. Thus
it can be said that the outside dimension of component 3 is approx.
equal to 20 .mu.m.
[0006] The foregoing conventional PWB undergoes pressing and
heating at 1-10 Mpa, 250-350.degree. C. and in 10-20 minutes before
completed. In other words, this pressing and heating process melts
the tin to be unified with silver, and connects the tin to
electrodes 8 of component 3 for fixing component 3 electrically and
mechanically. The conventional component-embedded PWB is disclosed
in, e.g. Japanese Patent Unexamined Publication No. 2003-86949.
[0007] The conventional PWB, however, has the following problem if
components 3 are densely mounted. For instance, as shown in FIG.
25, electronic components 3a-3e are mounted at a narrow pitch to
substrate 2c, and assume that an interval between the components
adjacent to each other is 100 .mu.m. FIG. 26A shows sectional views
of substrates 2c and 2d, and FIG. 26B shows an enlarged view of the
vicinity of components embedded. As shown in FIGS. 26A and 26B,
width W1 of frame 10a placed between electronic components 3a and
3b is found by equation 1.
W1=W0-W2.times.2 (1)
[0008] where W2 is, e.g. a distance between component 3a and
substrate 2c surrounding component 3a. In this case, since W0=100
.mu.m and W2=20 .mu.m, W1 becomes 60 .mu.m, i.e. the width of frame
10a is 60 .mu.m.
[0009] Thickness T1 of substrate 2c is 75 .mu.m, so that width W2
of frame 10a becomes smaller than thickness T1 of substrate 2c, and
it becomes physically difficult to manufacture this conventional
PWB.
[0010] To overcome this problem, holes 13 surrounding components
3a-3e mounted at narrow pitches can be provided as shown in FIG. 27
(plan view) and FIG. 28 (sectional view). In this case, however,
space 14a between components 3a and 3b cannot be filled
sufficiently with resin 15, so that air 16 sometimes remains. If
substrate 2c in this status undergoes soldering in a reflow-oven,
the reflow-temperature expands air 16 for applying heavy load
between components 3a and 3b. The load has the possibility of
damaging the connection of component 3, to be more specific, the
conduction of paste 6 is cut, or component 3 sealed with resin
produces cracks into which water leaks for rusting electrodes 8 and
causing defective insulation.
SUMMARY OF THE INVENTION
[0011] A component-embedded printed wiring board (PWB) of the
present invention comprises the following elements: [0012] a fluid
resin embedding section formed at a place corresponding to
electronic components embedded such that it covers the electronic
components; [0013] a resin flow-speed accelerator disposed in
parallel with a top face of a circuit board such that it surrounds
the fluid resin embedding section; and [0014] bonding resin
disposed between the resin flow-speed accelerator and the circuit
board. The fluid resin embedding section is filled up with the same
resin as the bonding resin. This structure enables the provision of
a component-embedded PWB of high connection reliability.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 shows a sectional view of a component-embedded
printed wiring board (PWB) in accordance with a first embodiment of
the present invention.
[0016] FIG. 2 shows a flowchart illustrating steps of manufacturing
the component-embedded PWB in accordance with the first
embodiment.
[0017] FIG. 3 shows a sectional view of the component-embedded PWB
in the step of flux application in accordance with the first
embodiment.
[0018] FIG. 4 shows a sectional view of the component-embedded PWB
in the step of cream-solder printing in accordance with the first
embodiment.
[0019] FIG. 5 shows a sectional view of the component-embedded PWB
in the step of mounting electronic components in accordance with
the first embodiment.
[0020] FIG. 6 shows a sectional view of the component-embedded PWB
in a reflow soldering step in accordance with the first
embodiment.
[0021] FIG. 7 shows a sectional view of the component-embedded PWB
in the step of layering pre-pregs in accordance with the first
embodiment.
[0022] FIG. 8 shows an enlarged view of an essential part of the
component-embedded PWB in the step of layering pre-pregs in
accordance with the first embodiment.
[0023] FIG. 9 shows a sectional view of the component-embedded PWB
in a unifying step in accordance with the first embodiment.
[0024] FIG. 10 shows a sectional view of the component-embedded PWB
in an evacuating step in accordance with the first embodiment.
[0025] FIG. 11 shows a sectional view of the component-embedded PWB
in the step of heating and softening in accordance with the first
embodiment.
[0026] FIG. 12 shows a sectional view of the component-embedded PWB
in a forcible flow-in step in accordance with the first
embodiment.
[0027] FIG. 13 shows a sectional view of the component-embedded PWB
in the step of cutting in accordance with the first embodiment.
[0028] FIG. 14 shows viscosity characteristics of epoxy resin in
accordance with the first embodiment.
[0029] FIG. 15 shows an enlarged view of a space formed around a
semiconductor element in accordance with the first embodiment.
[0030] FIG. 16A shows temperature characteristics in the unifying
step in accordance with the first embodiment.
[0031] FIG. 16B shows pressure characteristics in the unifying step
in accordance with the first embodiment.
[0032] FIG. 16C shows atmospheric-pressure characteristics in the
unifying step in accordance with the first embodiment.
[0033] FIG. 17 shows a flowchart illustrating the steps of
manufacturing a component-embedded PWB in accordance with second
and third embodiments.
[0034] FIG. 18 shows a sectional view of a hanging device in
accordance with the second embodiment.
[0035] FIG. 19 shows a sectional view of a decompressing and
layering device in accordance with the second embodiment.
[0036] FIG. 20 show a sectional view of the component-embedded PWB
used in a unifying step in accordance with the second
embodiment.
[0037] FIG. 21 shows a sectional view of a decompressing and
layering device in accordance with the third embodiment.
[0038] FIG. 22 shows another sectional view of the decompressing
and layering device in accordance with the third embodiment.
[0039] FIG. 23 shows a sectional view of the component-embedded PWB
in the forcible flow-in step in accordance with the third
embodiment.
[0040] FIG. 24 shows an exploded sectional view of a conventional
component-embedded PWB.
[0041] FIG. 25 shows a plan sectional view taken from over the
conventional component-embedded PWB.
[0042] FIG. 26A shows a sectional view of an essential part of the
conventional component-embedded PWB.
[0043] FIG. 26B shows an enlarged top view of vicinity of
components embedded in the conventional PWB.
[0044] FIG. 27 shows a plan view of an essential part of the
conventional component-embedded PWB.
[0045] FIG. 28 shows a sectional view of an essential part of the
conventional component-embedded PWB.
DETAILED DESCRIPTION OF THE INVENTION
Embodiment 1
[0046] The first exemplary embodiment of the present invention is
demonstrated hereinafter with reference to FIG. 1-FIG. 12. FIG. 1
shows a sectional view of a component-embedded printed wiring board
(PWB) in accordance with the first embodiment of the present
invention. FIG. 2 shows a flowchart illustrating the steps of
manufacturing the component-embedded PWB. FIG. 3-FIG. 12 detail
respective manufacturing steps of the component-embedded PWB in
accordance with the first embodiment. In these drawings, similar
elements to those of the prior art have the same reference marks,
and the descriptions thereof are simplified.
[0047] The construction of the component-embedded PWB in accordance
with the first embodiment is described with reference to FIG. 1.
Circuit board 101 shown in FIG. 1 is formed of thermosetting resin
in a multi-layer structure. Board 101 has inner via-holes (not
shown) which connect respective top faces to respective undersides
of each layer. The respective top faces have copper foils thereon,
and the foils form electric circuits respectively. Copper foil 145
is an example of conductive patterns which can employ printed
wiring patterns formed of metallic powder paste.
[0048] The top face of circuit board 101 has land patterns 104a and
104b thereon. Semiconductor element 105 mounted on the top face of
board 101 is coupled to land pattern 104a with solder bump 102.
Resistor 106 is coupled to land pattern 104b with solder 107. These
components are used as examples of electronic components; other
electronic components such as capacitors can be embedded.
[0049] Solder 107 and bump 102 employ lead-free solder, namely,
solder made of tin-, silver-, or copper-based metal, because these
materials do not contain harmful substances and their environment
load is low. Instead of solder 107 or bump 102, conductive adhesive
having a thermosetting property can be used. This adhesive has a
melting point higher than the solder, so that semiconductor element
105 or resistor 106 never comes off from circuit board 101 even in
a high temperature environment such that soldering is carried out
near these components.
[0050] Component embedding layer 108 is sandwiched by circuit board
101 and copper foil 145, and is filled with thermosetting resin.
Layer 108 tightly seals semiconductor element 105 and resistor 106
at their peripheries.
[0051] Layer 108 includes fluid-resin embedding section 108a made
of resin and covering both of semiconductor element 105 and
resistor 106, and substrate-included resin section 109 covering the
periphery of fluid-resin embedding section 108a. Substrate-included
resin section 109 is formed by layering plural substrates 109a,
plural resins 110b and 110c alternately. In this first embodiment,
plural substrates 109a are used as an example of a resin flow-speed
accelerator shaping like a board, and plural resins 110b and 110c
are used as an example of bonding resin.
[0052] Resin 110c is placed between the lower most substrate 109a
and circuit board 101, and resin 110d is placed between upper most
substrate 109a and copper foil 145. Substrate 109a of this
embodiment has a thickness of approx. 80 .mu.m, resin 110b has a
thickness of approx. 10 .mu.m, resin 110c and resin 110d have a
thickness of approx. 5 .mu.m.
[0053] The foregoing structure, i.e. layering substrates 109a one
after another, allows substrate 109a to accelerate a flow speed of
resins 110 layered on circuit board 101 along the surface of board
101 in a unifying step described later, so that the fluid-resin
embedding section has every nook and cranny filled with the resin.
As a result, no air remains. This structure, therefore, allows the
load produced by thermal expansion of the air not to damage the
connections, and improves the connection reliability. Resin 110 is
a collective term which includes resins 110b-110d, and
thermosetting resin such as epoxy resin is suitable for resin
110.
[0054] Fluid-resin embedding section 108a is formed of
thermosetting epoxy resin as is resin 110. Since embedding section
108a and resin 110 are made of the same resin, they have the same
thermal expansion coefficient with respect to temperatures, so that
a thermal change expands or contracts these two elements in the
same amount. As a result, damage or breakage rarely occur in the
boundary between embedding section 108a and resin 110.
[0055] Since resin 110c is disposed between substrate 109a and
circuit board 101, peel-off rarely occurs between component
embedding layer 108 and circuit board 101. On top of that, since
resin 110d is disposed between substrate 109a and copper foil 145,
peel-off rarely occurs between component embedding layer 108 and
copper foil 145.
[0056] Substrate 109a employs glass woven fabric, which prevents
embedding section 108a from being easily bent. Because if embedding
section 108a is formed of only epoxy resin, its flexural strength
is weakened. As a result, smaller expansion and contraction due to
thermal changes can be expected.
[0057] In this embodiment, glass woven fabric is used as substrate
109a; however, glass non-woven fabric or woven or non-woven fabric
of other fibers such as aramid fiber can be used instead. Epoxy
resin is used as fluid-resin embedding section 108a and resin 110;
however, other thermoplastic resin or thermosetting resin such as
unsaturated polyester resin can be used instead with an advantage
similar to the one obtained in this embodiment.
[0058] Respective steps of a method of manufacturing the
component-embedded PWB in accordance with the first embodiment are
demonstrated hereinafter with reference to FIG. 3-FIG. 16 following
the sequence of the steps shown in FIG. 2.
[0059] FIG. 2 shows a flowchart of manufacturing the
component-embedded PWB in accordance with the first embodiment, and
FIG. 3 shows a sectional view of the PWB in flux application step
111. In step 111, flux 112 is printed on land pattern 104a, to
which semiconductor element 105 (shown in FIG. 5) is to be mounted,
through a metal screen (not shown).
[0060] FIG. 4 shows a sectional view of the component-embedded PWB
in step 113 of printing cream solder in accordance with the first
embodiment. As shown in FIG. 2, cream-solder printing step 113
follows flux application step 111. In step 113, cream solder 107 is
printed on land pattern 104b, to which resistor 106 (shown in FIG.
5) is to be mounted, with squeegee 132 and screen 131. Cream solder
107 is used as an example of coupling and fixing member for
connection. Screen 131 is a metal screen made of stainless steel,
and recess 126 is formed on screen 131 at a place corresponding to
flux 112 applied. Recess 126 prevents flux 112 from adhering to
screen 131 during the printing of solder 107.
[0061] FIG. 5 shows a sectional view of the component-embedded PWB
in step 114 of mounting electronic components in accordance with
the first embodiment. As shown in FIG. 2, step 114 of mounting the
electronic components follows step 113 of printing the cream
solder. In step 114, semiconductor element 105 and resistor 106 are
mounted onto circuit board 101 at given places by an automatic
insertion machine. A plurality of solder bumps 102 are formed in
advance on underside 105a of semiconductor element 105.
[0062] FIG. 6 shows a sectional view of the component-embedded PWB
in reflow soldering step 115 in accordance with the first
embodiment. As shown in FIG. 2, reflow soldering step 115 follows
step 114 of mounting the electronic components. In step 115, cream
solder 107 is heated to a temperature higher than its melting
point, so that cream solder 107 is melted, whereby resistor 106 is
soldered to land pattern 104b, and bumps 102 of semiconductor
element 105 are soldered to land pattern 104a. In this embodiment,
reflow soldering step 115 is carried out in nitrogen atmosphere,
thereby preventing the surface of circuit board 101 from being
oxidized. The contact between board 101 and pre-preg 141a (shown in
FIG. 7) is thus improved.
[0063] After reflow soldering step 115, in-process items can be
washed in a step (not shown) of washing circuit board 101, so that
residue of flux 112 and solder balls are cleaned. On top of that,
O.sub.2 asher process and silane coupling process are recommended,
because these surface modifying processes improve the contact
between board 101 and pre-preg 141a.
[0064] In this embodiment, the reflow soldering is used because of
its excellent quality. The reflow soldering allows fixing of the
components soldered at given places due to its self-alignment
effect, so that components can be accurately fixed to board 101 and
the length of pattern-lines connected to these components becomes a
definite value. In other words, when the pattern lines are used as
an inductor, the inductance can produce a definitive value, so that
a given electrical performance can be expected. This is an
important matter for a high-frequency circuit.
[0065] FIG. 7 shows a sectional view of the component-embedded PWB
in step 116 of layering pre-pregs in accordance with the first
embodiment, and FIG. 8 shows an enlarged view of an essential part
of the foregoing component-embedded PWB. As shown in FIG. 2, in
hole-opening step 117, hole 142 is opened on pre-preg 141 for
receiving semiconductor element 105 and resistor 106 therein. In
pre-pregs layering step 116 following reflow soldering step 115,
pre-preg 141 with the hole is layered on circuit board 101.
Pre-preg 141 is a collective term which includes individual
pre-pregs 141a, 141b, and 141c. For instance, pre-preg 141a
includes resin 110c, substrate 109a and resin 110b. Pre-preg 141b
includes resin 110b, substrate 109a and resin 110b. Pre-preg 141c
includes resin 110b, substrate 109a and resin 110d. Pre-preg 141
employs substrate 109a made of non-woven fabric and impregnated
with epoxy resin 110c in advance to be unified. Substrate 109a is
used as an example of the resin flow-speed accelerator.
[0066] As shown in FIG. 8, pre-preg 141 is layered on circuit board
101, so that a layered unit, where substrate 109a, resins 110b,
110c and 110d are layered one after another, is completed on
circuit board 101. Pre-preg 141 has a thickness of approx. 120
.mu.m because substrate 109a has a thickness of approx. 80 .mu.m
and is impregnated with resin 110, thereby increasing the thickness
to approx. 120 .mu.m.
[0067] Clearances 144 are reserved between the outer walls of
resistor 106 and inner wall of hole 142, so that pre-preg 141 with
the hole can be layered on circuit board 101 with ease.
[0068] Since semiconductor element 105 and resistor 106 are mounted
on board 101 by reflow-soldering, the self-alignment effect due to
melting of cream solder 107 allows mounting of these elements at
given places accurately. In other words, clearances 144 can be
reduced in size because semiconductor element 105 and resistor 106
are mounted precisely, which allows resin 110 to flow into spaces
156, 157 with ease. In this embodiment, clearances 144 take the
maximum value of approx. 0.2 mm, so that even if resistor 106 is
mounted deviating from a predetermined place, pre-preg 141 can be
layered free from inconveniences such as collision.
[0069] Individual pre-pregs 141, which are impregnated with resin
110b, 110c and 110d respectively in advance, are used with respect
to substrate 109a. Thus hole 142 can be opened simultaneously
through substrate 109a and resin 110b, so that excellent
productivity can be expected. There is no need to layer substrate
109a and resin 110 individually, so that the number of layering
processes can be reduced, which also improving the
productivity.
[0070] On the upper most face of pre-preg 141, pre-preg 141d having
no hole 142 is placed, and copper foil 145 is provided on the
entire top face of pre-preg 141d. In layering step 116, resin 110c
of approx. 20 .mu.m thickness, substrate 109a of approx. 80 .mu.m,
and resin 110b of 40 .mu.m thickness are layered in this order from
the bottom on circuit board 101. Between the uppermost substrate
109a and copper foil 145, resin 110d of approx. 20 .mu.m thickness
is layered.
[0071] Pre-preg 141d and copper foil 145 are used in this
embodiment; however, a hardened circuit board can be used instead.
Any hardened circuit-board such as a single-sided board,
double-sided board, or multi-layer board can be used. Use of the
hardened circuit board can reduce a warp of pre-preg 141 caused by
thermal contraction in a cooling step described later.
[0072] Since there is a small space between semiconductor 105 and
resistors 106, only one hole 142 that accommodates all of a
plurality of electronic components is provided. When there are
enough spaces between respective electronic components, plural
holes can be prepared for accommodating each one of the components
respectively. In this case, however, clearances should be provided
between the hole and the respective components so that pre-preg 141
can be mounted. A depth of each hole can be changed in response to
a height of the respective components. These preparations reduce a
cubic volume to which the resin is to be embedded, so that the
resin can reach every nook and cranny.
[0073] In unifying step 118, circuit board 101 layered in step 116,
pre-preg 141 and copper foil 145 undergo heating and pressing at a
temperature slightly lower than the melting point of solder 107, so
that they are unified together. Step 118 is described hereinafter
following the sequence of the steps shown in FIG. 2.
[0074] FIG. 9 shows a sectional view of a unifying device used in
the unifying step in accordance with the first embodiment. The
unifying device includes platens 151 and 152, and circuit board 101
is mounted on platen 152. Platens 151, 152, and expandable walls
153 on both sides form air-tight container 154. A sucking device
(not shown) is coupled to airtight container 154. Heaters 160 are
embedded in platens 151, 152 for heating pre-preg 141.
[0075] Between driver 162 and platen 152, speed reducer 163 is
placed. Speed reducer 163 converts rotary motion into reciprocating
motion, and also reduces the rotating speed. A control circuit (not
shown) is coupled to driver 162 and heater 160, and controls the
timing that operates these two elements. Since resin 110 changes in
viscosity in response to temperatures, the temperature of heater
160 is controlled in order to obtain a given viscosity of resin
110.
[0076] Unifying step 118 using the foregoing unifying device is
detailed hereinafter. FIG. 10 shows a schematic sectional view of
the unifying device employed in an evacuating step in accordance
with the first embodiment. As shown in FIG. 2, the first step in
unifying step 118 is evacuating step 119 which follows pre-preg
layering step 116. In evacuating step 119, the component-embedded
PWB of which pre-preg 141 is layered on circuit board 101 is housed
in air-tight container 154. Platen 151 is fixed and platen 152 is
movable.
[0077] A sucking device sucks the air in airtight container 154
through vent hole 155 opened through platen 152, so that airtight
container 154 is decompressed to a substantially vacuum condition.
At this time, it is important to decompress the inside of hole 142
to a substantially vacuum condition, which allows resin 110 in
pre-preg 141 to be charged into hole 142, spaces 156 between
circuit board 101 and semiconductor 105, and space 157 between
circuit board 101 and resistor 106 in every nook and cranny in
forcible flow-in step 122 described later. The width of space 156
ranges from approx. 20 .mu.m to approx. 350 .mu.m, and the width of
space 157 ranges from approx. 10 .mu.m to approx. 40 .mu.m.
[0078] In order to simplify the description, one semiconductor
element 105 and two resistors 106 are used as examples of
electronic components in this embodiment; however, more electronic
components are actually mounted on circuit board 101. Considering
the productivity of the component-embedded PWB, a greater size of
circuit board 101 is preferable, and therefore, more clearances 144
and spaces 156, 157 exist practically.
[0079] In evacuating step 119, it is thus important to completely
suck the air existing in these numerous clearances 144 and spaces
156 and 157, because if the airs remain in pre-preg 141, voids tend
to occur. To overcome this problem, the following preparations are
carried out: In layering step 116, board-like pre-pregs 141 having
no viscosity at a room temperature, i.e. solid pre-preg, are
layered one after another in order to resist the occurrence of
voids. Then in evacuating step 119, hole 142 is evacuated before
pre-preg 141 is softened. These preparations allow the air in hole
142 to be sucked therefrom before pre-preg 141 becomes liquid and
has viscosity, in which status, pre-pregs 141 adhere to each other,
or pre-preg 141 adheres to circuit board 101. Evacuating step 119
is preferably completed at the latest before the temperature of
pre-preg 141 rises to the glass transition point, so that the air
in the spaces between respective pre-pregs 141 and the space
between pre-preg 141 and circuit board 101 can be sucked completely
therefrom. As a result, clearances 144 and spaces 156 and 157 can
be evacuated positively, thereby inhibiting the occurrence of
voids.
[0080] In this embodiment, heaters 160 start working to heat
platens 151, 152 simultaneously with the start of evacuating step
119, and driver 162 starts driving platens 152, so that a given
pressure is applied to the component-embedded PWB. This preparation
shrinks expandable walls 153, so that platen 152 is raised along
arrow mark A as shown in FIG. 9. Then as shown in FIG. 10, circuit
board 101, layered pre-preg 141 and copper foil 145 are compressed
completely between platens 151 and 152 by a predetermined pressure.
Heaters 160 work at approx. 110.degree. C., and a pressure of
approx. 40 kg/cm.sup.2 is used.
[0081] FIG. 11 shows a sectional view of the unifying device used
in a softening step in accordance with the first embodiment. As
shown in FIG. 2, evacuating step 119 is followed by heating and
softening step 120. Pre-preg 141 starts to be heated when platens
151 and 152 are brought into contact with circuit board 101 and
copper foil 145 through evacuating step 119. Resin 110 impregnated
into pre-preg 141 is softened by the heat from heater 160. Resin
110 is heated up to approx. 110.degree. C., and its viscosity is
lowered to approx. 2400 pas.
[0082] Pre-preg 141 is compressed by platens 151, 152, which thus
solidly contact the surface of copper foil 145. The heat from
heater 160 can thus be positively transferred to pre-preg 141, so
that a heating device excellent in energy efficiency and in saving
energy is obtainable.
[0083] FIG. 12 shows a sectional view of the unifying device in the
forcible flow-in step in accordance with the first embodiment. As
shown in FIG. 2, heating and softening step 120 is followed by
forcible flow-in step 122, in which respective pre-pregs 141 are
compressed into the thickness of approx. 90 .mu.m. At this time,
resins 110b, 110c, 110d of pre-pregs 141 flow along substrate 109a
in the direction of arrow mark B (shown in FIG. 8), and flows into
hole 142. Eventually clearances 144, spaces 156 and 157 are
entirely filled with the resin flowing out from resins 110b, 110c,
and 110d, which are named sometimes collectively as resin 110.
[0084] In order to increase the flow-speed of resin 110, the
viscosity of resin 110 is preferably kept at a low level for the
longest possible period. For this purpose, it is important to give
resin 110 a viscosity which turns resin 110 fluid as quick as
possible. In heating and softening step 120, the temperature of
pre-preg 141 is thus raised at a rate of 4.5.degree. C./minute. On
top of that, in forcible flow-in step 122, pre-preg 141 is kept at
110.degree. C. for 30 minutes and is compressed by the pressure of
40 kg/cm.sup.2.
[0085] These preparations lower the viscosity of pre-preg 141
quickly to a low enough level for the resin to start flowing in 15
minutes after the heating starts. In 25 minutes after the heating
starts, the viscosity becomes the lowest level, i.e. approx. 1500
pas. The temperature of 110.degree. C. is maintained for 50 minutes
after the heating starts. As discussed above, the lowest viscosity
is preferably maintained as long as possible, and for this purpose,
the viscosity of resin 110 is lowered as quick as possible, so that
clearances 144, spaces 156 and 157 start to be filled with the
resin as early as possible. At the same time, the temperature of
110.degree. C. is maintained for a given period in order to delay
as long as possible the progress of thermosetting reaction. As a
result, the viscosity is prevented from rising due to the progress
of reaction of thermosetting resin.
[0086] Since platens 151 and 152 sandwich circuit board 101 and
pre-preg 141 in the vertical direction before they are heated in
heating and softening step 120, circuit board 101 and pre-preg 141
are heated unevenly depending on the locations of heaters 160
embedded in platens 151, 152. Temperature differences thus tend to
occur on board 101 and pre-preg 141 due to this uneven heat. In
general, spaces 156, 157 are formed away from platens 151, 152.
Spaces 156, 157 eventually have some points of which temperatures
are lower than the temperature of resin 110. If resin 110 flows
into spaces 156, 157 before resin 110 and spaces 156, 157 have a
uniform temperature, the temperature of resin 110a, namely a tip of
the resin flow, lowers. As a result, resin 110a flowing into spaces
156, 157 is caused to have a high viscosity, and resin 110a does
not move halfway of the spaces in forcible flow-in step 122. This
problem invites insufficient fill-up of the resin into the spaces,
so that voids tend to occur.
[0087] The first embodiment thus keeps the temperature of
110.degree. C. for approx. 30 minutes in forcible flow-in step 122,
so that the temperatures of spaces 156, 157 and resin 110 become
uniform, thereby preventing resin 110 from stopping its flow into
the spaces caused by the temperature decrease. The movement of
platen 152 is stopped by stopper 161.
[0088] After spaces 156, 157 are filled up with resin 110, heating
and hardening step 123 hardens resin 110. In step 123, pre-preg 141
is heated to a temperature lower than a liquidus temperature of
solder bump 102 and solder 107, so that pre-preg 141 loses its
fluidity and is completely hardened. In the meantime, at the
liquidus temperature, solder becomes liquid completely by
heating.
[0089] In heating and hardening step 123, it is important to heat
pre-preg 141 so that it becomes hardened and loses its fluidity at
a temperature lower than the liquidus temperature of solder bump
102 and solder 107. Solder bump 102 and solder 107 employ lead-free
solder that has a melting point of approx. 270.degree. C., so that
the temperature at which resin 110 loses its fluidity is preferably
set not higher than 200.degree. C. in step 123. In this embodiment,
step 123 employs a pressure of 40 kg/cm.sup.2 so that resin 110 can
lose its fluidity at approx. 150.degree. C. The viscosity of resin
110 at this time is approx. 24000 pas.
[0090] As discussed above, heating and hardening step 123 makes
resin 110 lose its fluidity completely, and then raises the
temperature of resin 110 to 200.degree. C. for hardening resin 110.
Since resin 110 loses its fluidity at approx. 150.degree. C. in
step 123, connection between semiconductor 105 and circuit board
101, or connection between resistors 106 and board 101 never comes
off.
[0091] After pre-preg 141 is hardened, the step moves on to cooling
step 124, where moderate cooling is slowly carried out. The
component-embedded PWB sandwiched by platen 151 and platen 152 is
slowly cooled by controlling the temperature of heaters 160. This
cooling is done until the temperature reaches not higher than the
glass transition point (160.degree. C. by TMA measuring method).
Then water is poured into platens 151, 152 for quickly cooling by
the water. These preparations allow for a decrease in the
difference in shrinkage amount between copper foil 145 and resin
110 caused by different coefficients of linear expansion of these
two elements. As a result, warping of the component-embedded PWB
can be reduced, and conductors on board 101 are prevented from
peeling off from resin 110 at their interface.
[0092] FIG. 13 shows a sectional view of a cutting device in
cutting step 125 in accordance with the first embodiment. Cutting
step 125 cuts resin 172 flowing out to the outside of circuit board
101 through forcible flow-in step 122. In step 125, the
component-embedded PWB is cut by rotating dicing teeth 171, which
cuts not only surplus resin 172 but also both of circuit board 101
and resin 172. Because circuit board 101 is cut inside the edge,
the size of the component-embedded PWB becomes approx. a definite
size regardless of expansion or contraction of circuit board
101.
[0093] As discussed above, in unifying step 118, heating and
softening step 120 sharply heats the resin to be fluid, and heating
and compressing step 118a suppresses a temperature rise applied to
pre-preg 141 as well as circuit board 101 and maintains the
temperature. Step 118a compresses pre-preg 141 at the temperature
maintained so that circuit board 101 is unified with pre-preg 141
for completing component embedding layer 108. Meanwhile, step 118a
includes heating and softening step 120 and forcible flow-in step
122.
[0094] In unifying step 118, resin 110 flows into spaces 156 and
157. This movement is detailed hereinafter. First, the relation
between a temperature, pressure and viscosity of resin 110 is
described with reference to FIG. 14.
[0095] FIG. 14 shows viscosity characteristics of resin 110
measured by a viscosity tester. Lateral axis 201 represents
temperatures, vertical axis 202 represents viscosity. In FIG. 14,
curve 203 shows the viscosity on the assumption that the
temperature rises at a given rate both in steps 120 and 122. Curve
204 shows viscosity characteristics on the assumption that the
temperature rises more moderately in forcible flow-in step 122 than
in heating and softening step 120.
[0096] In the case of curve 203, i.e. in the case where the
temperature rises at a given rate both in steps 120 and 122, a
temperature rising speed becomes slower, so that the viscosity
lowers by a smaller amount. Minimum viscosity 217 thus eventually
becomes greater. As a result, in forcible flow-in step 122, resin
110 becomes resistant to flow into spaces 156, 157.
[0097] On the other hand, in the case of curve 204, i.e. in step
122 the temperature rises more slowly or steadily, and in step 120
the temperature rising speed becomes faster proportionately, the
viscosity lowers by a greater amount, and the minimum viscosity
becomes smaller.
[0098] In the case of curve 204, resin 110 is not viscous at room
temperature, and it becomes softer and lowers its viscosity as the
temperature rises. At temperature 206, the viscosity reaches
minimum viscosity 207, and it increases as the temperature rises
from temperature 206, so that the hardening of resin 110 is
quickened. When temperature 206 is approx. 133.degree. C., minimum
viscosity 207 of resin 110 becomes approx. 1150 pas.
[0099] The fluidity of resin 110 is determined by a pressure
applied to resin 110 and a viscosity, or a temperature of resin
110. For instance, as discussed in this first embodiment, when
platen 152 applies a pressure of 40 kg/cm.sup.2, resin 110 attains
a status of flow-start viscosity 217 at which resin 110 starts
flowing at temperature 211. In other words, resin 110 keeps a
board-like status at a temperature ranging from room temperature to
temperature 211 (first temperature range 213) and does not flow.
When resin 110 receives the pressure of 40 kg/cm.sup.2, flow-start
viscosity is 24000 pas, and temperature 211 at this time is approx.
90.degree. C.
[0100] Next, when the temperature exceeds temperature 211, the
viscosity of resin 110 lowers to minimum viscosity 207 at
temperature 206. Forcible flow-in step 122 is thus carried out in
temperature range 214 (second temperature range) ranging from
temperature 211 to temperature 206.
[0101] When step 122 is ended, resin 110 is hardened in heating and
hardening step 123, in which platen 152 keeps applying the pressure
of 40 kg/cm.sup.2. Epoxy resin 110 starts hardening gradually when
its temperature falls within temperature range 215 (third
temperature range) exceeding temperature 206, and its viscosity
reaches flow-start viscosity 212, where it loses the fluidity, at
temperature 216. Under the pressure applied in this embodiment,
resin 110 loses its fluidity at 150.degree. C., where its viscosity
is 24000 pas.
[0102] As discussed above, setting of the temperatures, such as a
smaller temperature-rise in forcible flow-in step 122 than that in
heating and softening step 120, and the temperature in step 122
falling between temperature 211 (flow-start viscosity 217) and
temperature 206 (minimum viscosity 207), can obtain the following
advantage: Resin 110 can maintain its viscosity, which allows resin
110 to flow into space 156 with ease at a specified pressure, for a
long time. As a result, resin 110 is moved forcibly by pressure, so
that hole 142 and space 156 are positively filled up with resin
110.
[0103] The flow of the epoxy resin into space 156 is demonstrated
with reference to FIG. 15 and FIGS. 16A-16C. FIG. 15 shows an
enlarged view of semiconductor element 105 in the forcible flow-in
step in accordance with the first embodiment. FIG. 16A-16C show
characteristics in unifying step 118 in accordance with the first
embodiment.
[0104] FIG. 10 shows resin 110 prior to compression, and resin 110
becomes fluid due to the compression by platen 152, and then its
tip, namely, resin 110a, flows into space 156. At this time, space
156 is so much smaller than clearance 144 that resin 110a can be
considered viscous fluid passing through a thin tube. Therefore,
eddy 221 occurs near corner 105b, thereby inviting pressure
loss.
[0105] The presence of solder bump 102 allows resin 110 to be
considered viscous fluid passing through a thick tube after passing
through the thin tube. In other words, resin 110 passing near
solder bump 102 passes through thin tubes and thick tubes
repeatedly, which invites a large amount of pressure loss, so that
the flow speed of resin 110 slows down. It is important to increase
the flow speed of resin 110 as much as possible so as not to stop
the flow of resin 110.
[0106] To achieve this objective, the present invention provides
fluid-resin embedding section 108a with substrate 109a made of
glass woven fabric, and a cross sectional area through which resin
110 flows is reduced in the area where substrate 109a exists. This
preparation allows resin 110 to resist moving within substrate 109a
along arrow mark B show in FIG. 8. Since pre-preg 141 has hole 142,
the compressing force in unifying step 118 is concentrated to
substrate 109a and resin 110. On top of that, compressed resin 110
flows through a space having a small sectional area and sandwiched
between substrates 109a. This mechanism allows resin 110 to flow at
a higher speed with respect to a compressed amount (or a
compressing pressure) by platen 152. As a result, inertia force
generated by a flow-speed of resin 110 becomes greater than viscous
force of resin 110, so that the flow-speed of resin 110 is
considered to be accelerated. As a result, the flow-speed of resin
110 becomes greater, so that resin 110 flows into clearance 144 and
space 156 more positively.
[0107] The relation between temperatures and pressures in unifying
step 118 in accordance with the first embodiment is detailed with
reference to FIGS. 16A-16C. FIG. 16A shows relations between time,
resin temperature and resin viscosity, FIG. 16B shows relation
between time and pressure, and FIG. 16C shows relation between time
and degrees of vacuum. In these drawings, lateral axis 231
represents the time (minutes), and first vertical axis 232
represents the temperature (.degree. C.) and second vertical axis
233 represents the viscosity (pas) of FIG. 16A. Vertical axis 234
of FIG. 16B represents the pressure (kg/cm.sup.2), and vertical
axis 235 of FIG. 16C represents the degree of vacuum (torr).
[0108] As shown in FIG. 16C, it is assumed that degree of vacuum
237 is achieved at time 236 in evacuating step 119. To be more
specific, time 236 spans approx. 4 minutes, and degree of vacuum
237 is approx. 37 torr. At the same time, the pressure starts being
applied to pre-preg 141, and the pressure reaches the specified
value of 40 kg/cm.sup.2 in one minute. Heaters 160 start heating
simultaneously.
[0109] In FIG. 16A, curve 238 shows the temperature of pre-preg
141, and curve 239 shows the viscosity of resin 110. In heating and
softening step 120 after evacuating step 119, the temperature is
raised to temperature 240 in order to make resin 110 fluid.
Temperature 240 is approx. 90.degree. C. in this embodiment, and
resin 110 is heated at a rate of approx. 4.5.degree. C./minute so
that the temperature reaches temperature 240 in approx. 15 minutes,
and the viscosity lowers to viscosity 248. The viscosity of resin
110 used in this embodiment becomes approx. 24000 pas at 90.degree.
C., and resin 110 starts flowing at this viscosity with respect to
the pressure of 40 kg/cm.sup.2 applied by platen 152.
[0110] Forcible flow-in step 122 is carried out after the viscosity
of resin 110 is lowered to a fluid level in step 120. In step 122,
the temperature is further raised to temperature 241 while pressure
249 continues to be supplied, and temperature 241 is maintained for
approx. 30 minutes to allow resin 110 to forcibly flow into space
156. In step 120, it is preferable to heat resin 110 as quick as
possible so that the viscosity lowers to not higher than viscosity
248, and in step 122, it is preferable to maintain the temperature
of resin 110 at temperature 241. These operations allow delaying
the progress of thermosetting reaction of resin 110 and maintaining
the viscosity at a low level for a long period.
[0111] The foregoing operations allow resin 110 to stay in fluid
status even 30 minutes after the viscosity passes across viscosity
248, and pressure 249 allows resin 110 to positively flow into
space 156. Temperature 241 is approx. 110.degree. C., and viscosity
242 is approx. 3550 pas.
[0112] A pressure applied to resin 110 at the start of being fluid
is preferably greater than that applied upon losing its fluidity,
because a greater pressure produces a greater flow-speed of resin
110 flowing into clearance 144 and space 156, which are thus
positively filled with resin 110.
[0113] Pre-preg 141 layered in layering step 116 is provided with
clearance 144 around or above semiconductor element 105 and
resistor 106. To be more specific, the pressure applied from platen
152 in unifying step 118a is concentrated to pre-preg 141 at its
face 146 (shown in FIG. 8) except hole 142, so that resin 110
receives a pressure greater than the pressure supplied from platen
152.
[0114] Hole 142 is not prepared for semiconductor element 105 and
resistor 106 individually, but it is prepared for surrounding all
these elements, so that hole 142 occupies almost a half area of
board 101. As a result, pressure as much as twice of the pressure
(40 kg/cm.sup.2) supplied from platen 152 can be applied to resin
110, namely 80 kg/cm.sup.2 is applied to resin 110. This mechanism
allows resin 110 to become fluid at a greater viscosity (or a lower
temperature) than flow-start viscosity 212, so that a fluidable
period can be prolonged. In forcible flow-in step 122, resin 110
can be positively charged into clearance 144 and space 156 with
more ease.
[0115] Heating and hardening step 123 follows forcible flow-in step
122 where space 156 is filled up with resin 110. In step 123, the
temperature of resin 110 is raised over temperature 245 at which
resin 110 loses it fluidity under pressure 249, so that resin 110
becomes non-fluid, i.e. does not move in the least. In this
embodiment, the viscosity at which resin 110 loses it fluidity
under pressure 249 is almost the same as viscosity 248 at which the
fluidity starts working, so that the viscosity is approx. 24000
pas. In heating and hardening step 123, the temperature of resin
110 is raised to 200.degree. C., and resin 110 is kept at this
temperature for approx. 60 minutes to be hardened completely.
[0116] Use of the method of manufacturing the component-embedded
PWB discussed above allows the flow-speed of resin 110 to be faster
by providing substrate 109a, so that resin 110 can flow into spaces
156, 157 between semiconductor element 105, resistor 106 and
circuit board 101 with ease. This manufacturing method thus allows
charging resin 110 positively into the spaces between semiconductor
element 105, resistor 106 and circuit board 101 without using any
intermediate members. This method does not require putting
intermediate members into spaces 156, 157 between semiconductor
element 105, resistor 106 and circuit board 101. In unifying step
118, pre-preg 141 and circuit board 101 are unified together, and
at the same time, spaces 156, 157 can be positively filled up with
resin 110. The present invention thus can provide the forgoing
method of manufacturing the component-embedded PWB.
[0117] The foregoing method does not need a step of putting
intermediate members or the intermediate members per se, so that an
inexpensive component-embedded PWB is obtainable. Further, in
forcible flow-in step 122, small spaces 156, 157 can be positively
filled up with resin 110, so that few voids occur and a reliable
component-embedded PWB is obtainable.
[0118] Complete fill-up of clearance 144, spaces 156, 157 with
resin 110 allows supplying the pressure from platen 152 both to
face 146 (shown in FIG. 8) and the resin charged into hole 142, so
that a greater area of the resin receives the pressure, and as a
result, resin 110 receives a smaller pressure. In this embodiment,
the pressure received becomes half. The presence of hole 142 having
clearance 144 with respect to semiconductor element 105 and
resistor 106 can advantageously prolong the fluid period of resin
110. On top of that, the pressure supplied to platen 152 can be
reduced when the resin starts being fluid, so that the driver can
be downsized, which makes the device smaller and inexpensive.
[0119] In this embodiment, platen 152 can apply a pressure of as
much as 40 kg/cm.sup.2 for compressing because of the following
structure: In layering step 116, pre-preg 141 is layered such that
hole 142 exists over semiconductor element 105 and resistor 106, so
that platen 152 cannot apply its pressure directly to semiconductor
element 105 or resistor 106. Since the foregoing great pressure can
be supplied, resin 110 is positively charged into clearance 144 and
space 156.
[0120] Since pre-preg 141 is made of thermosetting resin, once it
is heated and hardened, it never returns to plastic status even if
it is heated again. Thus once semiconductor element 105 is sealed
by resin 110, it keeps being fixed. The glass woven fabric is
impregnated with epoxy resin, so that when resin 110 becomes fluid
in heating and softening step 120 and forcible flow-in step 122,
pre-preg 141 can keep its shape as a substrate, so that a
component-embedded PWB having dimensional accuracy is
obtainable.
[0121] It is important to lower temperature 245 of resin 110, at
which temperature resin 110 loses its fluidity, lower than the
melting point of solder 107 under pressure 249. Because resin 110
should be hardened before solder 107 is melted through the heat
application in heating and softening step 123. To be more specific,
resin 110 stays hardened and covers solder 107, so that solder 107
never flows out when it is melted. This structure improves
reliability.
[0122] Temperature 246 (shown in FIG. 16A) in heating and hardening
step 123 is kept lower than the melting point of solder 107. In
other words, solder 107 employs high-temperature solder of which
melting point is higher than the temperature maintained in step
123. This preparation allows solder 107 not to be melted by the
heat supplied from step 123, so that a more reliable
component-embedded PWB is obtainable.
[0123] The temperature maintained in the forcible flow-in step is
low enough (e.g. 150.degree. C.) not to melt the solder which
connects and fixes semiconductor element 105 and resistors 106, and
yet circuit board 101 and pre-preg 141 are unified together at the
temperature maintained. This unification thus does not break the
connection or the fixation. As a result, semiconductor element 105
and resistor 106 can be kept in strong connection and fixation.
[0124] Semiconductor element 105 and resistor 106 are mounted on
circuit board 101, so that an inspection can be carried out for
this circuit board 101 with the components mounted. As a result, a
non-defective rate after completing the component-embedded PWB can
be improved.
[0125] In this embodiment, a plurality of pre-pregs 141 are layered
one after another; however, substrates 109a can be layered
independently and resins 110 can be layered also independently. In
such a case, the flow-speed of resin 110 can be changed
appropriately in response to a thickness of the component-embedded
PWB.
[0126] In forcible flow-in step 122, as shown in FIG. 16A,
overshooting temperature range 247 which temporarily exceeds
temperature 241 is provided. This preparation allows the
temperature in heating and softening step 120 to rise more quickly,
so that the viscosity of resin 110 lowers quickly. Thus the low
viscosity can be kept for a long period in forcible flow-in step
122, and the fluidity of resin 110 can be improved. The highest
temperature in overshooting temperature range 247 is 115.degree.
C., and this highest temperature is preferably set lower than
temperature 244 (125.degree. C. in this embodiment) at which resin
110 starts being hardened by the pressure supplied.
[0127] In layering step 116, the amount of resin 110 to be layered
on circuit board 101 should be a greater amount than a fill-up
amount of the resin charged into clearance 144, and spaces 156,
157. Because even if pre-preg 141 has dispersion in thickness,
substrate 109a has dispersion, or the condition such as pressure or
temperature has dispersion, clearance 144 and spaces 156, 157
should be positively filled up with resin 110. However, if too much
resin is supplied, a large amount of resin 110 sticks out
wastefully to the outside of board 101. To overcome this possible
problem, this embodiment uses substrate 109a, which accelerates the
flow-speed of resin 110, so that clearance 144 and spaces 156, 157
can be filled up with resin 110 leaving as little as possible left
over.
[0128] The surplus supply of resin 110 sometimes increases the
resin pressure in clearance 144 and spaces 156, 157 in unifying
step 118a, and the resin pressure remains as stress, which
generates warp. If the stress becomes excessive, it can damage the
components. To overcome this possible problem, the compression to
be done in unifying step 118a should be carried out such that resin
layers 110 can be formed between substrate 109a and board 101,
between substrates 109a themselves, and between substrate 109a and
copper foil 145. Such compression allows resin 110 supplied
excessively to flow toward the outside of board 101 with ease.
[0129] In other words, solid contact is not allowed between
substrate 109a and board 101, between substrates 109 themselves,
and between substrate 109a and copper foil 145. Thus when substrate
109a is compressed, surplus resin passes through these resin layers
110 toward the outside of board 101. This structure allows the
resin pressure in clearance 144 and space 156 to increase by a
smaller amount in unifying step 118a, so that warp or damage to the
component rarely occurs.
[0130] As discussed above, substrate-included resin section 109 is
formed by layering plural substrates 109a and plural resins 110b
and 110c alternately. Meanwhile, substrate 109a is used as an
example of the resin flow-speed accelerator, and both of resins
110b and 110c are used as examples of bonding resin. A fluid period
of resin 110 is prolonged or a pressure applied to resin 110 is
changed in response to the heating condition. As a result, resin
110 flows into hole 142 with ease, so that the spaces between
semiconductor element 105 and resistor 106 can be filled up
thoroughly with the resin without providing, for example, a
substrate which is conventionally disposed between semiconductor
element 105 and resistor 106. The complete fill-up of the spaces
with the resin allows covering semiconductor element 105 and
resistor 106 with the resin, so that these two components are
insulated from each other. As a result, the space between these two
components can be reduced, and a highly dense mounting of
electronic components can be expected, which allows downsizing of
the component-embedded PWB, and a module employing this PWB can be
also advantageously downsized.
Embodiment 2
[0131] The second embodiment of the present invention is
demonstrated hereinafter with reference to FIGS. 17-20. FIG. 17
shows a flowchart illustrating the steps of manufacturing a
component-embedded printed wiring board (PWB) in accordance with
this second embodiment. In FIGS. 17-20, similar elements to those
shown in FIGS. 1-12 have the same reference marks, and the
descriptions thereof are simplified here. In the first embodiment
previously discussed, six sheets of pre-pregs 141 are layered on
circuit board 101; however, in this embodiment, only one sheet of
pre-preg having a thickness of approx. 1 mm is layered on circuit
board 101. The respective steps are detailed hereinafter following
the sequence of the steps shown in FIG. 17.
[0132] In this second embodiment, as the first embodiment
demonstrates, semiconductor element 105 and resistor 106 are
mounted on circuit board 101, and they undergo the soldering in
reflow soldering step 115. After step 115, hanging step 300 is
prepared for hanging pre-preg 302 over circuit board 101. Hanging
step 300 is followed by decompressing and layering step 301.
Pre-preg 302 is used as an example of a sheet.
[0133] Hanging step 300, and decompressing and layering step 301
are demonstrated with reference to FIGS. 18 and 19. FIG. 18 shows a
sectional view of a hanging device in accordance with the second
embodiment. FIG. 19 shows a sectional view of a decompressing and
layering device in accordance with the second embodiment.
[0134] First, hanging step 300 is described. In FIG. 18, airtight
container 311 includes platen 152, guide 312 surrounding lateral
faces of circuit board 101, slope 313 disposed to the upper end of
guide 312, and opening 314 existing over guide 312. Airtight
container 311 is used as an example of an airtight device, and
platen 152 is used as an example of a compressing device.
[0135] Circuit board 101 is inserted into guide 312 of airtight
container 311. The clearance between guide 312 and board 101 is
approx. 0.5 mm on each side, and guide 312 determines the position
of board 101.
[0136] Pre-preg 302 is placed such that it covers opening 314.
Width 315 of pre-preg 302 should be greater than width 316 of guide
312 and yet smaller than opening size 313a of slope 313. Such
dimensions allow pre-preg 302 to be hung by slope 313 in hanging
step 300. Copper foil 145 is layered on pre-preg 302. To be more
specific, slope 313 hangs pre-preg 302 so that pre-preg 302 cannot
touch semiconductor element 105 or resistor 106 because this
hanging-up provides a clearance between the underside of pre-preg
302 and the top face of semiconductor element 105 or resistor 106.
Slope 313 thus works as a hanging device.
[0137] Pre-preg 302 used in this second embodiment employs epoxy
resin 317 in a liquid state, i.e. having a viscosity, at a room
temperature, because quicker lowering in viscosity needs less
heating, so that a component-embedded PWB can be produced with less
energy. Tip 302a of pre-preg 302 thus closely contacts slope 313,
so that an airtight space is formed by platen 152, guide 312, slope
313 and pre-preg 302. In other words, pre-preg 302 per se works as
a lid of airtight container 311. Epoxy resin 317 can be replaced
with thermosetting resin such as unsaturated polyester resin.
[0138] A sucking device (not shown) sucks the air from the airtight
space covered with pre-preg 302 working as a lid through vent hole
318 provided to guide 312. Decompressing the airtight space makes
the inside of container 311 negatively pressurized, so that
pre-preg 302 lowers along slope 313 and guide 312.
[0139] In this second embodiment, vent hole 318 is provided near
the lower end of guide 312, and it is preferable to provide hole
318 at a place lower than pre-preg 302 is supposed to sink through
decompression. This structure prevents pre-preg 302 from covering
vent hole 318 when the air is sucked through vent hole 318, so that
container 311 can be evacuated positively.
[0140] FIG. 19 shows a sectional view of a decompressing and
layering device in accordance with the second embodiment. As shown
in FIG. 19, pre-preg 302 stops lowering when it touches top face
105c of the semiconductor element or top face 106a of the resistor,
so that pre-preg 302 is layered over circuit board 101. Pre-preg
302 is thus held in a negatively pressurized status due to the
evacuating.
[0141] Evacuating the airtight container leaves no bubbles
containing air between pre-preg 302 and semiconductor element 105
or resistor 106, so that more solid contact can be expected between
pre-preg 302 and top face 105c of element 105 or top face 106a of
resistor 106. As a result, a reliable component-embedded PWB is
obtainable.
[0142] FIG. 20 show a sectional view of a unifying device to be
used in unifying step 303 which follows the compressing and
layering step in accordance with the second embodiment. In unifying
step 303, upper platen 321 heats, compresses, and cools pre-preg
302, thereby charging epoxy resin 317 into spaces 156, 157, and
unifying circuit board 101 and pre-preg 302 together.
[0143] The first step of unifying step 303 is heating and softening
step 304 which follows decompressing and layering step 301. In step
304, heaters 160 provided to platen 152 and upper platen 321 apply
heat to pre-preg 302 so that pre-preg 302 is softened to be fluid.
Since pre-preg 302 employs fluid material at a room temperature,
step 304 needs less heat, i.e. energy can be saved.
[0144] In forcible flow-in step 305 following step 304, epoxy resin
317 flows in as it does in the first embodiment. In this second
embodiment, it is also important to move platen 321 in order to
prevent epoxy resin 317 flowing into spaces 156, 157 from being
heated over the temperature, at which the resin starts hardening,
by frictional heat or pressure loss due to the flow-in. In step
305, epoxy resin 317 is forcibly charged into spaces 156, 157 at
approx. 100.degree. C., so that there is no need to charge an
intermediate member into spaces 156, 157 in another step. Epoxy
resin 317 starts hardening at a temperature ranging from
110.degree. C. to 150.degree. C. This embodiment uses epoxy resin
317 that starts a thermosetting reaction when the resin is held for
approx. 10 minutes in the foregoing temperature range.
[0145] In this second embodiment, pre-preg 302 does not have a
hole, so that spaces 331 formed around semiconductor element 105
and resistor 106 are greater than clearances 143, 144 formed in the
first embodiment. Thus the temperature in forcible flow-in step 305
is set at 100.degree. C., thereby charging epoxy resin 317
positively into spaces 331, 156, 157.
[0146] Step 305 is followed by heating and hardening step 306. In
step 306, epoxy resin 317 is heated to 150.degree. C. to be
completely hardened, and then hardened resin 317 is cooled
gradually in cooling step 124 and is cut in cutting step 125
following step 124.
[0147] In the second embodiment, it is also important to set a
temperature rising rate in step 305 smaller than that in step 304.
This preparation allows lowering the viscosity of pre-preg 141
quickly and minimizing the viscosity, so that the resin is
positively charged into the spaces in step 305.
[0148] Compressing and layering step 301 is carried out ahead of
unifying step 303, so that no bubbles containing air remain between
pre-preg 302 and semiconductor element 105 or resistor 106, and
more solid contact can be expected between pre-preg 302 and top
face 105c of semiconductor element 105 or top face 106a of resistor
106. As a result, a reliable component-embedded PWB is
obtainable.
[0149] Pre-preg 302 does not need a hole corresponding to
semiconductor element 105 and resistor 106 as it is needed in the
first embodiment, so that hole-opening step 117 in the first
embodiment is not needed here. As a result, an inexpensive
component-embedded PWB is obtainable.
[0150] In this second embodiment, holes in response to heights of
respective components are not needed, so that one sheet of pre-preg
302 can work sufficiently. Thus only one sheet of pre-preg 302 is
layered, which lowers the cost of the component-embedded PWB.
[0151] Further, pre-preg 302 can be just placed over slope 313, so
that layering work can be simplified, which also reduces the cost
of the component-embedded PWB.
[0152] In addition, vent holes 318 are provided to guide 312, so
that the clearance between board 101 and guide 312 can be narrowed.
Guide 312 thus accurately determines the position of board 101, and
also prevents pre-preg 302 from flowing out to the outside. This
structure allows epoxy resin 317 to flow into spaces 331, 156, 157,
thereby positively charging these spaces with resin 317 so as to be
free of voids.
Embodiment 3
[0153] In the third embodiment, another instance of the
decompressing and layering device described in the second
embodiment is demonstrated, and this other device can replace the
one used in the second embodiment. The respective manufacturing
steps in the third embodiment remain unchanged from those in the
second embodiment. Thus only this replaceable decompressing and
layering device is described hereinafter.
[0154] FIGS. 21 and 22 show sectional views of the decompressing
and layering device used in the decompressing and layering step in
accordance with the third embodiment. FIG. 23 shows a sectional
view of the component-embedded PWB in the forcible flow-in step of
the third embodiment. In these drawings, elements similar to those
used in FIG. 1-FIG. 20 sometimes have the same reference marks, and
in such cases the descriptions thereof are simplified.
[0155] In FIG. 21, platens 151, 152 and expandable wall 153 form
airtight container 154. Circuit board 101, on which semiconductor
element 105 and resistor 106 have been reflow-soldered, is mounted
to platen 152 at a given place. Airtight container 154 is used as
an example of an airtight device.
[0156] Platen 151 has holding gadget 401 mounted rotatably on shaft
402, and holding gadget 401 is urged inward by springs (not shown),
thereby pinching pre-preg 302 at both sides. Holding gadget 401 and
platen 151 hold pre-preg 302 such that pre-preg 302 confronts
circuit board 101.
[0157] Platen 151 and holding gadget 401 form a hanging device,
thereby preventing pre-preg 302 from touching semiconductor element
105 or resistor 106. This structure allows forming clearance 403
between underside of pre-preg 302 and top face of semiconductor
element 105 or resistor 106. A sucking device (not shown) sucks the
air from the airtight container through vent holes 155, so that the
container is decompressed and evacuated, which makes container 154
negatively pressurized, thereby raising platen 152.
[0158] FIG. 22 shows a sectional view of the decompressing and
layering device used in the decompressing and layering step in
accordance with the third embodiment. As shown in FIG. 21, due to
the evacuating, pre-preg 302 is halted keeping in touch with the
top faces of semiconductor element 105 and resistor 106, so that
pre-preg 302 is layered over the circuit board 101. Pre-preg 302 is
thus held in negative pressurized status due to the evacuating.
[0159] Thus no bubbles containing air remain between pre-preg 302
and semiconductor element 105 or resistor 106, and more solid
contact can be expected between pre-preg 302 and top face 105c of
semiconductor element 105 or top face 106a of resistor 106. As a
result, a reliable component-embedded PWB is obtainable.
[0160] FIG. 23 shows a sectional view of a unifying device used in
unifying step 303. As shown in FIG. 22, in unifying step 303, upper
platen 151 heats, compresses, and cools pre-preg 302, thereby
charging epoxy resin 317 into spaces 156, 157, and unifying circuit
board 101 and pre-preg 302 together. In this case, tip 401a of
holding gadget 401 stops when it touches platen 152. In other
words, holding gadget 401 also works as stopper 161 described in
the first embodiment.
[0161] In this third embodiment, holding gadget 401 is placed such
that it covers the entire periphery of pre-preg 302, so that
holding gadget 401 prevents fluid epoxy resin 317 from flowing out
to the outside in unifying step 303. Epoxy resin 317 thus flows
into spaces 331, 156, 157, thereby inviting no voids. As a result,
these spaces are positively filled up with resin 317. Since less
amount of pre-preg 302 flows out to the outside, the pre-preg can
be downsized proportionately, which can reduce an amount of
pre-preg 302, and an inexpensive component-embedded PWB is
obtainable. In general, the thermosetting resin cannot be reused
once it is hardened, so that the reduction of the amount of
pre-preg 302 can contribute to environmental protection.
[0162] When unifying step 303 ends its operation, holding gadget
401 moves outward (along the arrow mark shown in FIG. 22) for
releasing pre-preg 302, so that platen 151 can move upward and
circuit board 101 can be taken out.
[0163] Compressing and layering step 301 is carried out ahead of
unifying step 303, so that no bubbles containing air remain between
pre-preg 302 and semiconductor element 105 or resistor 106, and
more solid contact can be expected between pre-preg 302 and top
face 105c of semiconductor element 105 or top face 106a of resistor
106. As a result, a reliable component-embedded PWB is
obtainable.
[0164] Pre-preg 302 does not need a hole corresponding to
semiconductor element 105 and resistor 106 as it is needed in the
first embodiment, so that hole-opening step 117 in the first
embodiment is not needed here. As a result, an inexpensive
component-embedded PWB is obtainable.
[0165] In this third embodiment, holes in response to heights of
respective components are not needed, so that one sheet of pre-preg
302 can work sufficiently. Thus only one sheet of pre-preg 302 is
layered, which reduces the cost of the component-embedded PWB.
[0166] Further, pre-preg 302 can be just pinched by holding gadget
401, so that layering work can be simplified, which also reduces
the cost of the component-embedded PWB.
[0167] In this third embodiment, pre-preg 302 is hung; however,
circuit board 101 can be hung instead. In such a case, a similar
advantage to what is discussed above is also obtainable.
[0168] The component-embedded PWB of the present invention includes
the following elements: [0169] a fluid-resin embedding section
formed at a place corresponding to electronic components and
covering the components; [0170] a resin flow-speed accelerator
surrounding the fluid-resin embedding section and being disposed in
parallel with a top face of a circuit board; and [0171] bonding
resin disposed between the accelerator and the circuit board. The
embedding section is filled up with the same resin as the bonding
resin. The foregoing structure allows the accelerator to compress
the resin by a pressure applied when the component-embedded PWB is
pressed, so that the resin can flow along the PWB with ease. The
resin is thus charged into the fluid-resin embedding section in
every nook and cranny, so that no air remains. As a result, the
load produced by expanding the air does not damage the connections,
so that the quality of the connections improves.
[0172] The component-embedded PWB and the manufacturing method of
the same PWB of the present invention can advantageously provide
reliable connections in the component-embedded PWB. Use of the same
PWB as the boards, on which components are reflow-soldered, is
useful.
* * * * *