U.S. patent application number 09/735893 was filed with the patent office on 2001-06-28 for double-sided circuit board and multilayer wiring board comprising the same and process for producing double-sided circuit board.
Invention is credited to Inoue, Yasushi, Kaneto, Masayuki, Nagasawa, Megumu, Nakamura, Kei, Okeyui, Takuji, Ota, Shinya, Sugimoto, Masakazu.
Application Number | 20010004944 09/735893 |
Document ID | / |
Family ID | 26580184 |
Filed Date | 2001-06-28 |
United States Patent
Application |
20010004944 |
Kind Code |
A1 |
Nakamura, Kei ; et
al. |
June 28, 2001 |
Double-sided circuit board and multilayer wiring board comprising
the same and process for producing double-sided circuit board
Abstract
A double-sided circuit board of which a solder conductor is
prevented from deformation in a cycling test so as to maintain high
connection reliability, comprises an insulating layer 2 made of an
organic high molecular weight resin and a circuit 3 provided on
each side of the insulating layer 2, the circuits 3 on both sides
being electrically connected through via-holes filled with a
conductor 4 made of solder having a metal powder 6 dispersed
therein.
Inventors: |
Nakamura, Kei; (Osaka,
JP) ; Sugimoto, Masakazu; (Osaka, JP) ; Inoue,
Yasushi; (Osaka, JP) ; Nagasawa, Megumu;
(Osaka, JP) ; Okeyui, Takuji; (Osaka, JP) ;
Kaneto, Masayuki; (Osaka, JP) ; Ota, Shinya;
(Osaka, JP) |
Correspondence
Address: |
SUGHRUE, MION, ZINN, MACPEAK & SEAS, PLLC
2100 Pennsylvania Avenue, N.W.
Washington
DC
20037
US
|
Family ID: |
26580184 |
Appl. No.: |
09/735893 |
Filed: |
December 14, 2000 |
Current U.S.
Class: |
174/262 ;
174/263 |
Current CPC
Class: |
H05K 2201/0215 20130101;
H05K 3/4641 20130101; H05K 2203/0425 20130101; H05K 3/445 20130101;
H05K 2201/0305 20130101; H05K 3/4614 20130101; H05K 2203/1461
20130101; H05K 3/4038 20130101; H05K 3/3485 20200801; H05K 3/462
20130101; H05K 2201/0355 20130101 |
Class at
Publication: |
174/262 ;
174/263 |
International
Class: |
H05K 001/11 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 14, 1999 |
JP |
P. HEI. 11-354963 |
May 18, 2000 |
JP |
P. 2000-146796 |
Claims
What is claimed is:
1. A double-sided circuit board comprising: an insulating layer
comprising an organic high molecular weight resin; and wiring
conductors provided on both sides of the insulating layer, wherein
the wiring conductors are electrically connected through a
via-hole, and the via-hole is filled with a conductor made of
solder having a metal powder dispersed therein.
2. The double-sided circuit board according to claim 1, wherein the
metal powder has a melting point of 350.degree. C. or higher.
3. The double-sided circuit board according to claim 1, wherein the
metal powder is powder of Ni, Au, Ag, Cu, Fe, Al, Cr, Pd or Co, or
an alloy comprising at least one of these metals.
4. The double-sided circuit board according to claim 1, wherein the
solder comprises at least one of Sn, Pb, Sb, Ag, Cu, Bi and Zn, and
has a melting point of 150 to 350.degree. C.
5. A double-sided circuit board according to claim 1, wherein an
alloy layer with the solder is formed at the surface of the metal
powder.
6. The double-sided circuit board according to claim 1, wherein the
metal powder is present in an amount of 0.1 to 60% by weight based
on the solder.
7. The double-sided circuit board according to claim 1, wherein the
insulating layer further comprises a metal foil as a core.
8. The double-sided circuit board according to claim 7, wherein the
metal foil is an Ni--Fe-based alloy having an Ni content of 31 to
50% by weight and has a thickness of 10 to 100 .mu.m.
9. The double-sided circuit board according to claim 7, wherein the
metal foil is Fe, Ni, Cr, Al, Ti, Cu or Co, or an alloy comprising
at least two of them.
10. A multilayer wiring board, which comprises a plurality of
double-sided circuit boards according to claim 1 which are
integrally laminated via an adhesive layer interposed between every
adjacent circuit boards, wherein the adhesive layer has at least
one through-hole at a predetermined position in contact with the
wiring conductors of the adjacent two double-sided circuit boards,
and the through-hole is filled with a conductor made of solder by
which the wiring conductors of the adjacent double-sided circuit
boards are electrically connected.
11. A process for producing a double-sided circuit board according
to claim 1, which comprises the steps of: (1) providing at least
one through-hole in an insulating layer comprising an organic high
molecular weight resin; (2) pressing a mixture of a metal powder
and a solder powder at a predetermined mixing ratio into the
through-hole; (3) melting the solder powder in the insulating layer
into which the metal powder and the solder powder are pressed in
the through-hole, under pressure, to fill the through-hole with a
conductor of solder having the metal powder dispersed therein; and
(4) laminating both sides of the insulating layer from step (3)
with copper foil and melting the conductor of solder.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a double-sided printed wiring
board (i.e., double-sided circuit board, hereinafter sometimes
abbreviated as double-sided PWB) of which the wiring conductors on
both sides are electrically connected with solder having a metal
powder dispersed therein, a multilayer printed wiring board (i.e.,
multilayer wiring board, hereinafter sometimes abbreviated as
ML-PWB) comprising the same, and a process for producing the
double-sided PWB.
BACKGROUND OF THE INVENTION
[0002] With the recent tendencies for electronic equipment to have
a smaller size and higher performance, it has been demanded for
semiconductor devices constituting electronic equipment and ML-PWBs
for mounting the devices to have reduced size and thickness, higher
performance and higher reliability. To meet these demands, pin
insertion mount package is being displaced by surface mount
package, and, in recent years, a surface mount technology called
bare chip mount has been under study, in which non-packaged (bare)
semiconductor elements are directly mounted on a PWB.
[0003] Further, the increasing number of pins of semiconductor
elements to be mounted has increased the necessity of stacking a
plurality of PWBs. An ML-PWB can be produced by a build up method
comprising alternately building up, on one or both sides of a
substrate, insulating layers of a photosensitive resin and
conductor layers formed by plating or deposition. The build up
method is disadvantageous in that the production process is
complicated and involves many steps, the yield is low, and much
time is required.
[0004] In bare chip mounting, on the other hand, because silicon
chips having a thermal expansion coefficient of 3 to 4 ppm/.degree.
C. are directly mounted on a PWB having a thermal expansion
coefficient of 10 to 20 ppm/.degree. C. with an adhesive, stress
develops due to the difference in thermal expansion to impair the
reliability. The stress also causes cracks in the adhesive, which
results in reduction of moisture resistance. In order to relax the
stress, it has been practiced to use an adhesive having a reduced
elastic modulus thereby to disperse the stress imposed. However,
connection reliability achieved by such conventional techniques is
still insufficient. It is indispensable for securing further
improved reliability to diminish the thermal expansion coefficient
of the PWB itself.
[0005] Under these circumstances, the present inventors previously
proposed (1) a low-expansion double-sided PWB which comprises an
insulating layer of an organic high molecular weight resin having a
metal core and a wiring conductor provided on each side of the
insulating layer, the wiring conductor on both sides being
electrically connected via through-holes and (2) a low-expansion
ML-PWB which comprises a plurality of the double-sided PWBs
integrally laminated with each other via an adhesive layer
interposed between every adjacent PWBs, the adhesive layer having
through-holes at prescribed positions in contact with the wiring
conductors of the adjacent upper and lower double-sided PWBs, and
the through-holes containing a conductor made of solder by which
the wiring conductors of the upper and the lower double-sided PWBs
are electrically connected (see Japanese patent application No.
9-260201).
[0006] It has turned out that the above-mentioned double-sided PWB,
which has the wiring conductors on both sides thereof electrically
connected through via-holes, develops cracks at the corners in a
cycling test, which will lead to an electrical connection failure.
Further, where a plurality of the above-described double-sided PWBs
are superposed on each other to obtain an ML-PWB, the adhesive
layer connecting the upper and the lower PWBs is not allowed to
have the solder conductors provided at the positions corresponding
to the through-holes of the upper and the lower double-sided PWBs,
which limits the freedom of wiring design.
[0007] To solve these problems, the inventors proposed a
low-expansion double-sided PWB having high reliability and high
freedom of wiring design, in which the wiring conductors on both
sides thereof are electrically connected through via-holes filled
with a conductor made of solder (as of yet unpublished Japanese
Patent Application No. 9-199690). According to this technique,
however, where the insulating layer has a large thickness in
relation to the diameter of the via-holes, i.e., where the
via-holes have a high aspect ratio, the solder-filled via-holes
tend to undergo permanent deformation due to the stress accumulated
in a cycling test, which will lead to a failure to connect to the
wiring conductors.
SUMMARY OF THE INVENTION
[0008] An object of the present invention is to provide a
double-sided PWB (double-sided circuit board) and an ML-PWB
(multilayer wiring board) comprising the double-sided PWBs in which
a conductor made of solder is prevented from deformation in a
cycling test so that high connection reliability can be
maintained.
[0009] Another object of the present invention is to provide a
process for producing the double-sided PWB.
[0010] The present inventors have conducted extensive study seeking
a double-sided PWB of which the conductor made of solder is
prevented from deformation in a cycling test. As a result, they
have found that the above objects are accomplished by the following
invention.
[0011] The invention provides, in its first aspect, a double-sided
PWB comprising an insulating layer made of an organic high
molecular weight resin having on each side thereof a wiring
conductor, wherein the wiring conductors on both sides are
electrically connected through via-holes filled with a conductor
made of solder having a metal powder dispersed therein.
[0012] The invention provides, in its second aspect, an ML-PWB
which comprises a plurality of the double-sided PWBs integrally
laminateded with each other via an adhesive layer interposed
between every adjacent PWBs, the adhesive layer having
through-holes at prescribed positions in contact with the wiring
conductors of the adjacent upper and lower double-sided PWBs and
the through-holes being filled with a conductor made of solder by
which the wiring conductors of the upper and the lower double-sided
PWBs are electrically connected.
[0013] The invention provides, in its third aspect, a process for
producing the double-sided PWB comprising the steps of:
[0014] (1) providing at least one through-hole in an insulating
layer comprising an organic high molecular weight resin;
[0015] (2) pressing a mixture of a metal powder and a solder powder
at a predetermined mixing ratio into the through-hole;
[0016] (3) melting the solder powder in the insulating layer into
which the metal powder and the solder powder are pressed in the
through-hole, under pressure, to fill the through-hole with a
conductor of solder having the metal powder dispersed therein;
and
[0017] (4) laminating both sides of the insulating layer from step
(3) with copper foil and melting the conductor of solder.
[0018] According to the invention, plastic deformation of the
solder conductor is prevented by the hard metal powder dispersed in
the soft solder thereby to secure sufficient strength while
maintaining low connection resistance. Thus, deformation of the
solder conductor in a cycling test can be suppressed, and high
connection reliability can be retained.
[0019] In a highly preferred embodiment of the invention, the
insulating layer contains an Ni--Fe-based alloy foil as a core.
According to this embodiment, the presence of one low-expansion
Ni--Fe-based alloy layer (core) per two wiring conductor layers
brings the thermal expansion coefficient of the double-sided PWB as
a whole very close to that of silicon even where the wiring
conductors are made of copper. The lowered thermal expansion
coefficient of the double-sided PWB secures extremely high
reliability even in bare chip mount. BRIEF DESCRIPTION OF THE
DRAWINGS
[0020] FIG. 1 is a schematic cross section showing an embodiment of
the double-sided PWB according to the present invention.
[0021] FIGS. 2 through 5 illustrate the process for producing the
double-sided PWB of FIG. 1.
[0022] FIG. 6 is a schematic cross section showing another
embodiment of the double-sided PWB according to the present
invention.
[0023] FIGS. 7 through 12 illustrate a process for producing the
double-sided PWB of FIG. 6.
[0024] FIG. 13 is a schematic cross section of an embodiment of the
ML-PWB according to the present invention.
[0025] FIGS. 14 through 17 illustrate a process for producing the
ML-PWB of FIG. 13.
[0026] FIGS. 18 through 21 illustrate the process for producing the
double-sided PWB of Example 1.
[0027] FIGS. 22 through 24 illustrate the process for producing the
low-expansion double-sided PWB of Example 2.
[0028] FIGS. 25 to 29 illustrate the process for producing the
six-layered PWB of Example 3.
[0029] FIG. 30 illustrates the process for producing the
double-sided PWB of Comparative Example 1.
[0030] FIG. 31 is a schematic cross section of the double-sided PWB
of Comparative Example 1.
[0031] FIG. 32 illustrates the process for producing the
double-sided PWB of Comparative Example 3.
[0032] FIG. 33 is a schematic cross section of the double-sided PWB
of Comparative Example 3.
[0033] FIG. 34 is a schematic illustration of a metal powder and a
solder powder injected into a via-hole.
[0034] FIGS. 35 and 36 each schematically illustrate the cross
section of the via-hole of FIG. 34 after the solder powder is
melted.
DETAILED DESCRIPTION OF THE INVENTION
[0035] The organic high molecular weight resin which can be used as
an insulating layer preferably includes polyimide resins, epoxy
resins, and mixtures thereof for their heat resistance and
electrical characteristics. An adhesive sheet made of such an
organic high molecular weight resin is conveniently used. The
adhesive sheet preferably has a thickness of about 0.01 to 1.0 mm.
An adhesive sheet with a smaller thickness than about 0.01 mm tends
to have poor workability. With the thickness larger than about 1.0
mm, it may be difficult to completely fill the through-holes with
the metal/solder mixed powder, resulting in reduced
reliability.
[0036] The means for making the through-holes in the insulating
layer is appropriately chosen depending on the desired size of the
holes. For example, drilling, punching, laser machining and the
like can be adopted.
[0037] The composition of solder powder, which becomes a solder
conductor on melting, includes, but is not limited to, Sn--Pb
alloys, Sn--Ag alloys, Sn--Ag--Cu alloys, Sn--Ag--Cu--Bi alloys,
Sn--Ag--Bi alloys, Sn--Zn alloys, Sn--Cu alloys, Sn--Sb alloys, and
Sn--Au alloys. An optimum solder composition for the desired heat
resistance is chosen. The solder powder usually has a particle size
of 50 .mu.m or smaller, preferably 10 .mu.m or smaller. The solder
powder preferably has a melting point of 150 to 350.degree. C.
[0038] The metal powder, which is to be dispersed in a solder
conductor, preferably includes powder of Ni, Au, Ag, Cu, Fe, Al,
Cr, Pd or Co, and powder of alloys comprising at least one of these
metals. The metal powder usually has a particle size of 50 .mu.m or
smaller, preferably 10 .mu.m or smaller. The metal powder
preferably has a melting point of 350.degree. C. or higher.
[0039] It is preferred that the melting point of the metal powder
is higher by at least 5.degree. C. than that of the solder
powder.
[0040] Via-holes filled with a solder conductor having a metal
powder dispersed therein can be formed, for example, as follows.
The metal powder, the solder powder and an organic solvent are
mixed at a predetermined ratio into paste. The paste is applied in
excess on the openings of through-holes by printing. After removing
the organic solvent by drying, the mixture is injected (pressed)
into the through-holes by pressing down. While the mixed powder is
being injected, metal particles A and solder particles B rub
against themselves as well as mutually (see FIG. 34), whereby the
surface oxide film of these particles is destroyed. Then, excess
powder is removed from the surface, and the insulating layer is
heated under pressure at or above the melting point of the solder,
whereby the solder powder melts to form a conductor having the
metal powder dispersed therein. Thoroughly melting the solder
generally results in formation of an alloy layer D made from the
metal and the solder material on the surface of the metal particles
A as schematically depicted in FIG. 35. The alloy layer D is formed
by the solder material's diffusing and reacting. The alloy layer D
serves as an affinity between the metal particles A and the solder
conductor C to provide improved electrical and mechanical
characteristics. The growth rate of the alloy layer D depends on
the temperature and time. The alloy layer D gains in thickness with
time until the whole metal particle becomes an alloy D as shown in
FIG. 36, which is also included under the scope of the present
invention.
[0041] The metal powder is preferably used in an amount of 0.1 to
60% by weight based on the solder powder. In lower amounts, the
effect in suppressing deformation of the via-holes in a cycling
test is insubstantial. In higher amounts, the proportion of the
solder material is insufficient for binding the metal particles,
and the resulting conductor is so brittle as to develop cracks.
[0042] The mixing ratio of the organic solvent is decided according
to the dispersibility of the mixed powder in the resulting paste
and is preferably 1 to 70% by volume based on the mixed powder.
Alcohol solvents are suitable. The paste may be prepared by
previously plating the metal powder with solder and mixing the
plated metal powder with the organic solvent into paste.
[0043] The adhesive sheet (insulating layer) having the via-holes
is laminated with copper foil as a conductor layer on its both
sides, and the laminate is heated under pressure at or above the
melting point of the solder conductor to melt the solder conductor
thereby to secure the electrical connections between the copper
foil and the via-holes. The copper foil on each side is etched in a
conventional manner according to a desired circuit pattern to
produce the double-sided PWB of the present invention.
[0044] The insulating layer can contain a metal foil or a ceramic
material as a core so as to have a reduced thermal expansion
coefficient. The metal which can be used as a core includes Fe, Ni,
Cr, Al, Ti, Cu, Co, or an alloy thereof. In order for the metal
foil or the ceramic material to serve to suppress expansion of the
conductor layer and the insulating layer, their own thermal
expansion coefficient should be sufficiently low. In the case of an
Ni--Fe-based alloy foil, for instance, whose thermal expansion
coefficient varies with the alloying ratio, a preferred Ni content
ranges from 31 to 50% by weight, particularly from 31 to 45% by
weight. Out of this range, the alloy tends to have a fairly higher
thermal expansion coefficient than silicon chips. The metal foil
has a thickness of 10 to 300 .mu.m, preferably 10 to 200 .mu.m,
still preferably 10 to 100 .mu.m. With a thickness smaller than 10
.mu.m, the difference in thermal expansion between the double-sided
PWB and silicon chips cannot be reduced sufficiently.
[0045] The ML-PWB of the invention can be produced as follows. An
adhesive sheet having through-holes is stuck to one or both sides
of the double-sided PWB of the invention at right positions so that
the through-holes may correspond to desired positions of the
double-sided PWB. A solder paste is applied into the through-holes
of the adhesive sheet by printing, followed by heat melting the
paste to form solder bumps. A plurality of the double-sided PWBs
having solder bumps are stacked on each other at right positions,
and the laminate is press bonded under heat into an integral body.
The through-holes of the adhesive sheet may be at positions of the
circuit on the via-holes connecting the wiring conductors on both
sides of the double-sided PWB.
[0046] Serving as an insulating layer after stacking, the adhesive
sheet used in the production of the ML-PWB preferably includes a
sheet of polyimide resins, epoxy resins or mixtures thereof for
their heat resistance and electrical characteristics. The thickness
of the adhesive sheet is preferably about 0.01 to 1.0 mm. Too thin
an adhesive sheet has poor workability. If the thickness is too
large, it is difficult to completely fill the through-holes with
the solder paste, resulting in reduced reliability.
[0047] The through-holes can be made in the adhesive sheet by any
known techniques selected from drilling, punching, laser machining,
and the like according to the size of the openings. The adhesive
sheet having through-holes can be adhered temporarily on one or
both sides of the double-sided PWB by hot pressing. Alternatively,
the through-holes may be made by laser machining after the adhesive
sheet is temporarily stuck to one or both sides of the double-sided
PWB. Lasers which can be used include a carbonic acid gas laser, an
excimer laser, a YAG laser, etc.
[0048] Commercially available solder paste can be used to form
solder bumps. The size of solder bumps is 100 .mu.m or smaller,
preferably 50 .mu.m or smaller, still preferably 10 .mu.m or
smaller. The solder composition is not particularly limited and can
be selected in accordance with the heat resistance required of the
wiring board. After stacking, the solder bumps are bought into
contact with an opposite electrode to establish electrical
connections. If desired, the laminate may be heated at or above the
melting point of the solder either simultaneously with or after the
press bonding to form metallic joints.
[0049] The practice of the present invention will be described with
reference to the accompanying drawings.
[0050] FIG. 1 shows an embodiment of the present invention, in
which numeral 1 is a double-sided PWB composed of an insulating
layer 2 made of a polyimide resin having formed on both sides
thereof a circuit (wiring conductor) 3 made of a copper foil. The
circuits 3 on both sides are electrically connected by a via-hole
5a of the insulating layer 2 filled with a solder conductor having
a metal powder 6 dispersed therein.
[0051] The double-sided PWB 1 is produced, for example, as follows.
As shown in FIG. 2, through-holes 5a are made in a polyimide
adhesive sheet 5, which becomes an insulating layer 2, at
predetermined positions (positions where via-holes filled with a
solder conductor 4 are to be formed). As shown in FIG. 3, a mixture
comprising a metal powder 6 and a solder powder 7 at a prescribed
mixing ratio is pressed into the through-holes 5a and melted to
fill the through-holes 5a with a solder conductor having the metal
powder 6 dispersed therein (FIG. 4). A copper foil 8 is adhered to
both sides of the adhesive sheet 5, and the laminate is heated
under pressure at or above the melting point of the solder powder
to cause the solder to reflow, thereby securing the electrical
connections of the copper foils on both sides (FIG. 5). Each of the
copper foils is etched in a conventional manner to form a circuit
layer 3 (FIG. 1).
[0052] According to this embodiment, the solder conductor 4 is
prevented from plastic deformation owing to the metal powder 6
dispersed therein. The dispersed metal powder 6 and the solder form
an alloy layer, and the via-holes 5a filled with the solder
conductor 4 have low electrical resistance. Further, since the
circuit layers 3 on both sides are electrically and mechanically
connected to each other by the metal joints of the solder conductor
4, extremely high reliability is enjoyed.
[0053] A double-sided PWB 9 shown in FIG. 6, in which an insulating
layer 13 has a metal core 10, is produced, for example, as follows.
As shown in FIG. 7, through-holes 10a are made in an Ni--Fe alloy
foil 10 at predetermined positions (i.e., positions where via-holes
filled with a solder conductor 11 are to be formed). The foil
having the through-holes 10a is sandwiched in between a pair of
polyimide adhesive sheets (which become an insulating layer 13
together) to prepare a composite 12 (FIG. 8). As shown in FIG. 9,
through-holes 13a are made in the composite 12 at the same
positions as the through-holes 10a of the alloy foil 10, the former
being smaller than the latter. A mixture of a metal powder 14 and a
solder powder 15 is pressed into the through-holes 13a (FIG. 10),
the solder powder 15 is melted (FIG. 11), and a copper foil 16 is
adhered to both sides of the composite 12. The laminate is heated
under pressure at or above the melting point of the solder to cause
the solder to reflow, thereby to secure the electrical connections
of the copper foils 16 on both sides (FIG. 12). Each of the copper
foils 16 is etched in a conventional manner to form a circuit layer
16a (FIG. 6).
[0054] In the embodiment shown in FIGS. 6 to 12, the thermal
expansion coefficient of the composite 17 is governed by the Ni--Fe
alloy of the core and can therefore be adjusted by changing the
Ni/Fe alloying ratio or the thickness of the core.
[0055] FIG. 13 shows an example of the ML-PWB according to the
invention, which comprises a plurality of double-sided PWBs 18 each
having an insulating polyimide resin layer 20 containing an Ni--Fe
alloy foil 19 as a core and a circuit layer (wiring conductor) 21
made of a copper foil on each side thereof. In this particular
example, three double-sided PWBs 18 are stacked to provide six
circuit layers. Each double-sided PWB 18 has via-holes 18a filled
with a solder conductor 22 having a metal powder 23 dispersed
therein, through which the circuit layers 21 on both sides are
electrically connected. Numeral 24 represents a polyimide adhesive
with which adjacent two double-sided PWBs 18 are adhered to each
other. Numeral 25 is a solder conductor with which the circuit
layers 21 of adjacent two double-sided PWBs 18 are electrically
connected.
[0056] The ML-PWB of FIG. 13 can be prepared, for example, as
follows. Three double-sided PWBs 18 each having a polyimide resin
insulating layer 20 and a circuit layer 21 made of a copper foil on
each side thereof (shown in FIG. 13) and two adhesive sheets 26
(shown in FIG. 14) made of a polyimide adhesive are prepared. As
shown in FIG. 15, the adhesive sheet 26 is stuck to the upper side
of two out of three double-sided PWBs 18 in a right position with
its openings 26a mating with prescribed positions of the circuit
layer 21 of the PWB 18 (for example, the opening 26a shown in FIG.
15 is positioned where the solder conductor 22 has been formed). A
solder paste is applied to the openings 26a of each adhesive sheet
26 by screen printing and heat-melted to form solder bumps 27 on
the circuit layer 21. As shown in FIG. 17, the two double-sided
PWBs 18 having solder bumps 27 and another double-sided PWB 18 are
superposed on each other at right positions, and the resulting
laminate is heated under pressure to give a six-layered PWB having
three double-sided PWBs 18 united into one body shown in FIG. 13,
in which the adhesive sheets 24 correspond to the adhesive sheets
26, and the solder conductors 25 correspond to the solder bumps
27.
[0057] In the embodiment shown in FIGS. 13 to 17, although the
via-holes 18a of the insulating layer 20 have a high aspect ratio
because of the Ni--Fe alloy foil 19 as a core, plastic deformation
of the via-holes 18a can be suppressed by the presence of the metal
powder 23 in the solder conductor 22 thereby to maintain high
connection reliability. While the solder conductors 25 which
electrically connect every adjacent double-sided PWBs 18 do not
contain metal powder, they are free from the problem of plastic
deformation because the via-holes 26a have a small aspect
ratio.
[0058] The solder conductors 25 can be disposed at arbitrary
positions without being restricted by the positions of the
via-holes 18a filled with the solder conductor 22 having the metal
powder 23 dispersed therein. As a result, the freedom of wiring
design is broad, enabling high-density wiring.
[0059] Uniting the three double-sided PWBs 18 into one body and
electrically connecting the six circuit layers can be carried out
simultaneously in a single operation of heating under pressure. One
Ni--Fe alloy layer per two circuit layers makes it possible to
reduce the thermal expansion coefficient of the six-layered PWB as
a whole even where the circuits 21 are made of copper.
[0060] The present invention will now be illustrated in greater
detail with reference to Examples, but it should be understood that
the invention is not deemed to be limited thereto. Unless otherwise
noted, all the percents are by weight.
EXAMPLE 1
[0061] A 100 .mu.m thick polyimide adhesive sheet 30 was punched at
predetermined positions to make through-holes 30a having a diameter
of 100 .mu.m (FIG. 18). A paste prepared by mixing 30% of an Ni
powder 31 (average particle size: 10 .mu.m) and 70% of an Sn/Pb
solder powder (average particle size: 10 .mu.m) and kneading the
mixture with the same volume of an alcohol solvent was screen
printed on the through-holes 30a via a metal mask (thickness: 100
.mu.m; diameter of openings: 100 .mu.m). After the solvent was
evaporated, the printed powder was pressed into the through-holes
30a by pressing at 30.degree. C. and 10 MPa for 5 minutes. The
excess powder on the surface was removed by buffing. The sheet was
heated up to 200.degree. C. under pressure to melt the solder
powder to form via-holes filled with a solder conductor 32 having
the Ni powder 31 dispersed therein (FIG. 19). A 18 .mu.m thick
copper foil 33 was press bonded to each side of the adhesive sheet
30 at 175.degree. C. and 5 MPa for 60 minutes, followed by solder
reflow at 200.degree. C. and 5 Ma for 5 minutes (FIG. 20). The
copper foil 33 on each side was etched in a conventional manner to
produce a double-sided PWB 34 having a circuit 33a on each side
thereof (FIG. 21).
EXAMPLE 2
[0062] Holes having a diameter of 150 .mu.m were punched through a
100 .mu.m thick Ni--Fe alloy foil 35 (Ni: 36%; Fe: 64%; thermal
expansion coefficient: 1.5 ppm/.degree. C.) at predetermined
positions at a pitch of 300 .mu.m. A 50 .mu.m thick polyimide
adhesive sheet 36 (available from Nippon Steel Chemical Co., Ltd.)
was press bonded on each side of the foil at 200.degree. C. and 5
MPa for 60 minuets (FIG. 22). Through-holes 36a having a diameter
of 100 .mu.m were punched at the same positions as the holes 35a
(FIG. 23). A low-expansion double-sided PWB 39 having via-holes
filled with a solder conductor 38 having a metal powder 37
dispersed therein (FIG. 24) was produced by using the resulting
foil-cored insulating layer in the same manner as in Example 1. In
FIG. 24, numeral 40 is a circuit.
EXAMPLE 3
[0063] Holes having a diameter of 150 .mu.m were punched through a
100 .mu.m thick Ni--Fe alloy foil 35 (Ni: 36%; Fe: 64%; thermal
expansion coefficient: 1.5 ppm/.degree. C.) at predetermined
positions at a pitch of 300 .mu.m. A 50 .mu.m thick polyimide
adhesive sheet 36 (available from Nippon Steel Chemical Co., Ltd.)
was press bonded on each side of the foil at 200.degree. C. and 4
MPa for 60 minuets (FIG. 22). Through-holes 36a having a diameter
of 100 .mu.m were punched at the same positions as the holes 35a
(FIG. 23). A paste prepared by mixing 30% of an Ni powder (average
particle size: 10 .mu.m) and 70% of an Sn/Sb solder powder (average
particle size: 10 .mu.m; available from Nihon Genma K.K.) and
kneading the mixture with the same volume of an alcohol solvent was
screen printed on the through-holes 36a via a metal mask
(thickness: 50 .mu.m; diameter of openings: 100 .mu.m). After the
solvent was evaporated, the printed powder was pressed into the
through-holes 36a by pressing at 30.degree. C. and 10 MPa for 5
minutes. The excess powder on the surface was removed by buffing.
The sheet was heated up to 250.degree. C. under pressure to melt
the solder powder to form via-holes filled with a solder conductor
38 having the Ni powder 37 dispersed therein (FIG. 24). A 18 .mu.m
thick copper foil was press bonded to each side of the adhesive
sheet at 200.degree. C. and 5 MPa for 60 minutes, followed by
solder reflow at 250.degree. C. and 5 Ma for 5 minutes. The copper
foil on each side was etched in a conventional manner to produce a
low-expansion double-sided PWB 39 having a circuit 40 on each side
thereof (FIG. 24).
EXAMPLE 4
[0064] An polyimide adhesive sheet 41 (SPB-035A, available from
Nippon Steel Chemical Co., Ltd.) having through-holes 41a having a
diameter of 100 .mu.m punched (FIG. 25) was correctly positioned on
the low-expansion double-sided PWB 39 obtained in Example 3 and
press bonded at 175.degree. C. and 2 MPa for 30 minutes (FIG. 26).
The through-holes 41a of the adhesive sheet 41 were filled with a
solder paste (SQ10-11, available from Tamura Kakensha) by screen
printing. The solder was made to reflow at 220.degree. C., and the
flux was washed away to provide a double-sided PWB 43 having solder
bumps 42 (FIG. 27). In the same manner another double-sided PWB 44
having solder bumps 42 was prepared. The two double-sided PWBs 43
and 44 and a double-sided PWB 45 prepared in the same manner as in
Example 3 were superposed in this order at right positions (FIG.
28), and the laminate was press bonded at 175.degree. C. and 5 MPa
for 60 minutes to obtain an integral six-layered PWB 46 (FIG. 29),
in which numeral 47 indicates a solder conductor.
EXAMPLE 5
[0065] A polyimide adhesive sheet 41 (SPB-035A, available from
Nippon Steel Chemical Co., Ltd.) having through-holes 41a having a
diameter of 100 .mu.m punched (see FIG. 25) was correctly
positioned on the low-expansion double-sided PWB 39 obtained in
Example 3 and press bonded at 175.degree. C. and 20 MPa for 30
minutes (FIG. 26). The through-holes 41a of the adhesive sheet 41
were filled with an Sn/Sb solder paste (available from Nippon Genma
K.K.) by screen printing. The solder was made to reflow at
260.degree. C., and the flux was washed away to provide a
double-sided PWB 43 having solder bumps 42 (FIG. 27). In the same
manner another double-sided PWB 44 having solder bumps 42 was
prepared. The two double-sided PWBs 43 and 44 and a double-sided
PWB 45 prepared in the same manner as in Example 3 were superposed
in this order at right positions (FIG. 28), and the laminate was
press bonded into an integral body at 200.degree. C. and 5 MPa for
30 minutes, followed by solder reflow under pressure at 250.degree.
C. for 5 minutes to obtain an integral six-layered PWB 46 (FIG.
29), in which numeral 47 indicates a solder conductor.
COMPARATIVE EXAMPLE 1
[0066] A double-sided copper clad laminate 49 having a total
thickness of 50 .mu.m composed of a polyimide resin layer 48 and
copper foils 47 each having a thickness of 18 .mu.m (NEOFLEX-231R,
available from Mitsui Toatsu Chemicals, Inc.) was punched to make
through-holes 49a having a diameter of 100 .mu.m at predetermined
positions at a pitch of 300 .mu.m (FIG. 30). The inner wall of the
through-holes 49a was plated with copper to a deposit thickness of
10 .mu.m, and the copper foil 47 on each side was etched in a
conventional manner to prepare a double-sided PWB 51 having a
circuit 47a on each side (FIG. 31).
COMPARATIVE EXAMPLE 2
[0067] A double-sided PWB was prepared in the same manner as in
Example 1, except for using a paste prepared by kneading an Sn/Pb
solder powder (average particle size: 10 .mu.m) with the same
volume of an alcohol solvent in place of the paste containing the
Ni powder.
COMPARATIVE EXAMPLE 3
[0068] Through-holes 52a having a diameter of 100 .mu.m were
punched in a 100 .mu.m thick polyimide adhesive sheet 52 at
predetermined positions (FIG. 32). A conductive paste consisting of
85% of spherical copper particles having an average particle size
of 5 .mu.m as a conductive filler, 12.5% of a thermosetting epoxy
resin, and 2.5% of an acid anhydride curing agent was applied into
the through-holes 52a by screen-printing and cured by heating at
175.degree. C. for 60 minutes to form conducting via-holes 53 (FIG.
33). A copper foil was adhered to each side of the adhesive sheet
52 in the same manner as in Example 1 and etched in a conventional
manner to prepare a double-sided PWB.
[0069] Reliability of electrical connection through the via-holes
of the double-sided PWBs obtained in Examples 1 to 3 and
Comparative Examples 1 to 3 was evaluated in a thermal shock test
(in liquid; -55.degree. C..times.5 mins <.fwdarw.125.degree.
C..times.5 minutes). Table 1 below shows the number of cycles at
which a connection failure occurred. A change in resistivity
exceeding .+-.10% was regarded as a connection failure.
1 TABLE 1 Thermal Shock Test (cycle) Example 1 1000 Example 2 1000
Example 3 1000 Compara. Example 1 50 Compara. Example 2 100
Compara. Example 3 100
[0070] The double-sided PWB of Comparative Example 1 which has a
conventional via-hole structure develops a connection failure on
the 50th thermal shock cycle. In the double-sided PWB of
Comparative Example 2 in which the electrical connection between
the upper and lower circuits is made by a solder conductor
containing no metal powder, the solder-filled via-holes undergo
deformation with the thermal shock cycles and develop a connection
failure on the 100th cycle.
[0071] To the contrary, the double-sided PWBs of Examples 1 to 3
show no deformation of the via-holes until the 1000th cycle,
suppressing the resistivity change within .+-.10%. It is obvious
that these double-sided PWBs having a solder conductor having an Ni
powder dispersed therein exhibit high connection reliability
between the two circuits.
[0072] The thermal expansion coefficient of the (multilayer)
double-sided PWBs of Examples 2 to 5 having an Ni--Fe alloy foil as
a low-expansion core in the insulating layer per two wiring
conductor layers and the double-sided PWBs of Comparative Examples
1 to 3 and Example 1 having no metal foil was measured in a
temperature range of from room temperature (25.degree. C.) to
200.degree. C. The results are shown in Table 2 below.
2 TABLE 2 Thermal Expansion Coefficient (ppm/.degree. C.) Example 1
17.0 Example 2 4.0 Example 3 4.0 Example 4 4.0 Example 5 4.0
Compara. Example 1 17.0 Compara. Example 2 17.0 Compara. Example 3
17.0
[0073] It is seen from Table 2 that the double-sided PWBs having an
Ni--Fe alloy foil as a core (Examples 2 and 3) have an extremely
decreased thermal expansion coefficient.
[0074] The ML-PWB of Examples 4 and 5 prepared by using three
double-sided PWBs of Example 3 also have an extremely low thermal
expansion coefficient (4 ppm/.degree. C.) . Further, they exhibited
extremely high connection reliability at the via-holes, keeping the
resistivity change within .+-.10% even after 1000 thermal shock
cycles when tested under the same conditions as described above.
According to Examples 4 and 5, since any adjacent circuit layers
can be electrically connected through fine via-holes at arbitrary
positions, high freedom of wiring design is enjoyed, enabling
high-density wiring. Therefore, the low-expansion ML-PWBs of
Examples 4 and 5 are suitable for bare chip mount, promising high
reliability in electrical connection.
[0075] While the invention has been described in detail and with
reference to specific examples thereof, it will be apparent to one
skilled in the art that various changes and modifications can be
made therein without departing from the spirit and scope
thereof.
[0076] The entire disclosure of each and every foreign patent
application from which the benefit of foreign priority has been
claimed in the present application is incorporated herein by
reference, as if fully set forth.
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