JPH11346054A - Bonding method of circuit board and suspension board with circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method capable of bonding a flexible circuit board with bumps to a suspension board with a circuit with high performance in bonding strength and reliability. SOLUTION: A solder-plated bump 25 of a circuit board 2 is soldered to a gold-plated bonding pad E by melting and solidifying a solder layer, wherein a gold plating layer 19 is set as thick (average thickness) as 1.5 μm or above, preferably above 2.0 μm or more preferably 2.5 μm or above.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for joining circuit boards by soldering and a suspension board with a circuit to be bump-joined by the joining method.

[0002]

2. Description of the Related Art When a circuit board is joined by soldering, a bump is formed on one of the circuit boards, solder is plated on the bump in advance, and the solder plating bump is attached to a bonding pad of the other circuit board. May be soldered due to melting and solidification. For example, when joining a flexible circuit board for a relay cable to a suspension board with a circuit having excellent flying characteristics and electrical characteristics of an MR head mounted for reading and writing information on a magnetic disk, bumps are formed on the flexible circuit board. Then, the bump bonding method described above may be used. In this bump bonding, gold plating is applied to the bonding pad to prevent oxidation of the bonding pad.

When a surface to be soldered is plated with gold, the molten solder is eroded with gold, and an intermetallic compound (mainly A
It is known that uSn ₄ ) precipitates in the solidified solder and lowers the bonding strength as a so-called gold erosion phenomenon. For this reason, in the above bump bonding, the thickness of the gold plating layer of the bonding pad is set to the minimum thickness necessary for preventing oxidation of the bonding pad, and conventionally, the thickness of the gold plating layer is set to about 1.0 μm to 0.3 μm. I have.

[0004]

In recent years, the spread of personal computers has been remarkable, and further improvement in reliability has been demanded. The bump connection between the suspension board with the MR head mounted circuit and the flexible circuit board for a relay cable has been demanded. Is also one of the important issues.

The inventors of the present invention have conducted intensive studies to improve the strength and reliability of the bump bonding. As a result, unexpectedly, the thickness of the gold plating layer of the bonding pad was unexpectedly reduced to 0.3 μm or less. It has been found that when the thickness is considerably larger than 1.0 μm, the joining strength and stability can be remarkably improved. In order to investigate the cause of this unexpectedly good result, the cross section of the joint was observed with a magnified photograph (4000 times). As a result, the molten solder was pressed against the side due to the pressure applied by the bumps, and the side solidified. It was found that the metal composition was different from the solidified metal composition between the bump and the bonding pad base surface (nickel surface), and the latter was a gold-rich composition. From this observation result, the unexpected result is that the intermetallic compound (mainly AuSn ₄ ) causing the brittleness of the joint is pushed to the side, and the bump and the bonding pad base are pushed.
This is presumed to be due to the fact that the gold-rich layer between the surface and the metal surface is tough, and the strength of the joint is guaranteed by this gold-rich layer.

An object of the present invention is to provide a suspension board with a circuit capable of soldering and joining a flexible circuit board with bumps with excellent joining strength and reliability based on the above findings.

[0007]

According to the present invention, there is provided a method for bonding a circuit board, comprising the steps of:
A method of soldering to a gold plating bonding pad of the other circuit board by melting and solidifying the solder plating layer, wherein the thickness (average thickness) of the gold plating layer is 1.5 μm or more, preferably 2.0 μm or more, Preferably 2.5
It is a configuration characterized in that it is not less than μm.

[0008] One suspension board with circuit according to the present invention is the other circuit board joined by the above-mentioned joining method, and the thickness (average thickness) of the gold plating layer of the bonding pad is at least 1.5 μm, preferably 2 μm. 0.0 μm or more,
More preferably, the thickness is 2.5 μm or more.

A suspension board with a circuit according to the present invention is another circuit board to be joined by the above-mentioned joining method, wherein a bonding pad is composed of a nickel plating layer and a thickness (average thickness) on the nickel plating layer. 5 μm or more, preferably 2.0 μm or more, more preferably 2.5 μm
This is a configuration characterized by being formed by lamination with the above gold plating layer.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a perspective view showing an example of a suspension board with circuit 1 according to the present invention,
A is the MR head mounting part, C is the bonding pad part,
B indicates a circuit section between the MR head mounting section A and the bonding pad section C.

FIG. 2A is a sectional view taken along the line II in FIG. 1, and FIG. 2B is a sectional view taken along the line II in FIG. In FIG. 2, reference numeral 11 denotes a suspension metal plate, usually a stainless steel plate, but phosphor bronze, copper or the like can also be used. The thickness of the suspension metal plate is usually 5 to 50 μm. Reference numeral 12 denotes an insulating layer provided on the suspension metal plate 11, which may be made of, for example, a polyimide resin, a polyester resin, an epoxy resin, or the like. The insulating layer is patterned as shown in FIG. In this case, it is preferable to use a photosensitive polyimide resin. The thickness of the insulating layer 12 is usually 3 μm.
５０50 μm.

Reference numeral 13 denotes a conductor provided on the insulating layer 12, reference numeral 14 denotes a chromium thin film provided between the conductor 13 and the insulating layer 12, and a conductor material other than copper, such as aluminum or tungsten, may be used. The chromium thin film 14 is useful as a base for forming a conductor (by electrolytic plating or electroless plating). The thickness of the conductor 13 is usually 1 μm to 50 μm.
m, and the thickness of the chromium thin film 14 is usually 0.5 μm to 5 μm.
μm. Reference numeral 15 denotes an anti-migration barrier provided on the top and side surfaces of the conductor 13, which can be formed of a nickel plating layer or a palladium plating layer (either electrolytic plating or electroless plating is possible). .5 μm to 5 μm. The conductor width of the above-mentioned conductor circuit is usually 5 μm to 50 μm under the width including the migration prevention barrier, and the spacing between the conductors is usually 5 μm to 5 μm under the spacing between the migration prevention barrier outer surfaces.
It is 50 μm.

Reference numeral 16 denotes a cover, which can be made of polyimide resin, polyester resin, epoxy resin or the like, similarly to the insulating layer 12, and in particular, forms a pattern of the cover as shown in FIG. In this case, it is preferable to use a photosensitive polyimide resin. The thickness of the cover 16 is generally 1 μm to 25 μm, preferably 5 μm to 25 μm. The cover 16 can be made of the same material or different thickness as the insulating layer 12.

Reference numeral 17 denotes a hole provided in the cover 16 for forming a bonding pad, and is located just above the land at the end of the circuit conductor. Reference numeral 18 denotes a nickel plating layer provided on the bottom of the hole 17 (either electrolytic plating or electroless plating is possible), and 19 denotes a gold plating layer provided on the nickel plating layer 18 (either electrolytic plating or electroless plating is possible). )
The bonding pad is formed by the laminate E, and the thickness of the gold plating layer 19 is set to 1.5 μm or more, preferably 2.0 μm or more, and more preferably 2.5 μm or more. The upper limit of the thickness of the gold plating layer 19 is determined by the depth of the hole 17 of the cover 16 and the thickness of the nickel plating layer 18 and is usually 12 μm or less. The upper surface of the gold plating layer 19 is covered with a cover 16 with respect to the upper surface of the cover 16.
Is positioned at a height of ± 10% of the thickness t.

[0015] In suspension board with a circuit according to the present invention, the gold plating layer 19 of the bonding pads for the aforementioned bump bonding so as to contribute to the improvement of the deposition despite bonding strength of the intermetallic compound such as AuSn _4, It is characterized in that the thickness of the gold plating layer 19 is 1.5 μm or more, preferably 2.0 μm or more, and it can be manufactured, for example, as follows (provided that the conductor circuit is made of copper, an insulating layer, and a cover. (-: Pattern formation using photosensitive polyimide resin, migration prevention barrier: nickel plating layer, suspension board: stainless steel plate).

First, as shown in FIG. 3 (a), a photosensitive polyimide resin precursor 120 'is applied to one entire surface of a stainless steel substrate 11' previously processed into a desired shape, and then exposed through a photomask. , Developed and further heated
As shown in (b), an insulating layer 12 'is formed in a pattern. Next, as shown in FIG.
And a copper thin film 141 'are sequentially formed by sputtering.

Then, a negative pattern resist 21 'is coated on the conductor circuit pattern as shown in FIG. 3 (d), and is further coated on the copper thin film 141' as shown in FIG. 4 (a). A conductor 130 'is formed by copper electrolytic plating.
As shown in FIG. 4B, unnecessary copper thin film and chromium thin film other than the conductor circuit pattern portion are removed by chemical etching.
The conductor circuit 13 'is formed as shown in FIG. During this chemical etching, the copper conductor 130 'is also slightly but inevitably etched. For this etching, it is preferable to use an alkaline etching. For the etching of the chromium thin film [14 'in FIG.
A potassium ferricyanide-based etchant, a potassium permanganate-based etchant, a sodium metasilicate-based etchant, or the like can be used.

Next, FIG.
5 (d), a nickel thin film 15 'is coated on the surface of the conductor circuit 13', and a photosensitive polyimide resin is used. A cover 16 'is formed on the pattern of the circuit 13' and a bonding pad hole 17 'is formed. Then, after the above-mentioned electroless nickel plating thin film exposed in the hole 17 'of the cover is peeled off, electrolytic nickel plating 18' and electrolytic gold plating 19 'are sequentially applied to form a nickel-gold laminate. E ′ is formed, and the manufacture of the suspension board with circuit according to the present invention is completed.

FIG. 6A is a plan view of a main part of the flexible circuit board with bumps to be bonded to the bonding pads of the suspension board with circuit, and FIG. 6B is a plan view of FIG. (B) A cross-sectional view is shown, a conductor circuit 22 is formed on an insulating support film 21, and a cover 23 is coated thereon.
2, a metal bump 25, for example, a nickel bump is formed by swelling growth of nickel by electrolytic plating, and a solder 26 is plated on the bump surface by electrolytic plating or immersion. The size of the metal bump 25 of this flexible circuit board is usually 200 μm to 300 μm in diameter D.
m and height h are set to 80 μm to 150 μm. With respect to the diameter D of the bump 25, the diameter of the hole 17 in the bonding pad of the cover 16 is 1.0D to 3D, preferably 1.2D to 1.8D. In addition, Sn-Pb-based solder is used for the solder 26, and its plating thickness is usually 50 μm to 100 μm. The above metal bump 25
Is shaped to have a leg (a portion to be embedded in the cover 16) and a disk-shaped or bowl-shaped head having a diameter larger than the diameter of the leg. It is also possible to make the shape substantially equal. The material is nickel,
Use of copper, aluminum, etc. is also possible.

FIG. 7 shows a procedure for bonding the bumps of the flexible circuit board with bumps to the bonding pads of the suspension board with circuit according to the present invention. As shown in FIG. The bumps 25 are aligned with the corresponding bonding pads E of the suspension board with circuit 1 and then, as shown in FIG. 26) in step a) is melted, and then the hot plate 3 is detached to solidify the molten solder, thereby completing the joining according to the present invention. In this case, the soldering temperature is set to about 350 ° C., and the gold plating layer 19 of the bonding pad is eroded by the molten solder, and Au is solidified in the solidified solder.
Intermetallic compounds such as Sn ₄ , AuSn ₂ , AuSn, Au ₂ Pb, and AuPb ₂ are deposited.

However, in the suspension board with circuit according to the present invention, the thickness of the gold plating layer 19 of the bonding pad E is 1.5 μm or more, preferably 2.5 μm.
As shown in FIG.
As shown in (b), a gold-rich structure a is secured between the bump 25 and the bonding pad base surface 180, and the intermetallic compound of gold and tin is solidified by the precipitation of these intermetallic compounds. Even if it weakens the metal structure, it is pushed toward the side indicated by b in FIG. Thus,
Since the gold-rich solidified structure a is tough and the bump front surface 250 facing the bonding pad base surface 180 is sufficiently flat, the shearing force acting on the bonding portion is limited to the gold-rich solidified layer a and the upper and lower portions thereof. It is sufficiently uniformly dispersed at the bonding interfaces a ′ and a ″, and the strength of the bonding portion is dramatically improved.
It can also be confirmed from comparison of the results of high humidity exposure test.

[0022]

[Embodiment 1] Thickness 2 previously processed into a predetermined shape
A solution of the photosensitive polyimide resin precursor was applied on a 5 μm stainless steel substrate, and then heated and dried at 120 ° C. for 2 minutes to form a film of the photosensitive polyimide resin precursor. Next, the film is irradiated with ultraviolet light through a mask at an exposure amount of 700 mJ / cm ² , heated at 160 ° C. for 3 minutes, and then developed to form a negative image.
C. to form a polyimide resin insulating layer having a thickness of 10 .mu.m. Next, chromium and copper were formed in a thickness of 500 Å and 1000 Å, respectively, by sputtering on the entire surface of the stainless steel substrate on which the insulating layer was formed. Next, after laminating a commercially available dry resist film on a copper thin film at 110 ° C. in accordance with a conventional method, exposure and development are performed at an exposure amount of 80 mJ / cm ² ,
A reverse pattern resist was formed for a predetermined conductor circuit pattern. Next, a light adhesive seal was applied to the back of the stainless steel substrate.
Then, copper sulfate electrolytic plating was performed on the exposed copper thin film to form a 10 μm-thick copper-plated conductive circuit pattern, and then the resist was removed. Next, a normal electroless plating is performed, and a nickel thin film having a thickness of 0.1 μm is coated on the entire surface (three surfaces) of the conductor circuit.
Formed on the exposed surface of the stainless steel substrate. Then, a polyimide resin cover having a thickness of 10 μm is formed on the conductive circuit pattern on the stainless steel substrate using the photosensitive polyimide resin precursor in the same manner as the pattern formation of the insulating layer, and a hole for a bonding pad is formed. Was provided.
Then, the electroless nickel-plated thin film and the electroless nickel-plated thin film other than the circuit conductor exposed on the bottom surface of the bonding pad hole were immersed in a nitric acid-based stripping solution at room temperature. Finally, a suspension pad with a circuit according to the present invention is formed by forming a bonding pad comprising a laminate of a 7.3 μm-thick electrolytic nickel plating layer and a 2.7 μm-thick electrolytic gold plating layer on the conductor at the bottom surface of the bonding pad hole. Was manufactured. The number of circuit conductors of this suspension board with circuit is four.

The bumps of a flexible circuit board with bumps having a bump diameter of 250 μm, a bump height of 100 μm, and a solder plating thickness of 22 μm on the surface of the bumps formed on a bonding pad of the suspension board with circuit are formed on a hot plate at a temperature of 350 ° C. And crimped.

Example 2 The thickness of the electrolytic gold plating layer of the bonding pad of the suspension board with circuit was 4.4 μm, and the thickness of the electrolytic nickel plating layer was 5.6 μm.
The same as Example 1 except for the above.

Comparative Example 1 The thickness of the electrolytic gold plating layer of the bonding pad of the suspension board with circuit was 0.3 μm, and the thickness of the electrolytic nickel plating layer was 9.7 μm.
The same as Example 1 except for the above.

Comparative Example 2 The thickness of the electrolytic gold plating layer of the bonding pad of the suspension board with circuit was 1.4 μm, and the thickness of the electrolytic nickel plating layer was 8.6 μm.
The same as Example 1 except for the above.

Each of the bump joints of the example product and the bump joint of the comparative product (the number of samples for each was 100
), A 90 ° peel test was performed to examine the presence or absence of peeling at the solder joint interface. Nothing peeled off at the solder joint interface, and all of the bumps remained on the suspension board with circuit. Peeled from the flexible circuit board. Next, each sample after the 90 ° peel test was divided into 4 groups of 25 units, and the first group was divided into four groups.
Initial shear strength of the solder joint of the group, shear strength of the solder joint after 100 cycles of the second group thermal shock test, shear strength of the solder joint after 300 cycles of the thermal shock test of the third group, and When the shear strength of the fourth group after the high temperature / high humidity exposure test (85% humidity, 85 ° C., 72 hours) was measured, it was as shown in Table 1 (one cycle of the thermal shock test was − 40 ° C. × 30 minutes-125 ° C. × 30 minutes, measured values are average values). In Table 1, 550 gf or more means
With a shearing force of 550 gf, the narrow part of the bump is sheared and broken,
This means that the solder joint interface did not break.

Table 1 Example 1 Example 2 Comparative Example 1 Comparative Example 2 Gold plating layer thickness μm 2.7 4.4 0.3 1.4 Initial shear strength gf 550 or more 550 or more 400 550 or more After 100 cycles of thermal shock Shear strength gf of 550 or more 550 or more 319 389 After 300 cycles of thermal shock 550 or more 550 or more 271 325 Shear strength of 325 After high temperature / high humidity exposure test 550 or more 550 or more 384 463 Shear strength gf

As is apparent from Table 1, the suspension board with circuit according to the present invention in which the thickness of the gold plating layer of the bonding pad is 1.5 μm or more, preferably 2.0 μm or more, is intended to prevent the original oxidation prevention. It is apparent that bump bonding can be performed with extremely excellent strength and stability as compared with a conventional product having a gold plating layer thickness of 0.3 to 1.0 μm.

[0030]

In the suspension board with circuit according to the present invention, the flexible circuit board with bumps can be soldered to the bonding pads with excellent bonding strength and reliability, and the reliability of the suspension board with thin circuit mounted on the MR head is improved. This greatly contributes to increasing the number of disks and increasing the capacity and reliability of personal computers by promoting the spread of suspension boards with thin circuits.

[Brief description of the drawings]

FIG. 1 is a drawing showing an example of a suspension board with circuit according to the present invention.

2A is a sectional view taken along line II in FIG. 1, and FIG. 2B is a sectional view taken along line RO in FIG.

FIG. 3 is a drawing showing a previous stage of a method of manufacturing a suspension board with circuit according to the present invention.

FIG. 4 is a view showing an intermediate stage of a method of manufacturing a suspension board with circuit according to the present invention.

FIG. 5 is a view showing a subsequent stage of the method of manufacturing the suspension board with circuit according to the present invention.

FIG. 6 is a view showing a flexible circuit board with bumps to be joined to a suspension board with circuit according to the present invention.

FIG. 7 is a view showing a circuit board joining method according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Suspension board with circuit 13 Conductor circuit E Bonding pad 18 Nickel plating layer 19 Gold plating layer 2 Flexible circuit board with bump 25 Bump 26 Solder plating layer

Claims (3)

[Claims] 1. The method of claim 1, wherein the solder plating bump on one circuit board is
It is a method of soldering to the gold plating bonding pad of the other circuit board by melting and solidification of the solder plating,
A method for joining circuit boards, wherein the thickness of the gold plating is 1.5 μm or more. 2. A suspension board with circuit, which is another circuit board joined by the joining method according to claim 1, wherein the thickness of a gold plating layer of a bonding pad is 1.5 μm or more. 3. The other circuit board joined by the joining method according to claim 1, wherein a bonding pad is formed by laminating a nickel plating layer and a gold plating layer having a thickness of 1.5 μm or more on the nickel plating layer. A suspension board with a circuit, characterized in that: