JPH02191394A - Structure of soldering pad part of printed wiring board - Google Patents

Abstract

PURPOSE:To obtain satisfactory solder junction strength and long-term reliability by making a pad part have a twolayer structure and forming lower and upper layer parts by using respective specific silver pastes. CONSTITUTION:A pad part has a two-layer structure that is formed by a lower layer part 1 which uses a silver paste containing a silver powder 75-92wt.% and a thermo-setting resin 8-25wt.% as solid substances and is formed by an upper layer part 2 using the silver paste which contains a silver powder 91-99wt.% and a binder resin 1-9wt.% consisting of thermosetting and thermoplastic resins as the solid substances; besides, in such a case, the weight ratios between the above two resins are within ranges 10:90-80:20. Since the thermoplastic resin is combined with the binder resin in the upper layer part, a resin thin film is softened easily to flow by heat produced on soldering and the exposed surface of silver is allowed to secure the wettability of solder fully. Then solder junction strength and long-term reliability are obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to the structure of a pad portion for soldering a printed wiring board having a thick film circuit formed using a resin-type conductive paste.

[Prior art] In recent years, resin-type conductive pastes, in which conductive fillers are mixed and dispersed in resin, have been in demand for various electrodes on printed wiring boards and conductor circuits in response to the diversifying needs of the electronics industry. is growing. Among these, silver paste has 1) excellent conductivity and 2) excellent long-term stability. 3) Since it has advantages such as ease of handling, it is the mainstream of resin-type conductive paste.

A conductive circuit made of silver paste is generally formed by a screen printing method, and has a low specific volume resistivity of about 2×10″Ω·cm.

Further, as a binder resin for the silver paste, a thermosetting resin having excellent heat resistance is usually used.

[Problem to be solved by the invention] However, most of the surface of the conventional conductor circuit using silver paste as described above is covered with a thin film of thermosetting binder resin, and the probability of silver being exposed on the surface is low. There was a problem in that the solder did not adhere to the substrate due to its low resistance, and it could not be put into practical use as a printed wiring board as it was.

Therefore, attempts have been made to increase the silver content in the silver paste to make soldering possible, but even if the silver content is increased, the silver surface will not be covered with a thin film of thermosetting resin. This could not be avoided, and the solder joint strength was ultimately insufficient. Furthermore, in order to reduce costs, when using an insulating substrate with low heat resistance (such as polyester), it is necessary to use low-temperature solder with a low melting temperature. It was extremely difficult to solder with low-temperature soldering, including those that increased the temperature.

Moreover, when the silver content is increased beyond a certain range in order to expose the silver surface, the specific volume resistivity increases by 10-'Ω・c
There was a problem in that it becomes high when it is about m or more, making it unsuitable for use in conductor circuits.

The present invention has been made in view of the above points, and provides a solder that can provide sufficient solder joint strength and long-term reliability in printed wiring boards having resin-type thick film circuits, even when low-temperature solder is used. The purpose of the attachment is to provide the structure of the pad portion.

[Means for Solving the Problems] In the present invention, the soldering of a printed wiring board formed by applying and curing silver paste has a structure of a pad part having at least two layers, and 75% silver powder as a solid content. ~92wt%F, thermosetting resin 8~25wt
A lower layer is formed using a silver paste containing
T! k, using a silver paste containing 1 to 9 wH of a binder resin consisting of a thermosetting resin and a thermoplastic resin, and in which the weight ratio of the thermosetting resin and the thermoplastic resin is in the range of 10:90 to 80:20. By forming the upper layer,
The above tasks have been achieved.

[Function] In this invention, the lower layer of the pad portion is connected to the conductor circuit (it goes without saying that the lower layer may be formed integrally with the conductor circuit), and the binder resin is heated. Since it is made of curable resin, it has excellent heat resistance and adhesion. In addition, since there is no need to ensure solder wettability in the lower layer, the ratio of silver and binder resin is set within a range that can ensure both low specific volume resistivity and sufficient adhesion with the lower insulating layer. .

On the other hand, the upper layer often uses thermoplastic resin, which has conventionally been hardly considered for use in solder joints because its heat resistance and solvent resistance are inferior to thermosetting resins. Preferred thermoplastic resins include vinyl acetate, vinyl acetal, cellulose, and acrylic resins.

As mentioned above, even if the silver content is increased, it cannot be avoided that the thermosetting binder resin thinly covers the silver surface, but in the present invention, a thermoplastic resin is blended into the upper layer binder resin. Therefore, the resin thin film easily softens and flows due to the heat during soldering, thereby exposing the silver surface and ensuring sufficient solder wettability. Furthermore, since the thermoplastic resin is easily dissolved in the flux used during soldering, removal of the resin thin film (exposure of the silver surface) is facilitated.

However, here, in addition to the thermoplastic resin, a thermosetting resin is used, thermosetting resin/thermoplastic resin = 10/90 to 80/
The reason for blending at a ratio of 20 (weight ratio) is because if only thermoplastic resin is used as the binder resin for the upper layer, micro cracks and voids are likely to occur between the upper and lower layers during soldering. By adding a thermosetting resin, the adhesion with the lower layer is improved. Although these microcracks and voids are not a problem in the short term, they may develop into cracks in the long term, and it is necessary to prevent their occurrence. Preferred thermosetting resins include:
There are phenolic resins, urethane resins, and epoxy resins.

In the present invention, the binder resin in the upper layer is composed of a thermosetting resin and a thermoplastic resin in the above-mentioned specific weight ratio, thereby preventing microcracks that affect solder wettability, solder joint strength, and long-term reliability.

All problems of preventing voided occurrences have been solved. If the weight ratio of the thermosetting resin and the thermoplastic resin is outside the above-mentioned range, both solder wettability and long-term reliability cannot be satisfied.

Further, when the upper layer is composed of two or more layers, the weight ratio of the thermosetting resin and the thermoplastic resin for the binder resin may be the same for each layer or may be varied for each layer. However, from the viewpoint of solder wettability, it is preferable to increase the weight ratio of the thermoplastic resin in layers closer to the surface as the printing progresses.

[Example] Example 1 As shown in the cross-sectional view of Fig. 1, a silver paste containing phenol resin (thermosetting) lOwt% and silver powder 90wt% as solid content was placed on the insulating layer 4 made of polyester film. The lower layer part 1 was formed by a screen printing method. Then, on top of this lower layer 1, a solid content of epoxy resin (thermosetting)/cellulose resin (thermoplastic) = 10/90 (weight ratio) of binder resin containing 5 parts of binder resin and 95 parts of silver powder is contained. The upper layer 2 was formed by screen printing using a silver paste. Further, the surface other than the surface of the upper layer portion 2, which is the solder joint surface, was covered with an epoxy resin resist layer 3.

Low temperature solder (Bf type) is applied to the pad portion formed in this way.
and melted the solder at 140℃ in a reflow oven.
Solder wettability was visually observed. Thereafter, tin-plated wire was soldered to the pad portion, and the solder joint strength of the pad portion was measured using a tensile tester.

In addition, the pad portion was embedded in a resin and cured, then cut and polished, and its cross section was observed using a microscope.

The polyester film used for the insulating layer in this example has low heat resistance, and the insulating layer itself deteriorates at the melting temperature of Sn/Pb-60/40 solder (200°C).
Tested using only low temperature solder.

Example 2 Epoxy resin (thermosetting) was used as the binder resin in the lower layer, and the solid content in the upper layer was urethane resin (thermosetting)/acrylic resin (thermoplastic) = 50150.
(Weight ratio) Binder resin consists of 6 parts 0, silver powder 9 parts
A pad portion was formed on a polyester film in the same manner as in Example 1 except that it was formed using a silver paste containing 4 wt zero, and the solder wettability, bonding strength, and cross-sectional state were examined.

Example 3 In this example, the upper layer was composed of two layers as shown in FIG. First, an insulating layer 4 made of polyimide film is
As solid content on top, epoxy resin (thermosetting) 10+
yt! Then, a silver paste containing 90 wt% of silver powder was used to form the lower layer 1, and the solid content was epoxy resin (thermosetting)/polyvinyl butyral resin (thermoplastic) = 40/60 (weight ratio). The upper first layer 2a was formed using a silver paste containing 5 binder resin contents and 95 silver powder + VT96. Further, a second upper layer 2b was formed using the same silver paste used to form the first layer 2a, and the area other than the solder joint surface was protected with an epoxy resin resist layer 3 as in Example 1. .

The pad portion formed in this way is soldered with low-temperature solder (
melting temperature 140°C) and Sn/Pb-60/40 solder (
The solder wettability, bonding strength, and cross-sectional state were examined in the same manner as in the example mold using each of the molded specimens with a melting temperature of 200° C.).

Example 4 A polyester film was used for the insulating layer, and the lower layer was formed using a silver paste containing 15 wt. zero of epoxy resin (thermosetting) and 85 wt. of silver powder as solid contents. In addition, the upper layer contains epoxy resin (thermosetting)/
Polyvinyl acetate (thermoplastic) = 30770 (weight ratio) binder resin content 3wt%i,! l powder 97wtX
The first silver paste was prepared in the same manner as in Example 3 using a silver paste containing
layer, a second layer was formed. Thereafter, in the same manner as in Example 1, solder wettability, bonding strength, and cross-sectional condition were examined using low-temperature solder.

Example 5 A polyester film was used for the insulating layer, and the lower layer was formed using a silver paste containing epoxy resin (thermosetting) 15wt zero and silver powder 85+vt zero as solid content.

In addition, the upper layer is made of epoxy resin (thermosetting) as a solid content.
/Polyvinyl butyral (thermoplastic) = [i0/40
(weight ratio) of binder resin: 8 t * silver powder 92 w
The first layer and the second layer were formed in the same manner as in Example 3 using a silver paste containing t96. Thereafter, in the same manner as in Example 1, solder wettability, bonding strength, and cross-sectional state were examined using low-temperature solder.

Comparative example 1 Epoxy resin (thermosetting) 5wt with solid content in upper layer
%; The butt part was formed in the same manner as in Example 2, except that it was formed using a silver paste containing 95wt of silver powder, and the solder wettability, joint strength, and cross-sectional condition were adjusted using low-temperature solder. Ta.

Comparative Example 2 A pad part was formed in the same manner as Comparative Example 1, except that the upper layer part was formed using a silver base whose binder resin was made only of polyvinyl butyral resin (thermoplastic), and soldered using low-temperature solder. Wettability, bond strength and cross-sectional condition were investigated.

Comparative Example 3 The lower layer was made of epoxy resin (15wt$) as a solid content, the stomach was made of silver paste containing 85% of silver powder, and the upper layer was made of epoxy resin (thermosetting)/polyvinyl acetate (thermosetting) as a solid content. The lower and upper first and second layers were formed in the same manner as in Example 3, except that a silver paste containing a binder resin content of 3 wH and silver powder of 97 wt% F with a weight ratio of 90/10 (plasticity) was used. , low temperature solder and Sn/P
Using each of b-60/40 solders, solder wettability, bonding strength, and cross-sectional condition were investigated.

Table 1 shows the pad formation conditions for the above examples and comparative examples.
The test results for solder wettability, bonding strength, and cross-sectional condition are summarized in Table 2.

As shown in Table 2, in Examples 1 to 5, good solder wettability and cross-sectional condition were obtained, and the bonding strength was as high as 1.0 kg/mm2 or more when low-temperature solder was used. value was obtained. In particular, thermosetting resins/thermoplastic resins
Examples 3 and 4.5, which have a two-layer structure in which the upper layer part uses a binder resin of 10/90 to 80/20 (weight ratio), exhibit high bonding strength.

On the other hand, in Comparative Example 1 in which a thermosetting resin was used as the binder resin in the upper layer, when low-temperature solder was used, the resin thin film repelled the solder and the solder did not adhere to the zero-order upper layer. Comparative Example 2, in which only a thermoplastic resin was used as the binder resin, had good solder wettability and bonding strength even at low temperature soldering, but microcracks and voids were observed in the cross section. In addition, in Comparative Example 3 of 90710, in which the thermosetting resin/thermoplastic weight ratio of the upper layer binder resin was outside the range of the present invention, low temperature solder, Sn/Pb・6
In all cases where 0/40 solder was used, the cross-sectional condition was good, but the solder wettability and bonding strength were poor.

From the above results, it is not possible to solder using low-temperature soldering by simply layering a silver paste with a high silver content using a thermosetting resin as a binder, and the upper layer binder resin is thermoset. Low-temperature solder and Sn/
It has become clear that the solder wettability, joint strength, and cross-sectional condition of Pb-60/40 solder are all improved.

In the above embodiment, the pad portion was formed on the insulating film, but the base material on which the pad portion according to the present invention is formed is not particularly limited, and the present invention is applicable not only to flexible printed wiring boards. Needless to say, it is also applicable to rigid printed wiring boards. In addition, in the present invention, the entire circuit of the printed wiring board does not need to be formed with silver paste, and a thick film circuit using a resin-type conductive paste may be formed partially, or a circuit may be formed using a subtract method. Soldering is also applied when a conductor circuit including a pad portion is added later to a printed wiring board.

[Effects of the Invention] In the present invention, the soldering pad part has a structure in which a thermosetting resin and a thermoplastic resin are blended in a specific weight ratio on a lower layer part in which a thermosetting resin is used as a binder resin. Since the upper layer is made of binder resin and is laminated, the thin resin film covering the silver surface is easily removed by flux and heat during soldering. Therefore, the silver surface is exposed during soldering, resulting in very good solder wettability.
Bonding strength is also significantly improved.

In addition, since the upper layer also contains thermosetting resin at a specific weight ratio, it has excellent adhesion between the upper and lower layers, and soldering can sometimes cause micro-cracks between the upper and lower layers. There is no need to worry about the occurrence of voids or voids, and high long-term reliability can be ensured.

According to this structure of the pad part, good soldering is achieved even when low-temperature soldering is used. Therefore, from a cost perspective, a circuit is formed using a conductive paste using a material such as polyester with low heat resistance as an insulating substrate. It is extremely effective in reducing the effects of heat on mounted electronic components.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing an embodiment of the invention, and FIG. 2 is a sectional view showing another embodiment of the invention. [Description of symbols of main parts] 1...Lower layer part 2...Upper layer part 2a...Upper layer first layer 2b...Upper layer second layer 3...Resist layer 4...Insulating layer Agent Patent Attorney Masatoshi Sato

Claims (2)

[Claims] (1) In the structure of a soldering pad portion of a printed wiring board formed by applying a silver paste in which silver powder is mixed and dispersed in a binder resin in a predetermined shape and curing, the pad portion has at least a two-layer structure. , solid content: silver powder 75-92 wt%, thermosetting resin 8-25 wt%
A lower layer is formed using a silver paste containing silver powder, and on the lower layer, 91 to 99 wt% of silver powder is added as a solid content.
, using a silver paste containing 1 to 9 wt% of a binder resin consisting of a thermosetting resin and a thermoplastic resin, and having a weight ratio of the thermosetting resin to the thermoplastic resin in the range of 10:90 to 80:20. A structure of a soldering pad portion of a printed wiring board, characterized in that an upper layer portion is formed. (2) The structure of a soldering pad portion of a printed wiring board according to claim 1, wherein the upper layer portion consists of a plurality of layers.