JPH0783172B2 - Wiring board - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a wiring board, and more particularly to a wiring board provided with electrodes suitable for soldering microelectrodes called micro soldering.

[Prior Art] An electrode for solder connection in a conventional wiring board has a metal 2 for adhesion (mainly chromium), a metal 3 for diffusion prevention (for example, Cu, Cu + Cr, Ni, etc.) on a wiring layer 1 as shown in FIG. Al, Rh)
Are sequentially laminated, and in some cases, an antioxidant layer such as Au is also laminated. Among these, the thickness of the electrode and the solder wettability are determined by the properties of the diffusion preventing metal.

For example, since Cu melts into the molten solder in about 1 to several μm in one soldering, a Cu layer having a thickness of at least this thickness is required. Soldering causes solder melting twice, and Cu melts in the solder accordingly. Therefore, the thickness of the diffusion preventing metal that is actually required is 3 to 4 times the thickness that melts out by one soldering, and in the case of Cu, 3 to 4 μm or more is required.

When such a thick metal layer is formed on the substrate, breakage of the substrate or cracking of the metal layer itself is likely to occur due to thermal stress due to the difference in thermal expansion coefficient. Further, when a substance that protects the electrodes is formed on the electrodes, defects due to the step between the substrate and the electrodes are likely to occur. For materials other than Cu, the required thickness is a fraction of Cu, but poor solder wettability often results in poor connections. Also, Pd and Rh are very expensive.

To solve this problem, Cu, Ni, and Sn are sequentially formed on the wire in this order as in JP-A-57-235035 to improve the wettability of Ni.
There is an example of improvement by covering with Sn. As shown in this example, Au, S
It is known that the wettability is improved by forming a thin layer of n, solder or the like, but the number of steps is increased, and the material of such a thin layer is a fragile compound with the solder component metal. There is an example of making a connection, and there is a problem in connection reliability.

Further, in the application to a thin film circuit, the means for forming a Sn or solder layer is limited, and it is difficult to apply the vacuum evaporation method or the like. Therefore, conventionally, Cu is often formed thick as a diffusion preventing metal.

[Problems to be solved by the invention]

The above prior art does not consider application to a fine circuit,
Since only the electrodes to be soldered need to be formed very thick, this is one of the obstacles to soldering to fine circuits. In addition, the manufacturing cost is correspondingly high.

An object of the present invention is to provide a material for an electrode that can be soldered well, but can be thin as an electrode, thereby enabling soldering to a fine circuit and further reducing manufacturing cost. To do.

[Means for solving problems]

In addition to good wettability to molten solder, the above-mentioned object is to make a metal with a slow diffusion rate of solder component metal (for example, Sn, Pd, In, etc.) contact the entire electrode or even the electrode, and stop the diffusion of the solder. It is achieved by using it for a desired portion (a part of the electrode). The main component metal of the solder,
Table 1 shows the diffusion coefficient with general electrode constituent metals and the activation energy required for diffusion.

Al, Cu, and Ni are most used as the material of the electrode in view of the physical properties such as the values in Table 1, the properties of the oxide layer, and the price. However, these materials have various problems. For example, Al forms a strong oxide layer instantaneously, and thus often causes poor solder wetting. Further, Cu is extremely weak against oxidation, requires careful attention to the heat process, and has a high solder diffusion rate, so that it is necessary to form the electrode thick. Ni is solder diffusion Although the speed is low, the solder wettability is low and the soldering conditions are often limited. Therefore, it is necessary to select the material of the terminal that most suits the conditions depending on the target.
At present, no material has been found that satisfies the wettability and the diffusion prevention property with a single metal. Therefore, as a result of studying the use of an alloy as an electrode material, it was confirmed that no brittle mesophase (intermetallic compound) was generated and that the main component metal (S
n, Pb, In, etc.) does not form brittle compounds,
The Ni-Cu alloy of the present invention has been found as an alloy satisfying the requirements of good wettability with respect to solder, excellent corrosion resistance, and easy formation of a shape as an electrode.

This alloy is generally known by the names such as Constantan,
As shown in the state diagram of FIG. 1, it is a solid solution alloy. In addition, it has superior acid resistance and higher strength than Ni, so it is used in large amounts in various chemical industries. It may also be used as a current adjusting resistance material. Therefore, there is no problem in terms of material supply and price.

[Action]

Corrosion resistance is improved by using an alloy of Ni and Cu.
The progress of oxidation is very slow even in the atmosphere of about 350 ° C., and there is no particular problem in a normal manufacturing process or a soldering process.

This advantage is mainly due to the fact that a thin film made of Ni oxide covers the surface, but a thin film made of Ni oxide can be removed by cleaning with dilute sulfuric acid of several% concentration at the time of solder connection. A clean surface is easily obtained by removing. In addition, the wettability with respect to the solder is very good, and if the soldering flux is used, the solder will be instantly wet without any problem.

Therefore, it was clarified that it has solder wettability comparable to that of Cu electrodes and has corrosion resistance equivalent to that of Ni. Furthermore, the results of examining the diffusion rate of the solder are shown in FIG. 2 and FIG.
Shown in the table. This experiment is a result of evaluating how long the solder passes through a metal film having a predetermined thickness. When 63Sn / 37Pb solder, which has a high diffusion rate, is used and is connected at, for example, 250 ° C., it is clear that the diffusion rate of the solder is about 1/100 that of Cu.

Therefore, in the case of soldering under the above conditions, an electrode of Ni-Cu alloy may be an electrode having a thickness of one tenth of that of Cu. Actually, it is better to set it to about 1/10 or less, considering the margin.

[Examples] Examples of the present invention will be described below with reference to the drawings.

Example 1 Various methods can be considered for actually forming a Ni—Cu alloy as an electrode. Below, these methods and characteristics are described.

Hot dipping The substrate is dipped in a molten Ni-Cu alloy. It is difficult to apply in terms of thickness control and heat resistance of the substrate.

Thermal spraying Molten Ni-Cu is sprayed onto the substrate. There are large restrictions on the heat resistance of the substrate and restrictions on the thickness of the Ni-Cu film.

Vapor plating Vapor deposition and sputtering are typical methods, which are advantageous because a Ni-Cu film can be uniformly formed on the substrate and the substrate temperature can be low.

As a result of the above consideration, vapor-phase plating is the most promising, so Ni-Cu electrodes were formed by this method.
Above all, sputtering is effective as a method for uniformly forming a thin film. However, alloy sputtering
It is generally said that the composition varies greatly.

As a result of considering the cause of the large fluctuation, it is thought that the alloy film can be stably formed if the target used for sputtering is sufficiently cooled, and based on this, the Ni-Cu alloy target was manufactured and the substrate by the magnetron method was used. A film was formed on top. The discharge characteristics and composition analysis results at that time are shown in FIG. 3 and Table 3. From the results of FIG. 3 and Table 3,
It was revealed that the composition of the target was reproduced as it was in the Ni-Cu thin film on the substrate, and it was proved that the Ni-Cu alloy film can be formed by this method.

The Ni-Cu alloy thin film obtained in this example has a specific resistance
It is about 40 to 50 μΩ · cm, and this value is equivalent to the intermetallic compound of Sn and Cu (generated during solder connection at the Cu electrode), so the resistance increase at the connection part is also the same as before. Conceivable.

Example 2 First, as shown in FIG. 4 (1), a metal film 1 to be wiring such as Al and Cu is formed on a substrate 5 by vacuum deposition, plating, foil sticking, etc., and shown in Example 1. As shown in Fig. 4 (2), the Ni-
The Cu alloy film 3 is formed to a thickness of about 0.01 μm to several μm. At this time,
It is necessary to take care so that an oxide layer is not formed on the surface of the metal film 1 to be the wiring. Such an oxide layer sometimes deteriorates the connection strength for soldering and adversely affects the electrical connection.

After the film forming process is completed, a desired wiring pattern is formed by using a method such as photoetching as shown in FIG.
At this time, an iodine-based solution or a cupric chloride-based solution can be used as the etching solution for the Ni-Cu alloy and can be easily removed at room temperature. Furthermore, when the wiring layer 1 is Cu, the two layers are simultaneously etched by these liquids, so that the process is greatly shortened. In this case, Cu, which has almost no oxidation resistance, is replaced by Ni.
-The structure is protected by Cu alloy, which is also advantageous in terms of reliability.

To ensure the wetting of the Ni-Cu surface to the solder,
Au6 may be electrolessly soldered at this stage as shown in FIG. 4 (4). On the surface of the Ni-Cu alloy, an Au coating film can be formed without any problem by using a commercially available gold plating solution.

The electrode thus formed was subjected to solder connection and the connection strength was examined.As a result, the composition of the electrode was 63Ni / 40Cu and 63Sn /
When using 37Pb solder, an average breaking strength of 5.2 kg / mm ² was obtained. Furthermore, all fractures occurred in the solder part, and a value close to the ideal connection strength was obtained. Compared with the case where this value is soldered to the Cu electrode, the breaking strength is 1.3 times or more. Therefore, the Ni-Cu alloy electrode needs only a small electrode thickness,
In addition to achieving the intended purpose of getting well with solder,
It has been revealed that it has an excellent property that the connection strength is also high.

[Advantages of the Invention] According to the present invention, the thickness of the electrode to be soldered can be 1/10 to 1/10 of the conventional thickness, which facilitates soldering to a fine circuit and also reduces manufacturing cost. It can be reduced. Further, both the connection strength and the solder wettability are higher than those of conventional electrode materials, and the reliability of the solder connection portion is improved.

Such good characteristics are from 80Ni / 20Cu (at%) to 20Ni / 80
This is remarkable in the range of Cu (at%). In this range, the more Cu, the better the solder wettability, and the more Ni, the slower the solder diffusion. In addition to this, considering the connection strength, 70Ni / 3
The range of 0Cu (at%) to 40Ni / 60Cu has the best properties.

[Brief description of drawings]

FIG. 1 is a state diagram of a Ni—Cu alloy of the present invention, FIG. 2 is a diagram showing solder diffusibility in a solder diffusion preventing metal, and FIG. 3 is a condition for forming a Ni—Cu alloy film based on an embodiment. FIG. 4 and FIG. 4 are views showing a wiring board manufacturing process according to an embodiment, and FIG. 5 is a sectional view of a conventional soldering electrode. 1 ... Wiring layer, 2 ... Adhesive metal 3 ... Diffusion prevention metal 3 '... Diffusion prevention metal (Ni-Cu alloy) 4 ... Solder, 5 ... Substrate 6 ... Wettability improving layer, 7 ... Liquidus line 8 ... Solidus line

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tsuneaki Kamei, 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Inside the Hitachi, Ltd. Institute of Industrial Science (72) Inventor Mori Morita Totsuka-ku, Totsuka-ku, Yokohama-shi, Kanagawa ▼ 216, Machi Hitachi Co., Ltd. Totsuka Factory (56) Reference JP-A-55-122666 (JP, A)

Claims (2)

[Claims] 1. A wiring board having desired wiring, comprising:
Ni is the metal that prevents the diffusion of the electrodes to be soldered at least.
A wiring board which is an alloy composed of (nickel) and Cu (copper). 2. The alloy comprising Ni (nickel) and Cu (copper) in the composition range of 70Ni / 30Cu (at%) to 40Ni / 60Cu (at%). The wiring board according to item 1.