JPH0533187A - Method for controlling whisker in tinning - Google Patents

Abstract

PURPOSE:To control tin whiskers by a relatively low-cost means without introducing a metal other than tin as a plating film by preplating or alloy plating and to obtain a pure tin film having high reliability by tinning on a fine pattern. CONSTITUTION:When a fine Cu or Cu alloy pattern is tinned, it is first tinned in >=0.15mum thickness and all the resulting pure tin layer on the Cu base is converted into a Cu-Sn diffusion layer by heating. This diffusion layer is further tinned to form a pure tin layer of 0.15-0.8mum thickness.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for suppressing tin-plated whiskers that cause a short circuit when tin-plating a fine pattern of copper or copper alloy.

[0002]

2. Description of the Related Art In general, tin-plated coatings have a glossy appearance and are excellent in corrosion resistance and solderability, and are therefore widely used in electronic and electrical equipment. It is known that so-called genuine tin whiskers are generated in these tin-plated films, and particularly in fine patterns such as the inner lead portion of the film carrier having a small line width and gap width, the occurrence of whiskers causes a short circuit. Various measures have been taken conventionally.

Examples of means for suppressing tin whiskers are Cu, Ni, Pb, Sn-Pb, Sn-Ni, S.
Substrate plating such as n-Cu, reflow treatment after plating, annealing treatment after plating, tin plating is replaced with other metal or alloy plating, tin plating is replaced with solder plating containing lead of several percent or more, etc. Are known.

However, in the method (1), since an undercoating process is added, the process becomes longer and the cost increases. If you do not thicken the plating first, the plating thickness of pure tin will decrease due to copper-tin diffusion, and even if you uniformly plating first, there will be uneven plating thickness after reflow. However, there is a problem that the tin plating film surface is oxidized. Although the above method can prevent whiskers from being generated for a short period of time, it cannot be a complete measure to prevent whiskers from being generated in the long term of about 2 to 3 months. As a method of and, gold plating and solder plating may be performed, but gold plating is expensive, and in the case of a film carrier, the bonding temperature and pressure with IC must be set higher than that of tin plating. However, there is a problem that it is difficult to control the composition and thickness of the plating film.

An object of the present invention is to provide a method for suppressing tin plating whiskers which is inexpensive and highly reliable, without using a metal other than tin as a plating film unlike the undercoating or alloy plating method. ..

[0006]

According to the present invention, when tin plating is applied on a fine pattern of copper or copper alloy, first, tin plating having a thickness of 0.15 μm or more is applied, and then heat treatment is performed to form the pure tin layer. The above problems are solved by forming a Cu-Sn diffusion layer with a copper base material, plating tin on the Cu-Sn diffusion layer, and setting the pure tin plating thickness to 0.15 to 0.8 [mu] m.

The present invention will be described below in the order of steps with reference to the drawings. FIG. 1 is a process explanatory view of the present invention. In FIG. 1, (a) shows a copper base film carrier in which a copper layer 2 is formed on a polyimide layer 1, and (b) shows this film carrier. The state which formed the tin plating layer 3 by the plating process is shown. The tin plating layer 3 has a thickness of 0.15 μm or more. If the thickness is less than 0.15 μm, the whisker suppressing effect becomes insufficient. Then 7
Heat treatment is performed at 0 ° C. to 200 ° C. for 5 minutes to 1 hour, and all the tin plating layers 3 are changed to copper-tin diffusion layers (alloy layers) 4 (c). Thereafter, as shown in (d), a pure tin layer 5 is further applied on the tin diffusion layer 4, and the thickness of the pure tin layer 5 on the surface of the plating film is 0.15 to 0.8 μm (the plating thickness is Measured with a Coulter film thickness meter). Plating thickness is 0.15μ
When the thickness is 0.8 m or more, bonding becomes difficult, and when it is 0.8 μm or more, plating sagging occurs, which causes a short circuit.

Whether or not the plating according to the present invention can be distinguished from the ordinary plating, that is, the tin-plated film obtained by annealing the tin plating, is determined by x-ray diffraction and A
When examined by ES analysis or the like, the value of the x-ray diffraction intensity ratio of Cu ₃ Sn and Cu ₆ Sn ₅ is 1: in the plating according to the present invention.
While it is 0.3 to 0.5, it is 1: in normal plating.
It was 1 to 1.5. Further, while the area around the plated surface according to the present invention is a pure tin layer, copper is diffused up to the area near the surface medicine in normal plating (AES analysis), and it was found that both plated films can be clearly distinguished.

In the present invention, it is considered that the tin whiskers are not generated from the plating film for the following reason. That is, one of the driving forces for the growth of tin whiskers
One is said to be the growth of a copper-tin diffusion layer (alloy layer) (internal stress of the plating film increases). However, in the present invention, since the copper-tin diffusion layer is positively formed, even if a retin-plated layer is attached thereon, further copper-tin diffusion does not proceed or it is difficult to proceed, so tin whiskers grow. I think it's getting harder. It is also considered that tin whiskers are less likely to sprout due to the surface-regulating effect of the surface of the underplating due to re-tinning, and tin whiskers are less likely to occur.

In the present invention, tin plating may be formed by either electrolytic plating or electroless plating.
It is preferable to use electroless plating because there is little variation in plating thickness. Further, the copper-tin diffusion layer may be formed by an annealing treatment or a reflow treatment.

[0011]

As described above, according to the present invention, it is possible to suppress tin whiskers by a relatively inexpensive means without introducing a metal other than tin as a plating film unlike the undercoating or alloy plating method. It is possible to obtain a plated film of only tin plating with high reliability in a fine pattern.

[0012]

[Example 1] Film carrier (IL number: 152)
As an electroless tin plating solution on top, TINPOSTjLT
-34 (manufactured by Shipley Far East Co., Ltd.) was used, and electroless tin plating was performed at 60 ° C. for 7 minutes.
After annealing at 130 ° C. to 200 ° C. for 1 to 2 hours, electroless tin plating was performed again at 60 ° C. for 1 minute to obtain a sample. These tin plating annealing conditions,
Table 1 shows the tin plating thickness (measured with a fluorescent x-ray film thickness meter, Cocool film thickness meter) and the like, and Table 2 shows the number of tin whiskers generated in the IL part (inner lead part) after 1 to 5 months. Here, for the tin plating thickness, a fluorescent x-ray film thickness meter is used, and in this case, Sn in the Cu—Sn diffusion layer falls within the measured value. Further, the pure tin plating thickness was measured using a Cocool film thickness meter, but in this case, Sn in the Cu—Sn diffusion layer does not enter the measured value.

[0013]

[Table 1]

[0014]

[Table 2]

From the results shown in the above table, a plating film was produced by the annealing treatment after the base tin plating so that the pure tin layer was not left in the plating film (annealing temperature of 180 ° C. or higher), and the tin-plated sample was obtained. No occurrence of tin whiskers (5 months after plating). Further, it is understood that the thicker the Cu—Sn diffusion layer thickness after the second tin plating is, the more the tendency of suppressing the generation of tin whiskers is. On the other hand, No. It can be seen that when the pure tin layer remains after the annealing treatment as in the samples 1 to 5, whiskers are generated after the subsequent retinning.

[0016]

[Example 2] Film carrier (IL number: 152)
As an electroless tin plating solution on top, TINPOSTjLT
Using -34 (manufactured by Shipley Far East Co., Ltd.), electroless tin plating as a base material was performed at 60 ° C. for 0.5 to 7 minutes. After an annealing treatment at 110 ° C. to 180 ° C. for 1 hour, electroless tin plating was performed again at 60 ° C. for 1 minute to obtain a sample. Table 3 shows the annealing conditions for these tin platings, the tin plating thickness (measured by a fluorescent x-ray film thickness meter, a Cocool film thickness meter), etc. Show.

[0017]

[Table 3]

[0018]

[Table 4]

As a result, even if the thickness of the base tin plating is thin, an annealing treatment is performed to form a plating film that does not leave a pure tin layer in the plating film, and if tin plating is performed again, tin can be used even 90 days after plating. No outbreak of whiskers was seen. On the other hand, No. It can be seen that when the pure tin layer remains after the annealing treatment as in the sample of No. 8, whiskers are generated after the subsequent retinning.

[Brief description of drawings]

FIG. 1 is an explanatory view showing a plating process according to the present invention.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location C25D 7/00 G 6919-4K

Claims (1)

Claim: What is claimed is: 1. When tin plating is applied to a fine pattern of copper or copper alloy, first, tin plating having a thickness of 0.15 μm or more is applied, and then heat treatment is performed to completely convert the pure tin layer to a copper base material. And a tin-plated tin-plated layer having a pure tin plating thickness of 0.15 to 0.8 μm to suppress tin-plated whiskers.