How to analyze and improve the gold surface of chemical nickel in the gold process

How does chemical nickel work in the gold process to deal with gold surface color analysis? How did he improve these problems? Take a look at some of the introductions below! Let you know more about the meaning of the existence of chemical nickel! Let's learn together!

Chemical nickel gold is one of the surface treatments of PCB. If the copper surface is abnormal, the quality of gold plating will be abnormal, such as leakage plating, bismuth nickel gold, and gold surface discoloration. The next step will be to solve the problem of chemical nickel gold pretreatment. In this paper, the gold surface is colored thick and thin, and the heterochromatic problems are concentrated in the IC and BGA positions, as shown in Figure 1. This article will analyze the causes of this problem, and give improvement measures to finally solve such gold-faced problems.

Second, reason analysis

2.1 Principle of Shenjin Reaction

When the nickel plate is plated on the surface of the PCB and placed in the gold bath, the nickel surface is attacked by the bath to dissolve nickel ions. The two electrons thrown are obtained by gold cyanide ions and deposited on the nickel surface. The gold layer, the reaction mechanism is as follows:
Anodic reaction: Ni→ Ni2++2e- E0=0.25V
Cathodic reaction: Au(CN)2-+e-→Au+2CN- E0=0.6V
Total reaction formula Ni + Au(CN)2- → Ni2+ + Au + 2CN-
From the above reaction mechanism of the immersion gold, it can be concluded that the reaction is a typical displacement reaction. The potential of the total reaction is -0.35V, and the contact between Ni and Au+ can be spontaneously carried out. Theoretically, after the nickel surface is completely covered with Au, The precipitation of gold stops. In fact, due to the large number of pores on the surface of the gold layer, the nickel under the porous metal can still dissolve and eject electrons, and the gold continues to precipitate on the nickel, but the rate will be lower and lower until it is terminated.

2.2 Analysis of Causes of Gold Faces

2.2.1 Analysis of Nickel Thick Slices of Normal IC and Abnormal IC

From the above measurement of nickel thickness, there is no significant difference between the IC color difference and the normal IC nickel thickness, which indicates that the cause of IC color is not caused by immersion gold plating (nickel thickness is thin), that is, such IC exposure Nickel is not related to nickel thickness.

2.2.2 Normal IC and heterochromatic IC gold-nickel thickness measurement

From the above abnormal IC and normal IC gold thickness test data, it is concluded that there is no significant difference between the abnormal IC and the normal IC nickel thickness, but the abnormal IC gold thickness is 0.82 micro-inch different from the normal IC gold thickness, so it can be judged that the IC color is different. It is caused by the color of nickel being too thin.

2.2.3 SEM and EDS analysis of normal IC and heterochromatic IC nickel layer

From the above nickel surface SEM and EDS analysis, there is no significant difference between the IC color difference and the normal IC site nickel surface crystal structure and P content, indicating that the cause of IC nickel exposure is not caused by abnormal nickel surface crystal.

2.2.4 Analyze the possible causes of gold-colored heterochromatic through the fishbone diagram, as shown in the following figure.

2.2.5 Reason Filtering

(1) On-site follow-up finds boards produced at the same time and under the same conditions. Some boards have different colors, and some boards have no color, so people, machines and environmental factors can be excluded.
(2) In the field follow-up, it was found that the plates with different colors occurred on the boards with BGA and IC, and the different color plates were mainly concentrated on the variegated inks, so the reason was locked in the materials and methods.

Three, experimental verification

3.1 Experimental Process

Incoming → horizontal sandblasting → upper plate & rarr; degreasing & rarr; micro etch & rarr; pre-dip & rarr; activation & rarr; post immersion & rarr; sulphide nickel & rarr; chemical gold & rarr; gold recycling & rarr; Board

3.2 Experimental parameters

3.3 Experimental Plan

Four, experimental verification results and analysis

Compared from the results of the five sets of experiments in Table 4:
(1) There is still a heterochromatic phenomenon in increasing the gold concentration of the immersion gold cylinder to 1.2g/L, indicating that the gold concentration is not the cause of the IC or BGA color difference;
(2) There is still a heterochromatic phenomenon when the activation time of the immersion gold is increased to 120S, indicating that the gold activation time is not the cause of the IC or BGA color difference;
(3) Compared with the immersion gold and the external immersion gold in the factory, there is a BGA or IC color difference phenomenon, indicating that the immersion gold syrup is not the main cause of the heterogeneous color of the gold plate;
(4) The solder mask process uses different inks to screen the gold and has a gold color difference, indicating that the ink is not the cause of the gold surface color;
(5) In the solder mask process, the plate treated with the aluminum plug hole has no gold surface discoloration phenomenon after the immersion gold, and the gold surface discoloration phenomenon occurs after the immersion gold of the aluminum plate plug hole is used, indicating that the solder resist aluminum The chip plug hole greatly improves the BGA and IC gold surface color difference; the aluminum plate plug hole and the non-aluminum chip plug hole hole slice comparison are shown in Fig. 3 and Fig. 4.

V. Conclusions

The problem of the different colors of BGA or IC gold surface of Shenjin board has been plagued by our company for a long time. We have also tried many times in the production process, but they have not completely eliminated the purpose, and then passed the reaction principle of Shen Jin. The abnormal plate was carefully analyzed, and through the comparison of the test plate and the mass production verification, it was finally found that the BGA or IC gold surface color can be completely solved by the resistance welding aluminum plate plug hole.