Abstract: In this paper, a kind of strippable adhesive is used to replace the special adhesive tape as the protective layer for gold plating and hot air leveling, and the process of using strippable adhesive is studied.

Key words: Peelable adhesive masking ratio

1、 Preface

Pin gold plating and hot air leveling are two important processes in PCB production, and some parts that do not need processing should be protected. In the past, special tape was often used as the protective layer for plating and hot air leveling. However, this method has the disadvantages of high cost, long production cycle (for mass production), and often residual glue affecting the surface appearance. At present, some enterprises along the coast have used peel adhesive instead of adhesive tape, which has the protective effect of special adhesive tape in gold plating and hot air leveling. The strippable adhesive also has the following advantages: the cost is greatly reduced, the operation is simple, there is no residue and stain, and it is convenient for batch production. The cost of each PCB has been placed in a very important position for waiting for employment. Everyone is concerned about reducing the cost, so the strippable adhesive must have a certain application prospect. Our factory has also carried out some exchanges on this.

2、 Some characteristics of strippable adhesive

Peelable adhesive is a kind of single component silk screen protective ink, with solid content of 100% and color of blue viscous liquid. It can be used as a protective layer to protect some non electroplating lines during electroplating and the process of soldering tin and lead. Peelable adhesive is used as a protective layer instead of adhesive tape. Because it is a temporary coating, it should be completely stripped in the end.

3、 Process flow

Process flow of using strippable adhesive:

4、 Key points of operation

To make peeling easier, clean the printed board before coating to remove grease and stains. Use special cleaning agent when cleaning.

As the strippable adhesive is a viscous liquid with a certain viscosity, it is generally coated by screen printing. During operation, use wire mesh below 18T and polyurethane scraper with 60 degree fillet to ensure that the strippable adhesive has a certain thickness to facilitate stripping.

Generally, no thinner is used during coating. In order to make the adhesive have a certain thickness, the attack angle of the scraper shall be 45-55 degrees during coating. In order to ensure 1% of the hole masking rate, the cutting speed is required to be slower than usual, otherwise it will not play a role in protection.

The coating shall achieve the following effects: the strippable glue on the board surface shall be uniform and have a certain thickness; The part of glue flowing into the hole is two-thirds to one-third of the hole depth. When printing the first side, remember not to flow into the other side to form a "rivet". Otherwise, it is difficult to peel.

The degree of drying and curing also has a great impact on peeling. Generally, the peeling adhesive is cured in a hot air circulation oven at 140 ℃ for 20-30 minutes. After curing, the peeling adhesive should have high internal stress and elasticity. Over curing and insufficient curing can not achieve the best peeling effect.

5、 Conclusion

Through many tests, we found that the strippable adhesive has stable performance and good adhesion with the solder mask. To solve the problem of difficult peeling, we changed the screen printing method, changed the screen with different mesh size, used the scraper with different hardness, and adjusted the curing time, which played a good role and improved the peeling effect.

During the gold plating of pins, the peelable adhesive can protect the holes and parts that do not need electroplating, and no gold plating solution is found to penetrate; It can also protect the gold-plated pin part in the process of hot air leveling. Peelable adhesive is not found to fall off under high temperature, and there is no tin lead sticking to the gold pin, so it can be seen that the strippable adhesive can withstand high temperature.

Compared with the special protective tape, the cost of peelable adhesive tape is greatly reduced, and it has good performance, so it is also widely used.