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Abstract: This article uses a peelable adhesive instead of a special tape as the protective layer for gold plating and hot air leveling, and studies the application of the peelable adhesive process in PCB production.
I. Introduction
Pin gold plating and hot air leveling are two important processes in the production of printed circuit boards, and some parts that do not need to be processed should be protected. In the past, special adhesive tapes were commonly used as the protective layer during plating and hot air leveling, but this method has disadvantages such as high cost, long production cycle (for mass production), and residual glue residue that affects the appearance of the board surface.
At present, some PCB companies along the coast have used peelable adhesive instead of tape, which has the protective effect of special tape in gold plating and hot air leveling. And the peelable glue also has the following advantages: the cost is greatly reduced, the operation is simple, there is no residual traces and stains, and it is convenient for mass production. For the printed circuit board industry, the cost has been placed in a very important position, and reducing costs is everyone's concern. Therefore, peelable adhesives must have certain application prospects.
2. Some characteristics of peelable glue
peelable glue is a one-component screen printing protective ink with a solid content of 100% and a blue viscous liquid. It can be used as a protective layer during electroplating to protect parts of non-plated circuits and solder lead. The role of peelable glue is to replace tape as a protective layer. Because it is a temporary coating, it must be completely peeled off at the end.
Three, operation points
In order to make peeling easier, the printed circuit board should be cleaned before coating to remove grease and stains. Use special cleaning agent when cleaning. Since the peelable glue is a viscous liquid with a certain viscosity, its coating is generally by the method of screen printing. Use wire mesh below 18T and a 60-degree rounded polyurethane squeegee to ensure that the peelable glue has a certain thickness in order to facilitate the peeling. Generally, no thinner is used during coating. In order to make the glue have a certain thickness, it is required that the angle of attack of the doctor blade be between 45-55 degrees during coating. In order to ensure a one-percent masking rate, the cutting speed is required to be slower than usual, otherwise it will not be able to protect it.
The following effects should be achieved during the coating: the peelable glue on the board surface is even, and there must be a certain thickness; the part of the glue flowing into the hole is two-thirds to one-third of the depth of the hole, remember not when printing the first side Flow into the other side to form a "rivet". Otherwise, it will be difficult to peel off.
The degree of drying and curing also has a great influence on peeling. Generally, it is cured in a hot air circulating oven at 140°C for 20-30 minutes. After curing, the peelable adhesive should have high internal stress and elasticity. Neither over-curing nor under-curing can achieve the best peeling effect.
Four. Conclusion
Through many experiments, it is found that the performance of the peelable adhesive is very stable, and the adhesion to the solder mask is good. In response to the problem of difficult peeling, we changed the PCB screen printing method, changed screens of different meshes, used different hardness squeegees, and adjusted the curing time, which played a very good role and improved the peeling effect.
During the process of gold-plating the pins, the protective effect of the peelable glue on the holes and the parts that do not need to be electroplated is good, and there is no penetration of the gold plating solution; it can also protect the gold-plated pins during the hot air leveling process of the PCB, and no peeling is found at high temperatures. The glue falls off, and there is no tin and lead sticking to the gold pins, which shows that the peelable glue can withstand high temperatures. Compared with the special protective tape, the peelable adhesive has a greatly reduced cost and its own good performance, so its application is also quite extensive.
