Test Coupons as an Aid to Process Control of the PCB Manufacturing and Assembly Processes

A high percentage of circuit boards manufactured in the electronics industry are of an irregular shape and are produced on a standard panel outline to facilitate assembly handling. The unused pieces of circuit board pass through the same processes as the useful parts and are normally discarded. This excess material could, among other things, be used to evaluate the quality of a bare board or an assembly. This paper will highlight the usefulness of designing test patterns on this excess material, namely test coupons, in terms of how these can be used to monitor all of the manufacturing and assembly process steps. It will also show how these coupons can be used to make the board easier to assemble and how they may actually lead to an improvement in the quality of the assembly and an increase in production yields. Suggestions will be made as to the types of test pattern that can be used, as well as how these patterns can be utilised as process control checkers. Since the test coupons are incorporated in the board design, a quality check of 100% of the boards that are being processed is possible. This would highlight any board‐to‐board variation if it were present. It would also allow for destructive testing to be carried out, without damaging any of the working product. The applications of these patterns are wide ranging. They can be used to check bare board quality—etch definition, layer registration, plating quality, solder mask definition etc. They can also be used to monitor the assembly processes for SMT and conventional PTH assembly types—cut and clinch quality, paste printing quality, onsertion accuracy, reflow/flow soldering quality and assembly cleanliness, among others. Many of these applications are examined in this paper.