Closed‐loop Soldering

This paper discusses some basic ideas about process development and control in Part I and applies them to soldering in Part II. Because it is possible to understand how design, materials and process affect the product, it is unnecessary and inappropriate to resort to the statistical‐correlation methods that are applied to complex processes. A process qualifies for the label ‘closed loop’ only if the design and materials going into.it are controlled. The types, degree and sophistication of control needed for a process are to be judged by consistency of the product. For soldered assemblies, the product is evaluated by visual inspection, and the adequacy of process development and control depends on the adequacy of inspection. Inspection can be improved if it is regarded as a process. It can also be improved if inspectors understand which features are important and which can be ignored safely, i.e., by understanding their causes and associated risks. Much of the criticism of visual inspection, and perception of need for automated inspection, derive from a failure to distinguish clearly enough between material and process variables, between the two types of inspection (product‐oriented and materials/process‐oriented) and between appearance and risk. Properly controlled visual inspection is well suited for evaluating the soldering process. The most important visual attribute to look for in solder inspection is the contour of the fillet, because this is what reveals the quality of wetting, and wetting is the most important physical attribute of the connection in determining its strength and reliability. Wetting depends on just two basic requirements, heat transfer and solderability, and these are discussed in some detail. Causes of non‐ideal texture and lustre of the solder are given, but these attributes do not affect reliability, nor is measuring solder purity important. Additional factors which do affect reliability relate more to design and materials than to process. Failure to deal with these factors can result in solder defects that are undetectable by any inspection technique. The answer to this problem is therefore not automated inspection to find more kinds of defects than visual inspection can, but control of design and materials, as well as process, to prevent them entirely.