Another category would include the cases where the components are barely solderable, due to their quality. Those ones are relatively easy to verify by the solderability tests (particularly wetting balance) and they can be caught by incoming inspection.
The third group is the most difficult to deal with, as each case may require a comprehensive analysis, in order to establish the root cause for the problem and so the corrective actions can be implemented. The case below is a typical example of the solderability issues from that group.
The ProblemA customer reported a solderability problem observed after assembly (SMT process) on the board side of an ENIG finished PCB. The majority of the assembled boards were affected. One of the assemblies was submitted to SENTEC for root case analysis. Typical appearance of the affected solder joints is shown in Fig. 1.
The AnalysisAn area of the board with some of the component exhibiting the problem was cut out and inspected in a scanning electron microscope (SEM) equipped with an X-ray microanalysis system (EDS). The inspection revealed that areas of the solder pads not covered with solder exhibit the presence of an abnormally thin layer of electroless Ni. To shed more light on the root cause of the problem one of the components was cross-section. The results of the analysis confirmed that the layer of electroless Ni (E-Ni) is uneven and it is almost missing in some areas of the solder pad. The images in Figures 2 through 4 illustrate the findings:
Fig. 2 - An overall appearance of the joints - scanning electron microscopy (SEM);
Fig. 3 - An enlarged light microscopy image of the area of interest where the presence of a thinned layer of electroless Ni is evident;
Fig. 4 - A close up SEM image of the area where a typical structure of a good solder/solder pad interface was formed (a well defined layer of Ni-Sn intermetallics is evident in the right part of the image) and the area where no wetting between the solder and the pad (left part of the image) was evident.