A customer called the EMPF Helpline to obtain a visual assessment of their printed circuit boards that were assembled using a new Pb-free process and then subjected to accelerated aging tests.
A customer called the EMPF Helpline and explained their qualification strategy for a new Pb-free process. It consisted of manufacturing a series of boards using their lead free process, followed by environmental testing, and finally post-visual inspection and microsectional analysis. Their goal was to determine how well the joints survived exposure to environmental tests such as temperature cycling, and mechanical shock and vibration. The customer requested that the EMPF perform visual inspection to IPC standards as well as microsectional analysis of solder joints.
The boards were inspected at the EMPF. Excessive flux residue, burn marks, lack of vertical fill, board measling, and scratches are all symptoms of poor electronics manufacturing procedures, however, only some will ultimately affect the board reliability. The customer wanted to determine the relationships between quality and reliability before and after testing.
The manufacturer specified that the joints be cleaned and brushed with alcohol. In this case, the customer had procedures in place that were not being followed properly. Multiple boards and multiple joints were found with flux residue similar to that shown in Figure 3-1. Under high voltage application, these areas may develop ionic migration and ultimately cause a short circuit.
Burn marks were also noted indicating excessive tip temperature and extended duration of heat application. There were several instances of an unknown white residue also noted (Figure 3-2). Upon closer inspection and a further analysis, additional defects indicated a lack of attention to standard electronics manufacturing procedures. These ranged from a component in the meniscus (Figure 3-3), to scratches on the boards (Figure 3-4), to mis-formed leads on components (Figure 3-5), and over crimped leads and wiring at joint (Figure 3-6). These defects normally do not affect the board reliability, but are process indicators and should be eliminated from future production.
There were also boards that had measling (Figure 3-7). This is a condition that indicates a localized delamination of the epoxy fiber laminate. It can lead to reliability issues, especially with multilayer boards. Internal connections between vias can develop cracks and ultimately cause an open circuit in the field, resulting in a non-functioning assembly.
Another detrimental manufacturing defect detected that can affect long term reliability was an unacceptable fill of many of the vias (Figure 3-8). Without a properly formed joint, the mechanical strength of the joint is severely reduced. Over the duration of a severe environmental exposure, cracks will initiate and propagate, and ultimately cause an open circuit. Closer inspection of the image revealed that the unacceptable fill was present on the wide power trace of the component. The metal had acted as a heat sink, and reduced the solder flow to the point where it was not sufficient to complete the joint. The customer was advised of thermal profiling changes that would help eliminate this condition, improve long term reliability, and complete their intended goal of product qualification.