Close inspection of Pb-free boards showed plenty of room for improvement.

A company’s qualification strategy for a new Pb-free process consisted of manufacturing a series of boards using a Pb-free process, followed by environmental testing, and finally post-visual inspection and microsectional analysis. The goal was to determine how well the joints survived exposure to environmental tests such as temperature cycling, mechanical shock and vibration. The customer requested visual inspection to IPC standards and solder joint microsectional analysis.

Under inspection, the boards were found to have excessive flux residue, burn marks, lack of vertical fill, board measling and scratches. These are symptoms of poor electronics manufacturing procedures; however, only some ultimately affect board reliability.

The customer wanted to determine the relationships between quality and reliability before and after testing. The manufacturer specified that joints be cleaned and brushed with alcohol. In this case, the customer had procedures in place that were not being followed. Multiple boards and joints were found with flux residue similar to that shown in Figure 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. In addition, several instances of an unknown white residue were observed (Figure 2). Upon closer inspection and further analysis, additional defects indicated a lack of attention to standard manufacturing procedures. These ranged from a component in the meniscus (Figure 3), scratches on the boards, misformed component leads, and overcrimped leads and wiring at the joint (Figure 4). These defects normally do not affect board reliability, but are process indicators and should be eliminated from future production.

Certain boards also had measling (Figure 5). This condition 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.

Unacceptable via fill (Figure 6) is another defect that can affect long-term reliability. Without a properly formed joint, the joint’s mechanical strength is severely compromised. During 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 heatsink and reduced 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 meet the intended goal of product qualification.

The American Competitiveness Institute (aciusa.org) is a scientific research corporation dedicated to the advancement of electronics manufacturing processes and materials for the Department of Defense and industry. This column appears monthly.

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