Lead-free wave soldering solder defects

In eutectic Sn-Pb wave soldering, formation of solderballs on the surface of a PCB is a low-occurrence defect. For lead-free wave soldering, however, the number of solderball defects is substantially higher than with eutectic Sn-Pb. The higher operating temperatures required in lead-free soldering softens the solder resist on boards which makes solderballs more prone to stick to the board surface. In addition, the reduced wettabihty and increased oxide formation found with many lead-free solders increases the chance of solderball formation. 

Fig. 36 shows the evaluation board used to study the lead-free wave soldering process, and Table 7 lists the components and their finishes addressed in these studies. A comparative defect summary for several proposed lead-free solders was prepared (Table 8) as a result of the evaluation conducted, as were joint strength measurements (Table 9). Several observations were made based on the studies that are listed in Table 10. 

Panasonic has experienced similar results when lead-free solder assembly was phased in for TV production. The defect levels dropped from a historical rate of 3% with eutectic Sn-Pb to a 0.07% level with Sn-3.5Ag-0.5Bi-8In solder paste and Sn-3.0Ag-0.5Cu wave solder. 

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