PB-Free Wave Soldering of Thick Printed Circuit Boards Using No-Clean VOC Free Flux

Abstract

There has been an increasing focus on surface mount technology, and the miniaturization of electronic assemblies. However, wave soldering still remains an integral part of the Printed Circuit Board (PCB) assembly process. Hole fill is an important aspect in wave soldering. It is caused by the capillary action of molten alloys, as the PCB traverses across the molten wave. The advent of Pb-free materials has tightened the process windows for wave soldering. This is primarily because Pb-free alloys have higher melting points. One way to enhance hole filling action is to increase the operating temperatures of the molten wave. This step, however, could result in the disintegration of surface mount or through hole devices, board discoloration and warpage. Flux selection is an important process step for Pb-free wave soldering as it enhances hole-fill and cleans the soldering surface of the oxides prior to the PCB hitting the wave. It is critical to use the ‘optimal’ flux in Pb-free wave soldering processes. The objective of this study is to determine the ‘best’ flux for the Pb-free wave soldering of 2.16mm (0.085″) thick PCBs with Ni/Au surface finish. The Pb-free solder in this application is SAC387 (95.5%Sn, 3.8%Ag, and 0.7% Cu) with VOC-free no clean water based flux. Under different conveyor speeds, the experimentation evaluates the ‘best’ flux among three candidates. Complete (100%) inspection using X-ray laminography equipment detects the percentage of hole fill and other defects such as bridging, flux residue, and solder balling. The ‘best’ flux should have the least number of defects.