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

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.

An Experimental Study of Lead-Free Wave Soldering Using SAC 305 and No-Clean Flux

The transition to lead-free assembly will have a significant effect on wave soldering operations. Since the wetting ability of lead-free solder is usually less than that of tin-lead solder, it can result in unacceptable hole fills and inconsistent top side wetting - especially in the case of thick Printed Circuit Boards (PCBs). Presently, there is very little data available on lead-free wave soldering with tin-silver-copper (SnAgCu or SAC) alloy and no-clean flux chemistry. Although some researchers and consortia recommend tin-copper (SnCu) for lead-free wave soldering, demonstrating the feasibility of using the SAC alloy for wave soldering operation can aid manufacturers to use the same alloy for both reflow and wave soldering operations. In this study, SAC 305 alloy and no-clean flux were evaluated in terms of percentage of hole fill and solderability on a 93 mil thick test vehicle with Immersion Silver (ImmAg) surface finish. The evaluation was performed on a nitrogen equipped wave soldering equipment. It has 4 preheating zones (3 convection bottom heaters and 1 infrared top heater) that provides good control to develop the required preheat profile. A partial factorial experiment was conducted to study the main effects of solder pot temperature, topside preheat temperature and conveyor speed on wave soldering performance. Wave soldering was performed after two reflow cycles. A 100% visual inspection was done for all the through hole components using a 10X microscope to determine top side wetting, percentage of hole fill, bridging, flux residue and solder balling. Thickness of the hole fill was also measured using digital X-Ray equipment. The data generated from this experiment was used to determine the 'optimum' lead-free process parameters for wave soldering using a SAC 305 alloy with a no-clean flux chemistry. The 'optimized' process parameters were then used to evaluate boards with Organic Solderability Protective (OSP) and Electroless Nickel Immersion Gold (ENIG) surface finishes. The designed experiments approach adopted to determine the optimum process settings and the research findings are explained in detail.