Lead-free Solders and PCB Finish Effects on Solder Joint Reliability

As we move forward into the lead-free era there are numerous possible combinations of solder alloys and PCB finishes. This is further complicated by wide variations in processing resulting in different microstructural properties. To judge the relative impact this might have in accelerated life testing during thermal cycling. Test vehicles were assembled with combinations of three alloys (SnAgCu, SnAg and SnPbAg), two board finishes (ENIG and immersion Ag), various component sizes and two substrate materials (FR4 and CTE matched), plus a conformally coated PCB was included. The degree of damage was monitored using continuity testing, push-off shear testing and microsectioning. The salient points from these studies were that there were significant (measurable) difference between the reliability of joints soldered with SnAgCu and SnPbAg alloys. There was no evidence to suggest that thermal pre-treatment was detrimental to the long-term reliability of the joint. Additional reflow passes or extended thermal ageing did produce some differences in reliability, but these were component specific. The reliability of joints made using SnAgCu solder was significantly better than that of joints made using the simple binary SnAg alloy. There was no evidence to suggest that thermal pre-treatment (reflow, wave or isothermal ageing) was detrimental to the long-term reliability of the joint. The results suggest that ageing and microstructure are not linked in a straightforward way. Lead-free processing can affect the reliability of plated-through holes (vias). The latter generated cracks when subjected to more than 500 thermal cycles. Conformal coating improved the joint reliability performance, possibly by a retaining action of the more pliable coating, preventing complete detachment. Comments are offered on the techniques used to assess solder joint reliability