Effect of SAC composition on soldering performance

Tombstoning and voiding have been plaguing the surface mount assembly industry for decades. Recent global moves toward lead-free soldering and extensive adoption of microvia technology further aggravate the problems. In this study, tombstoning and voiding at microvia behavior were studied on a series of SnAgCu lead-free solders, with an attempt to identify a possible "composition window" for controlling these problems. Eutectic SnPb was included as the baseline. Properties may be related to problems such as alloy surface tension, alloy melting pattern, and solder wetting behavior were also investigated in order to assess the critical characteristics required to harness these problems. Results indicate that tombstoning of SnAgCu is affected by the solder composition. At vapor phase soldering, both wetting force and wetting time at a temperature well above the melting point have no correlation with the tombstoning behavior. Since tombstoning is caused by unbalanced wetting force, the results suggest that the tombstoning maybe dictated by the wetting at the onset of paste melting stage. A maximal tombstoning rate is observed at 95.5Sn3.5Ag1Cu. The tombstoning rate decreases with increasing deviation in Ag content from this composition. DSC study indicates that this is mainly due to the increasing presence of pasty paste in the solders, which is expected to result in a slower wetting speed at the onset of solder paste melting stage. Surface tension plays a minor role, with lower surface tension correlates with a higher tombstoning rate. SnAgCu composition with a Ag content lower than 3.5%, such as 2.5Ag, is more favorable in terms of reducing tombstoning rate with minimal risk of forming Ag₃Sn intermetallic platelet. On the other hand, the voiding rate at microvia was studied with the use of simulated microvia, and was the lowest with 95.5Sn3.8Ag0.7Cu and 95.5Sn3.5Ag1Cu, and increased with further decrease in Ag content. Results indicated that voiding at microvia was governed by via filling and exclusion of fluxes. The voiding rate decreased with decreasing surface tension and increasing wetting force which in turn was dictated by the solder wetting or spreading. Both low surface tension and high solder wetting prevented the flux from being entrapped within- microvia. A fast wetting speed might also facilitate reducing voiding. However, this factor was considered not as important as the final solder coverage area. Overall, composition of SAC alloys affects tombstoning and voiding at soldering, mainly through physical effects. In general, composition deviates away from ternary eutectic SnAgCu in Ag content, particularly with a Ag content lower than 3.5Ag, exhibits a greater solid fraction at onset of melting thus results in a lower tombstoning rate, presumably due to slower wetting speed. On the other hand, composition devices away from ternary eutectic SnAgCu also exhibits a lower surface tension which results in an easier solder spread or solder wetting, and consequently results in a higher tombstoning rate and a lower voiding.