Effect of SnAgCu composition on soldering performance

Abstract

PurposeTombstoning and voiding have been plaguing the surface mount assembly industry for decades. The recent global move toward lead‐free soldering and the extensive adoption of microvia technology further aggravate the problems. The present study investigates the impact of SnAgCu (SAC) alloy composition on these important issues.Design/methodology/approachIn this study, tombstoning and voiding at microvias are studied for a series of SAC lead‐free solders, with an attempt to identify a possible “composition window” for controlling these problems. Properties which may be related to these problems, such as alloy surface tension, alloy melting pattern, and solder wetting behaviour, were investigated in order to assess the critical characteristics required to control these problems.FindingsThe results indicate that the tombstoning of SAC alloys is greatly influenced by the solder composition. Both the wetting force and the wetting time at a temperature well above the melting point have no correlation with the tombstoning frequencies. Because the tombstoning is caused by imbalanced wetting forces, the results suggest that the tombstoning may be controlled by the wetting at the onset of the paste melting stage. A maximum tombstoning incidence was observed for the 95.5Sn3.5Ag1Cu alloy. The tombstoning rate decreased with increasing deviation in Ag content from this composition. A differential scanning calorimetry (DSC) study indicated that this was mainly due to the increasing presence of the pasty phase in the solders, which result in a slower wetting speed at the onset of solder paste melting stage. Surface tension plays a minor role, with lower surface tension correlating with a higher tombstoning rate. The voiding rate at the microvias was studied by employing simulated microvias. The voiding level was lowest for the 95.5Sn3.8Ag0.7Cu and 95.5Sn3.5Ag1Cu alloys, and increases with a further decrease in the Ag content. The results indicate that voiding at microvias is governed by the via filling and the exclusion of fluxes. The voiding rate decreased with decreasing surface tension and increasing wetting force, which in turn is dictated by the solder wetting or spreading. Both low surface tension and high solder wetting prevents the flux from being entrapped within a microvia. A fast wetting speed may also facilitate reducing voiding. However, this factor is considered not as important as the final solder coverage area.Research limitations/implicationsIn general, compositions which deviate from the ternary eutectic SAC in Ag content, particularly with a Ag content lower than 3.5Ag, exhibit a greater solid fraction at the onset of melting, resulting in a lower tombstoning rate, presumably due to a slower wetting speed. The SAC compositions with an Ag content lower than 3.5 per cent, such as 2.5Ag, resulted in a lower tombstoning rate with minimal risk of forming Ag3Sn intermetallic platelets. On the other hand, ternary eutectic SAC exhibits a lower surface tension resulting in an easier solder spread or solder wetting...

Reliability of lead-free solder joints with different PCB surface finishes under thermal cycling

“…Comparing the results with the other investigators' studies [9,10,23], it is found that there are some discrepancies between our results and their findings. Huang et al [9] found that solder joints with ENIG finishes on PCB revealed longer lifetime than those with OSP finishes.…”

“…Huang et al [9] found that solder joints with ENIG finishes on PCB revealed longer lifetime than those with OSP finishes. The predominant failure mode was the tearing of PCB trace in the OSP samples.…”

“…The dispersing particles would prevent the coarsening of solder microstructure and improve the shear strength. Huang et al [9] compared the pull strength of solder joints with OSP and ENIG finishes on PCB, and found that the OSP joints had higher strength. It is believed that the precipitates inside the bulk solder promote the reliability performance of the OSP solder joints.…”

“…Moreover, area array package: chip scale package (CSP), or ball grid array (BGA), and Cu or NiAu (UBM) were employed in the previous investigations [9,10,23], which were different from this study (leaded TSOP and FeNi). The difference in device, UBM, sample configuration, material properties (such as CTE, mechanical properties) is considered as the main cause for the discrepancies.…”

“…Thus, different surface finishes on PCBs, which lead to the difference in interfacial reactions and IMCs formation, would have an effect on the reliability performance of solder joints. Some investigators [9,10] compared the thermal fatigue lifetime, and adhesive strength of solder joints with OSP and NiAu finishes during thermal cycling test. The results revealed that the lifetime of solder joints with NiAu finishes was better than those with OSP finishes.…”