A Study of SnAgNiCo vs Sn3.8AgO. 7Cu C5 Lead Free Solder Alloy on Mechanical Strength of BGA Solder Joint

RoHS Directive for environmental and health concerns have resulted in significant activities to find substitutes for lead-contained solders for microelectronics. The potential candidates such as Sn-Ag1 and Sn-Ag-Cu1 eutectic solders with melting temperatures of 221degC and 217degC, respectively are the most prominent solders because of their excellent mechanical properties as compared with that of eutectic Sn-Pb solder2. Other candidates as drop-in replacements for eutectic Sn-Pb solder, such as Sn-In-Zn alloys, may have melting point close to 185degC, though not eutectic, and an acceptable solidification range but have received only limited attention1. Among the many possible lead-free solder alloy candidates, three commonly used alloys to meet automotive thermal cycling requirement are Sn3.0Ag0.5Cu, Sn3.8AgO.7Cu and Sn4.0Ag0.5Cu. However, industry has found these commonly applied solder alloys to have certain level of ball drop problem which affects production yield, product quality as well as customers satisfaction. This paper reports a study which was conducted on BGA lead-free C5 solder joint system to compare SnAgNiCo solder alloy versus the conventional Sn3.8AgO.7Cu solder alloy, with the objective to strengthen the solder joint so as to eliminate ball drop encountered on the conventional Sn3.8AgO.7Cu solder alloy. This study showed that SnAgNiCo C5 solder system performed better than Sn3.8AgO.7Cu in terms of joint strength and intermetallic brittle mode failure rate. Experimental works were carried out to observe the melting properties, micro structure and elemental analysis that were obtained by Differential Scanning Calorimetry (DSC), SEM and EDX respectively. Shear and pull strength was measured by Dage which is representative of solder joint strength between the C5 solder sphere and Cu/Ni/Au pad finishing. Drop Tests were done per Tray & Packing methods to gauge solder joint performance against impact force. A comprehensive study was done to study the effect of microstructure and interface intermetallic of both solder system at ambient, high temperature storage (HTS) at 150degC for 168 hours and 504 hours, and 6x multiple reflow towards the joint integrity. Microstructure studies on SnAgNiCo solder reveals that formation of rod shape Ag3Sn IMC distributed across the solder surface helps to act as dispersion hardeningthat increases the mechanical strength for the SnAgNiCo solder. Through the microstructure study with SEM, it was also found that the addition of Ni and Co into the SnAg solder alloy also helps to minimize growth of grain size and prevent increase in intermetallic thickness after thermal aging. EDX analysis confirmed that in SnAgCu solder/Ni interface, Cu- rich IMC formed on top of the Ni-rich IMC. For SnAgNiCo system, only Ni-rich IMC is found. Therefore, it is highly suspected that the presence of Cu-rich IMC posed a detrimental effect on the joint strength and tends to cause brittle joint failure. Both of the effect is then showed in ball pull result that at Time Zero after assembly and after 6x reflow, SnAgCu solder has 90% and 100% brittle mode failure, where SnAgNiCo solder has 0% and 5% for the respective read points. This result correlates with missing ball responses after packing drop tests which shows no failure for SnAgNiCo alloy. Thus, SnAgNiCo lead-free solder is a potential candidate for lead-free solder joint improvement for overall lead-free package robustness.