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A Comparison Study on SnAgNiCo and Sn3.8Ag0.7Cu C5 Lead Free Solder System
Keywords: BGA packaging, lead-free C5, brittle solder joint
A study was conducted on BGA lead-free C5 solder joint system to compare SnAgNiCo versus the conventional Sn3.8Ag0.7Cu solder alloy, with the objective to resolve ball drop issue encountered on the conventional Sn3.8Ag0.7Cu solder alloy. This study showed that SnAgNiCo C5 solder system performed better than Sn3.8Ag0.7Cu in terms of joint strength and brittle mode failure. Experimental works were carried out to observe the melting properties, microstructure 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 the intermetallic (IMC) strength between the C5 solder sphere and Cu/Ni/Au pad finishing. Drop Tests were done per ASE & FSL methods to gauge solder joint performance against impact force. A comprehensive study was done to study the effect of microstructure and interface intermetallics of both solder system at ambient, high temperature storage (HTS) at 150C for 168 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 hardening that increases the mechanical strength for the SnAgNiCo solder. 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 after 6x reflow, SnAgCu solder has 100% brittle mode failure, where SnAgNiCo solder has only 5%. This result correlates with missing ball responses after packing drop tests. Thus, SnAgNiCo lead-free solder is a potential candidate for lead-free solder joint improvement for overall lead-free package robustness.
Eu Poh Leng, Senior Staff Engineer (R&D Engineer III)
Freescale Semiconductors Malaysia Sdn Bhd
Petaling Jaya, Selangor 47300,
Malaysia


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