Effect of Germanium addition to Sn3.5Ag Lead Free Solder System for Overall BGA Package Robustness Improvement

摘要

A study was conducted on BGA lead-free C5 solder joint system to assess the effect ofrnGermanium addition to Sn3.5Ag solder alloy through comparison study of Sn3.5Ag vs.rnSn3.5AgGe. The main objective of this study is to find a way to resolve solder surface oxidationrnafter thermal processes, while determining if there’s any adverse effect on the solder joint byrnGermanium addition. The result of this study showed that addition of Germanium in Sn3.5Ag C5rnsolder system resolved solder surface oxidation after thermal processes, without any negativernimpact to the solder joint and solderability. Experimental works were carried out to observe thernmelting properties and solder surface morphology by Differential Scanning Calorimetry (DSC) andrnSEM. Solder surface oxidation was measured by EDX. Shear and pull strength was measured byrnDage which is representative of the intermetallic (IMC) strength between the C5 solder spherernand Cu/Ni/Au pad finishing. Solderability test was conducted per Jedec standard. Tray andrnPackaging Drop Tests were done to gauge solder joint performance against impact force. Arncomprehensive study was done to study the effect of microstructure and interface intermetallic ofrnboth solder system at ambient, high temperature storage (HTS) at 150℃ for 24, 48, 96, 168, 504rnand 2000 hours and multiple reflow of 1x, 2x, 3x, 6x towards the joint integrity. In each read point,rnSn3.5AgGe has significantly higher ball shear and ball pull strength. EPMA microstructurernanalysis after cross-sectioning on bulk solder and IMC revealed traces of Germanium thatrncontributed to the significant increase in ball shear and ball pull strength, while did not cause anyrnbulk solder and IMC morphology changes. Both solderability and drop test passed. In conclusion,rnaddition of Germanium in Sn3.5Ag lead-free solder is able to resolve surface oxidation problemrnafter thermal processing, with improvement in solder joint strength for overall lead-free packagernrobustness.