BGA lead-free C5 solder system improvement by Germanium addition to Sn3.5Ag and Sn-3.8Ag-0.7Cu solder alloy

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 221?C and 217?C, 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 Pb-Sn solder, such as Sn-In-Zn alloys, may have melting point close to 185?C, though not eutectic, and an acceptable solidification range but have received only limited attention due to various reasons & concerns¹. In semiconductor packaging industry, lead-free solders such as Sn-Ag¹ and Sn-Ag-Cu¹ have been widely applied in mass production of ball grid array products. Such alloys are often used for C5 solder system. However, one of the major challenges is oxidation after thermal processing such as reflow, burn-in, data retention bake and hot temperature testing. A study was conducted on BGA lead-free C5 solder joint system to assess the effect of germanium (Ge) addition to Sn3.5Ag and Sn3.8Ag0.7Cu solder alloy. The main objective of this study is to find a way to resolve solder surface oxidation after thermal processes, while determining if there's any adverse effect on the solder joint by Ge addition. Experimental works were carried out to observe the melting properties and solder surface morphology by differential scanning calorimetry (DSC) and SEM. Solder surface oxidation was measured by EDX. 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. Solderability test was conducted per Jedec standard. Tray and Packaging Drop Tests were done 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 150?C for 24, 48, 96, 168, 504 and 2000 hours and multiple reflow of 1x, 2x, 3x, 6x towards the joint integrity. Overall, Ge doped alloys had significantly higher ball shear and ball pull strength. EPMA microstructure analysis after cross-sectioning on bulk solder and IMC revealed traces of Ge that contributed to the significant increase in ball shear and ball pull strength, while did not cause any bulk solder and IMC morphology changes. Solderability test passed. Drop tests had comparable performance as non Ge doped alloys. In conclusion, addition of Ge in Sn3.5Ag and Sn3.8Ag0.7Cu lead-free solder alloys is able to resolve surface oxidation problem after thermal processing, with improvement in solder joint strength for overall lead-free package robustness.

1. Introduction

The introduction of lead-free solder in recent years has spurred the development of new processes and materials. The effort to replace lead-based solder with lead-free solder was because of the toxicity of lead that is hazardous to human and the environment.