Sn3Ag0.5Cu無鉛銲錫添加稀土元素Ce之球格陣列構裝界面反應與錫鬚成長研究

Abstract

近年來許多相關研究報導在無鉛銲錫添加微量稀土元素可改善其潤濕性與機械性質，中國大陸有意將其納入China ROHS規範，然而在運用到真實的接點時，還有許多效應仍待進一步確認，以評估此無鉛銲錫的取代性。因此本論文使用Sn3Ag0.5Cu-XCe(X=0.1,0.25, 0.5,1.0wt%)無鉛銲錫之球格陣列構裝接點作為研究，第一部份探討添加不同含量稀土元素添加對銲錫熔點、界面介金屬反應及高溫時效後推球強度之影響，第二部份為錫鬚成長特性研究，以更進一步建立此稀土添加之無鉛銲錫的錫鬚成長機構。 研究結果顯示，添加稀土元素Ce可以減緩Ag3Sn顆粒粗大化並抑制時效後推球強度下降，有效改善此銲錫的機械性質，其中以0.25 wt%添加效果最好。在固固反應中，稀土元素添加提高了Cu6Sn5與Cu3Sn成長活化能，使Cu6Sn5與Cu3Sn的成長受溫度影響較大。在添加稀土元素後，錫球內部可觀察到CeSn3介金屬，CeSn3氧化後可觀察到錫鬚形成，導致此種無鉛銲錫在使用上將因可靠度而受到限制。 本研究中的錫鬚型態可分為細長狀(fiber)、柱狀(column)、粗短花狀(flower-cluster)與山丘狀(hillock)，其中細長狀錫鬚中又分為直徑較細的TypeI錫鬚(0.1-0.5μm)與較粗的TypeII錫鬚(1μm)兩種。而提高相對濕度會使錫鬚型態由細長狀轉變成短花狀，提高溫度轉變成山丘狀，提高溫度也會使細長狀錫鬚數目減少、成長速度變快、直徑變大與平均長度變短，而低溫度5℃下與真空10-2torr下則可抑止CeSn3氧化與錫鬚生成。本研究亦藉由橫截面與氧化性質歸納出此稀土添加之無鉛銲錫的錫鬚成長機構為CeSn3氧化導致體積膨脹與外來氧原子擴散，使周圍銲錫基地與CeSn3區域承受壓應力所致。

Recently, the addition of a trace amount of rare earth elements is reportedly able to considerably improve the wetting and mechanical properties of mast of these Pb-free solder, related researches have just gotten underway is very recent years and China government are willing to arrange this Ce-doped solder into China ROHS. However, to apply this Ce-doped solder into a real package production, a lot of these efforts are still to be clarified through further experiments so that this Ce-doped solder will be confirmed as the Pb-free solder replacer. In the first part of this study is concerned with the Sn3Ag0.5Cu-XCe(X=0.1, 0.25, 0.5, 1.0wt%) Pb-free solder joints in Ball Grid Array (BGA) Package, while focusing on the melting point, interfacial intermetallic reactions and shear test after high temperature aging. The second part of this study is to clarify the mechanism of tin whisker formation through the whisker growth properties. From the experimental results, it is evidenced that the Ce-dope is effectively to slow down the coarsening of Ag3Sn precipitates in the solder matrix and the degradation of bonding strengths after high temperature aging. For the solid interfacial reactions, Ce-dope increases the growth activation energy of Cu6Sn5 and Cu3Sn, which implies that the growth reaction of Cu6Sn5 and Cu3Sn-intermetallics is much more sensitive to the aging temperature. In the microstructure of solder matrix, Ce-dope results in the formation of precipitated CeSn3 clusters in the reflowed solder matrix and the oxidation of CeSn3 clusters makes the tin whisker grow. However, this Ce-doped solder will be restricted by the reliable issue of tin whisker growth. In this study, the morphology of whiskers observed can be summarized as fiber, column, flower-cluster and hillock. Different diameter fiber whiskers are Types I and II, which can coexist on the same CeSn3 oxide layer. In addition, the morphology of whiskers will transfer from fiber into flower-cluster whisker with increasing related humidity and into hillock whisker with increasing temperature. The growth rate and diameter of fiber whisker will be increased by increasing temperature, but the average length decreased. Lower temperature at 5℃ and 10-2 torr vacuum can inhibit the whisker formation. Through the relationship between cross section observation and oxidation behavior at various temperatures, the mechanism of abnormal tin whisker formation in this Ce-doped solder can be attributed to the compressive stress induced by the diffusion of oxygen into the CeSn3 precipitate clusters in the solder, which squeezes the Sn atoms in the Ce-depleted region of CeSn3 phase out of the oxide layer.