Ag-4Pd銀合金線超音波打線接合之材料特性分析

Abstract

為改善金鋁銅線在打線接合上的應用，本研究從超音波接合參數、製程與高溫時效的方向著手改善銀合金線應用上的可靠度。將線徑為25.6μm的銀合金線接合於鋁墊發現，相同參數下，銀合金線的拉線強度為鋁線的1.6~2倍左右。經時效退火後，發現Ag2Al為穩定相。經參數優化後，線徑為25.6μm的銀合金線的拉線強度近11gf、界面強度近87.67MPa。然而，優化後界面的機械式接合增加、平整度也降低。此情況可藉由短時間加熱鋁墊可改善界面強度。 在不同基板上進行超音波接合，鋁墊因硬度較低，使線徑為195μm銀合金線的參數範圍極小。鎳鋁墊因鎳銀不互溶且鎳硬度比銀大，使銀合金線接合時易破壞基板。金鎳銅墊因金銀完全互溶且金硬度略小於銀，使銀金固溶體可潤滑基板、避免破壞基板結構。 比較不同線材(線徑為200μm鋁線、195μm銀合金線、195μm 4N銅線、195μm 6N銅線)接合於金鎳銅墊發現，銀合金線有最大的拉線強度，界面強度略小於銅線。經過150℃、500小時的高溫時效後，銀合金線的拉線強度僅次於6N銅線；界面強度雖仍比4N和6N銅線小。但是，因金銀固溶體生成，使界面強度下降幅度比銅線小、機械性質相對穩定。經時效退火後，粗線徑的異質超音波接合的界面平整度低、介金屬化合物生長與水平分布不均勻，界面強度下降、可靠度降低。

Abstract To improve the application of the gold aluminum copper wire bonding, this study focuses on improving the reliability of silver alloy wire in the ultrasonic wire bonding, in the aspect of bonding parameters, process and annealing. In the same parameters, the silver alloy wire (ϕ = 25.6μm) bonded to the aluminum pad. It is found that the pull strength of silver alloy wire is about 1.6 to 2 times than that of aluminum wire. After annealing, this study has found that found Ag2Al is stable phase. After optimizing the parameter, the wire pull strength of the silver alloy wire (ϕ = 25.6μm) is near 11gf, and the shear strength is near 87.67MPa. However, the optimized interface has more mechanical bonding, the flatness of the bonding interface decreases. This situation could be improved, since heating on aluminum pad for a short-time could improve the shear strength. When ultrasonic wire bonding on different substrates, aluminum pads cause short parameter range of the silver alloy wire (ϕ = 195μm) according to its lower hardness. Because silver is nickel-immiscible and softer than nickel, the nickel substrates were easily damaged by silver alloy wire during the ultrasonic wire bonding. Gold-nickel-copper pads is a workable choice for silver alloy wire on ultrasonic wire bonding. Gold is completely miscible with silver and its hardness is slightly less than that of silver, so gold-silver solid solution can lubricate the substrate to avoid damaging the structure of the substrate. Comparing different wire (aluminum wire with the diameter of 200μm, silver alloy wire with the diameter of 195μm, 4N copper wire with the diameter of 195μm and 6N copper with the diameter of 195μm) bonding to the gold -nickel-copper pad, the wire pull strength of the silver alloy wire is maximum, and the shear strength of the silver alloy wire is slightly smaller than the copper wire. After 500 hours of annealing at 150 ℃, the wire pull strength of the silver alloy wire is slightly less than the 6N copper wire ,and the shear strength of the silver alloy wire is less than the 4N and 6N copper wire. However, since gold and silver generate a solid solution, the decline of the shear strength is smaller than the copper wire. This means the mechanical properties of the silver alloy wire is relatively stable. After high-temperature annealing, the flatness of the interface has positive correlation with the diameter of the bonding wire. The shear strength and reliability get worse because the uneven bonding interface result in inhomogeneous growth of the IMCs.