銀合金銲球凸塊覆晶組裝可行性評估

Abstract

近年來由於金價高漲，迫使產界與學界積極開發取代金打線的材料，銀合金線的發明，不管是在成本或可靠度上，皆展現優秀的性質，也為封裝產業提供一個更佳的選擇。然而半導體上游微顯影技術不斷提升，使得日常生活3C產品性能不斷進步，為了延續摩爾定律，3D IC技術被發明，其封裝技術中，需利用金屬凸塊。但現今主流技術如電鍍法，造成汙染時有所聞，而Stud Bump技術不需用到電鍍技術，符合現今環保觀念，但傳統使用材料為金，本研究評估將金轉換為銀合金，利用銀合金線製造出高可靠度的銀合金凸塊。

本研究第一部分為評估現今封裝打線電化學性質，由極化曲線的腐蝕電位與腐蝕電流判斷其抗腐蝕性，結果得知鋁線抗腐蝕性最差，鍍鈀銅線燒結成球後，會有鈀層擴散入內的情形，至於銀加入貴金屬金與鈀後，抗腐蝕性大幅提升，抗腐蝕性順序為:Ag-8Au-3Pd > Ag-3>Pd>Pd-Coated Cu > Al。 本研究第二部分為模擬Stud Bump界面，傳統金凸塊與銲錫(Sn-3Ag-0.5Au)反應後，金會大量擴散入銲錫中，生成AuSn4，導致「金脆現象」，導致界面破壞，而銅與銲錫反應後，介金屬層生成厚度太薄，且介金屬會有裂縫，整體可靠度不佳，銀合金與銲錫反應後，界面單純，只有Ag3Sn，整體厚度介於金與銅之間，介金屬成長穩定，但在200℃長時間時效後，錫原子會越過Ag3Sn，與銀合金金或鈀原子結合形成AuSn4或PdSn4，且界面會出現裂縫。

These years, the higher gold price has forced a number of groups in industry and academy to develop new wire bonding materials to replace the gold wire. The newly developing Ag alloy wire has a great deal of advantages include either cost or reliability and is a better approach for the electronic packaging industry doubtlessly. Thanks to the continually improving lithography of upstream, the function of 3C products in our daily life keeps on improving. To prolong Moore’s Law, 3D IC was invented, and the metal bumps are necessitated during packaging. However, the prevailing method such as electroplating has raised a few pollution issues. In contrast, Stud Bump rules out electroplating and fulfills the environment-friendly need. This research assesses the possibility if the Ag alloy wire could substitute the traditional gold wire to make highly reliable metal bumps. In this research, the first part is to assess the electrochemical property of current bonding wire, the corrosion potential and current on the polarization curve decide the corrosion resistance. The results show that the Al wire has the poorest corrosion resistance, and after EFO burning, the Pd layer of Pd-coated Cu wire will diffuse inside the core. Different to them, Ag can gain much higher corrosion resistance after alloying with noble metals Pd and Au. The corrosion resistance trend is : Ag-8Au-3Pd > Pd-Coated Cu > Ag-3Pd > Al. Second part is the simulation of Stud Bump interface. In the results, Au will diffuse into Sn layer abundantly to form AuSn4 after the reaction between traditional gold bump and solder (Sn-3Ag-0.5Au), gold embrittlement and the interface will result crack. It also shows that in the reaction between Cu and solder, the formation breaks with cracks because the IMC is too thin, and the reliability is poor. In contrast, the interface between Ag alloy and solder after reacting is simple - the only IMC Ag3Sn forms stably, and the average thickness is between the two reactions above. However, after aging for a long time at 200℃, Sn will go across Ag3Sn and form AuSn or PdSn4 with Au and Pd, furthermore, and results in cracks.