以Au-Sn為銲料的功率集成模塊接合技術之開發

Abstract

IGBT功率模組能有效增加電動車中電能轉換動能之效率，因此學、業界相當重視此元件之接合與製造技術。同時隨著電動車的快速發展，越來越高的電流密度，導致了IGBT工作溫度的快速上升。從而更高效率和工作溫度的SiC基IGBT越來越受到大家的關注。然而，長時間的高工作溫度，也對銲點質量要求更加苛刻。因此,對於高性能IGBT來說，一個具有良好高溫熱穩定性和高可靠度的銲點是現在研究的重點。 本研究通過固液擴散接合技術成功的將Au-20Sn (wt. %)銲料應用於高功率車用IGBT 電子元件與DBC基板的組裝，改善接合良率與降低生產成本。研究發現Ni/Au-20Sn/Ni的結構在組裝完成後就能忍受278 ℃以內的高溫，同時經過高溫熱儲藏一段時間後，當銲點中殘留的AuSn被完全消耗，銲點可以耐受的溫度會升高到522 ℃。經過針對三組不同熱處理溫度下銲點微觀結構發展和界金屬成長動力學的討論，以確定最佳的工業製程。我們發現在300 ℃，60 s的組裝條件後追加240 ℃，100 h熱處理的製程有很好的應用前景以達成低溫接合高溫使用的目的。 同時本研究也對Ni/Au-20Sn/Ni界面反應進行了深入探討，研究發現Ni原子趨向沿著初生的(Ni,Au)3Sn2晶界處擴散進入銲點與AuSn反應生成(Ni,Au)3Sn2，當銲點中AuSn完全被消耗後，沿(Ni,Au)3Sn2晶界擴散的Ni原子則會佔據晶界處(Ni,Au)3Sn2中Au原子位置生成低溶Au量的(Ni,Au)3Sn2。通過穿透式電子顯微鏡的分析，這兩種Au含量不同的(Ni,Au)3Sn2的化學組成和晶體結構被準確鑑定。同時，更進一步應用高解析穿透式電子顯微鏡分析,被取代的Au原子會先析出在(Ni,Au)3Sn2的晶界處然後會在(Ni,Au)3Sn2內部聚集以大塊的純Au相形式析出。此外，經過240 ℃長時間的熱處理，極少量的Ni3Sn出現在(Ni,Au)3Sn2與Ni的界面處。 在銲點機械性質方面，雖然Au5Sn在190 ℃（IGBT工作溫度約200 ℃）發生高低溫相轉變時會引入2.04 %的體積改變。但研究發現在經過300 ℃，60 s的組裝條件後，銲點中較脆的Au5Sn低溫相會完全轉化為高溫相並且一直以高溫相形式存在於銲點中。通過微奈米硬度機對銲點在三組不同溫度下介金屬機械性質進行量測，討論了Ni濃度對於介金屬造成之影響。最後對不同熱處理時間試片進行剪力測試及破壞金相分析。

IGBT (Insulated Gate Bipolar Transistor) power IC module can efficiently increase the conversion efficiency of transforming electricity to kinetic energy, and consequently has attracted increasing amount of attentions. For electrical vehicle applications, the power density is relatively high, and consequently an IGBT has to be operated at a high service temperature. Therefore, silicon carbide-based IGBT has attracted increasing amount of attentions. However, long-term high-temperature operations require quality bonding interfaces that exhibit high temperature stability and reliability. Accordingly, searching for alternative bonding materials that can withstand high service temperatures is an urgent priority in order to realize the full potential of SiC IGBT technology. In this study, the eutectic Au-Sn (20 wt. % Sn) is successfully used to assemble IGBT chips and direct-bond-copper substrates by using solid liquid interdiffusion (SLID) bonding in order to improve production yield and lower production cost. It is found that the Ni/Au-20Sn/Ni structures can withstand the highest service temperature of 278 ℃ after bonding. As the aging time increases, once the AuSn was nearly exhausted, the whole joint itself can theoretically withstand a highest service temperature of 522 ℃. During the subsequent isothermal aging at 150, 200, and 240 ℃, the microstructure evolution and growth kinetics of intermetallic compounds are investigated. It is found that 300 ℃ bonding for 60 s followed by aging at 240 ℃ for 100 h is a highly promising recipe to realize the objective of low-temperature bonding for high-temperature applications. Interfacial reactions of Ni/Au-20Sn/Ni are investigated. It is found that Ni would diffuse along the grain boundary of the original (Ni,Au)3Sn2 to form a new (Ni,Au)3Sn2 at the interface of (Ni,Au)3Sn2 and AuSn. After the residue AuSn had been consumed, Ni diffusion along the grain boundary of (Ni,Au)3Sn2 to form a thin layer of Au-lean Ni3Sn2 at grain boundaries. The exact composition and crystal structure for these two kinds of (Ni,Au)3Sn2 are identified by using transmission electron microscopy. According to HRTEM analysis, the occupied Au atoms precipitates within the interface of (Ni,Au)3Sn2 grain then form a large pure Au in the matrix of (Ni,Au)3Sn2. In addition, Ni3Sn formed at the interface of (Ni,Au)3Sn2 and Ni after long time aging at 240 ℃. It was pointed out Au5Sn exists in two crystal structures with an isomeric transformation from ζ to ζ' at the 190 ℃ (IGBT working temperature closed to 200 ℃). This isomeric transformation in Au5Sn would result in 2.04 % of volume shrinkage. The results of this study show that all the brittle ζ' phase of the Au-Sn eutectic alloy has transformed into ζ phase after bonding. Thereafter the mechanical properties of intermetallic compounds in this system were investigated by nanoindentation. The influence of Ni concentration on the mechanical properties of Au-Sn IMC ware investigated.