藉由銲料中銀含量的調變與表面處理材料之選擇探討錫銀銅焊接系統於高低溫循環測試下之失效機制與可靠度

Abstract

對晶圓級晶片尺寸封裝（Wafer-Level Chip Scale Packaging, WLCSP）的電子產品而言，在使用時即長期處於高低溫冷熱循環（thermal cycling）的狀態下，基板與晶片之間熱膨脹係數的差異，導致其封裝中之接點必須承受熱與機械疲勞。而熱循環過程中所產生的起始裂縫則是受到材料之間熱膨脹係數的不匹配所致。 一般銲料(solder bump)中銀元素的添加與凸塊底金屬層(under bump metallization, UBM)材料的表面處理，經常影響到熱循環後之熱性質表現與可靠度。藉由調變銲料中銀元素的含量將直接影響熱循環後的結果。此外，不同表面處理材料將影響銲料中的微結構與介金屬化合物的形成，進而影響接點可靠度。透過表面處理材料的選擇，熱循環過程中所產生的再結晶情形亦會受到影響。此有關銀元素與表面處理材料在熱循環後的可靠度議題，必須詳細討論。 為了解決上述的問題，本研究採用有機保護膜(Organic Soldering Preservative, OSP)與無電鍍鎳浸金(Electroless Nickel Immersion Gold, ENIG)兩種表面處理材料，分別添加高銀含量（SAC396）與低銀含量（SAC396）於銲料中，嘗試比較不同試片情形在熱循環下的裂縫生成，銲料中的各元素的分佈與晶粒結構。韋伯分布(Weibull distribution)的結果顯示，有機保護膜的表面處理相較於無電鍍鎳浸金的表面處理有較佳的熱循環表現。同時高銀含量的銲料也較低銀含量的銲料在熱循環下有更穩定的熱性質表現。 為了探討每一組試片其失效模式，利用場發射掃描式電子顯微鏡(Field-Emission Scanning Electron Microscope, FE-SEM)觀察裂縫擴展之情形與介金屬化合物的生成，另分別透過場發射式電子微探儀Field-Emission Electron Probe Micro-Analyzer, FE-EPMA)偵測裂縫周圍的元素分佈情形，與電子背向散射繞射(electron back scattered diffraction, EBSD)以獲得晶粒取向與晶界的資訊。藉此完成在熱循環下的機制探討，並找出具有潛力的表面處理材料與相對應的銲料，組合成具有優良封裝結構的材料系統。

When the electronic devices are in operation, the Wafer Level Chip Scale Packaging (WLCSP) interconnects are subjected to thermo-mechanical fatigue that is generated by temperature cycles reacting with a substrate and a chip that have different coefficients of thermal expansion (CTE). Usually, the locations of crack initiation are strongly related to the CTE mismatch between Si chip and solder alloy. In general, there are some ways to enhance the thermo-reliability, including the selection of pad finish and Ag addition in solders. Adjusting Ag content in solders would influenced the performance of thermal cycling. Besides, the joint properties with dissimilar microstructures and IMCs formation will differ between joints with various surface finishes. By the way of pad selection, the situation of recrystallization and other phenomena occurred during thermal cycling are affected. Consequently, the solder joint reliability regarding Ag effects and substrate effects under thermal cycling and related mechanism are needed to be well addressed. As for these critical concerns, this study investigated the correlations among crack, elemental distribution and grain structure in Ni(V)/SAC/OSP and Ni(V)/SAC/ENIG solder joints both with SAC396 and SAC105 solder alloy, respectively. The results of thermal cycling test shows that the joints with OSP substrate exhibit longer lifetime during thermal cycling test, as compared to that of ENIG substrate. Meanwhile, the joints with SAC396 solder alloy demonstrates superior thermal stability than that of SAC105 solder alloy. To probe the mechanisms regarding the failure more in each condition, this study applied FE-SEM to observe the crack propagation, surrounding IMCs precipitates in solders and interfacial IMCs. Moreover, FE-EPMA and EBSD were used to detect the elemental distribution and grain orientation, near the cracks, respectively. It is demonstrated that the solder joints with OSP substrate and SAC396 solder content exhibit superior thermal stability. The related mechanisms were proposed and addressed.