SnCu/Ni系統中溫度效應對臨界銅濃度之影響

Abstract

摘要 銲料是目前最常使用於電子封裝產業的接合材料。近年來最常用於銲接的銲料為 Pb-Sn 合金，為了保護環境促進無鉛化的發展，已經有許多關於無鉛銲料的研究與討論。事實上， Sn-0.7wt.% Cu為常見的銲料之一，常用於波銲製程和覆晶封裝上。除了銲料禁 Pb 外，現在連印刷電路板和元件表面處理都開始走向無鉛製程。而 Ni 為常見的表面處理層，其他還有 Au/Ni 和 Pd/Ni，其目的是當作一擴散阻障層，避免銲料與 Cu 過度反應。而 Sn-Cu/Ni 及 SnAgCu/Ni 為常見的界面反應，為了評估銲點可靠度，因此了解其界面反應是很重要的。已知SnAgCu 銲料與 Ni 接合時，界面反應生成物對於銲料中 Cu 濃度相當敏感( Cu-Concentration Effect )。由於一般的銲點，其銲料中的 Cu 會與界面生成物耦合，隨生成物生長其銲料中的 Cu 濃度將隨之下降( Solder-Volume Effect )。因此銲料中的 Cu 濃度隨反應而變，不再維持一定值，此現象隨銲點體積縮小越趨明顯。可了解當銲料中 Cu 濃度隨含銅介金屬相之生成而下降，改變了銲料中Cu 濃度進一步使得原先界面平衡相跟著改變。因此藉由平衡相圖可有助於了解其界面反應的機制。 從文獻得知由熱力學計算方法，可預測臨界銅濃度與溫度有一相依性。本研究主要是排除體積參數，藉由實驗證實溫度與臨界銅濃度間確有一關係存在。實驗將不同組成的 Sn-Cu 合金與 Ni 在160, 180, 200, 225℃ 下反應，實驗結果顯示溫度的確會影響臨界銅濃度，證實臨界銅濃度將隨溫度下降而降低。

Abstract Solder is the most common interconnecting material for electronic packaging and assembly. The Pb - Sn solders are the most used solder for over 50 years. Environmental concern for Pb toxicity has propelled the search for a Pb-free replacement. Many Sn-based alloy series have been developed as candidates of Pb-free solders. In particular, the eutectic Sn-0.7Cu is widely used in wave-soldering and flip chip applications. The push for lead-free solder assembly has led to many investigations into PCB materials and finishes. Nickel is used in several lead-free finishes, such the Au/Ni and Pd/Ni, as a solderable diffusion barrier to prevent the rapid reaction between solder and the Cu conductor. Therefore, the Sn-Cu/Ni and the SnAgCu/Ni contact will be often encountered with the prevailing of Pb-free solders, and the knowledge of their interfacial reactions is crucial for the reliability assessment of the solder joints. In our previous studied, the reaction product(s) between SnAgCu solders and Ni are very sensitive to the Cu concentration in the solders (“Cu-Concentration Effect”). In a real solder joint, Cu in solder is incorporated into the reaction product(s), and as the amount of the product(s) increase, Cu in solder is gradually consumed. Consequently, the Cu concentration is no longer constant during the reaction, especially true for a small joint. This will change the Cu concentration in solder, and consequently the equilibrium intermetallic at the interface may also change (“Solder-Volume Effect”). In short, the solder-volume effect is a particular type of expression of the Cu-Concentration effect. To understand the mechanism of the interfacial reaction, the equilibrium phase relationship is a major tool. In this study, the reaction between Sn-xCu ( x = 0.1, 0.2, 0.3, 0.4 wt.% ) solder alloys and Ni at 160, 180, 200, 225℃ was studied to reveal the effect of temperature on the critical Cu Concentration in solders. Recently, using thermodynamic calculation, predicted that the critical Cu concentration was in fact a strong fuction of temperature. It is experimentally verified that this critical Cu concentration indeed decreases with decreasing temperature.