Study of Electromigration in Flip Chip Solder Joints under Extra High Current Density with Temperature Control

隨著電子產品微小化且具備更強大功能之需求，封裝方式朝著高I/O數發展已為必經之路。於新型態封裝技術中，覆晶(flip-chip)封裝技術佔有極大的技術優勢，目前已成為高階封裝技術的最佳選擇。然而電子元件朝著輕薄短小的趨勢發展，將導致覆晶銲點內電流密度的提升。在此情況下，電遷移對覆晶銲點可靠度之影響將不容忽視，並且是一個非常重要的研究主題。 本研究將探討在超高電流密度(3.5×10^4 ~ 6.0×10^4 A/cm^2)下電遷移對覆晶銲點之影響。在過去的研究中，通電衍生的焦耳熱效應，使得晶片的溫度與所施加的電流密度互依互存、相互影響，因此兩變數無法獨立控制。而在超高電流密度之下，將更受到焦耳熱效應的影響，使用不同的電流密度亦將造成不同的晶片溫度，兩變數將同時影響實驗結果。然而若能分別控制晶片溫度與電流密度兩參數，將能有效釐清電遷移對覆晶銲點之影響及其機制，並得知兩參數各自扮演的角色及其重要性。因此，本研究將利用一水冷式散熱控溫裝置，來達到不同電流密度下獨立精確控制晶片溫度的效果。實驗所使用的試片為Ni(2 micron)/Sn2.6Ag/Cu之覆晶結構。 實驗的結果顯示未加裝散熱模組的試片，通電過程中晶片的溫度不斷地快速上升，當電流密度為4.5×10^4 A/cm^2時，因為大量焦耳熱的關係，導致試片在短短10分鐘便快速失效。加裝散熱模組的測試中，試片的晶片溫度可於通電的過程中保持定值，並且在相同的電流密度下，試片有較長的壽命(935小時)。由此結果證實，加裝散熱模組的確能有效分別控制晶片溫度與電流密度。 藉由散熱模組的輔助，本研究亦進一步探討電流密度及晶片溫度兩參數各自對覆晶銲點通電測試的影響。在固定晶片溫度下，覆晶銲點的壽命隨著電流密度下降而有所延長；在固定電流密度下，不同晶片溫度將造成覆晶銲點內微結構發展機制有所改變。To meet the requirement for decreasing packaging size, the electron current density through flip chip solder joints has to increase with every new generation of devices. At this condition, electromigration in flip chip solder joints become a serious issue. In this research, electromigration in flip chip solder joints under extra high current density (3.5×10^4 ~ 6.0×10^4 A/cm^2) is studied. At such a high current density level, due to Joule heating, the joint temperature is strongly coupled to the applied current density. Accordingly, it is highly desirable to have the capability to decouple the chip temperature and the current density. In this study, a water cooling module with a PID controller was devised for controlling the chip temperature. The configuration of flip chip solder joints used in this research was Ni(2 micron)/Sn2.6Ag/Cu structure. Without the cooling module, the chip temperature increased rapidly with the applied current. When the current density reached 4.5×10^4 A/cm^2, a rapid failure caused by excessive Joule heating was observed only after 10 min of current stressing. With the cooling module attached, the chip temperature kept constant during current stressing, and the joint exhibited a much longer life (935 h) under 4.5×10^4 A/cm^2. It was successfully demonstrated that the cooling module was able to de-couple the applied current density and the chip temperature. Since the chip temperature and the current density are successfully de-coupled, the effects of these two parameters can be studied independently. At constant chip temperature test, the failure time for flip chip solder joints increased when the current density decreased. At constant current density test, the mechanism was different when the chip temperature changed