焦耳熱效應對覆晶銲錫電遷移可靠度之影響

Abstract

本研究主要是對於影響覆晶銲錫接點可靠度的電遷移(eleectromigration)與熱遷移(thermomigration)現象來做探討。當電流密度與溫度梯度達到一個臨界值時，會產生電遷移(eleectromigration)與熱遷移(thermomigration)的現象，本研究中觀察到伴隨這些現象的產生，可以發現UBM的消耗與介金屬化合物(Intermetallic compound)產生，最後因為電流擁擠效應(current crowding effect)在鋁導線與銲錫接點處生成孔洞造成後來電阻產生改變，在最後階段接點溫度越來越高，並在鋁導線與UBM入口發現一熱點(hot spot)的產生，造成整個接點的破壞導致可靠度下降的問題。 本研究中使用紅外線熱像儀來量測覆晶封裝銲錫接點在加速電遷移之下的溫度分佈、利用X-ray可以清楚的觀測孔洞在覆晶銲錫接點中的形狀，並且觀察到孔洞如何的生成與推進、利用凱文銲錫球結構（Kelvin bump probes）的精確性，確實分辨出電遷移造成的不同階段孔洞成長、最後利用3-D電腦模擬來分析出覆晶結構之焦耳熱效應。經實驗的結果發現鋁導線的尺寸大小對於焦耳熱效應扮演相當重要的角色。在相同的通電參數下，較窄寬度、較長的鋁導線覆晶接點具有比較嚴重的焦耳熱效應與較高的溫升。這些結果顯示出，由於鋁導線對整個迴路是最主要的發熱源，理論上可以造成的熱遷移溫度梯度也會比較嚴重，所以mean-time-to-failure (MTTF)和鋁導線的尺寸有著相當大的關聯。在本研究中發現，在通電的過程當中，孔洞的形成時間(incubation time)大約在20小時左右，結果顯示出，在電遷移的效應之下，孔洞的形成在很初期的階段就已經產生，這可以提供對於孔洞在銲錫中的成長機制有更深一層的了解。除此之外，由3-D電腦模擬來分析出覆晶結構之焦耳熱效應，並且發現在鋁導線與UBM入口在通電的過程中會有一個熱點(hot spot)產生，並且會隨著外加電流的提高而有更高的溫昇，這個熱點的存在對於覆晶結構有加速破壞的影響。特別是在陰極端產生加速產生孔洞導致覆晶結構沿著界面破壞。根據上述之結果，來提出降低電遷移與熱遷移的現象，並減緩熱點(hot spot)對於元件中造成的影響來提昇覆晶銲錫接點的可靠度。

Joule heating effect in solder joints was investigated using thermal infrared microscopy and modeling in this study. It is found that the dimension of the Al-trace and silicon chip has significant influence on the electromigration failure time. With the increase of applied current, the temperature increased rapidly due to Joule heating. Furthermore, modeling results indicated that a hot spot existed in the solder near the entrance point of the Al trace, and it became more pronounced as the applied current increased. The temperature difference between the hot spot and the solder was as large as 9.4 □C when the solder joint was powered by 0.8 A. This hot spot may play an important role in the initial void formation during electromigration. In additional, x-ray microscopy and Kelvin bump probes were employed to investigate void nucleation and propagation during electromigration in solder joints. The shape of the voids at various stages can be clearly observed. The voids appeared to be quite irregular when they propagated to deplete the contact opening. The growth velocity was measured to be 1.3 □m/h at early stages under 6.5□103 A/cm2 at 150□C, and it decreased to 0.3 □m/h at later stages. Formation of intermetallic compound (IMC) and compositional changes at the interface of solder/IMC on the chip side were speculated to be responsible for the retarded growth rate at later stages.