Failure Mechanism of Electromigration for Flip-Chip SnAg3.5 Solder Joints
Tung Liang Shao
|摘要:||本文探討覆晶無鉛錫銀銲錫接點於150℃的溫度下，不同的通電平均電流密度1×104 A/㎝2及5×103 A/㎝2之電遷移行為及其破壞機制。在上述不同的通電平均流密度狀況下，觀察到不同的失效模式。當通電平均電流密度為1×104 A/㎝2時，銲錫接點於晶片側陰極端或陽極端皆會發生破壞。然而，當通電平均電流密度為5×103 A/㎝2時，銲錫接點破壞只會發生在晶片側陰極端。而為了更進一步瞭解在銲錫接點的電流聚集效應，利用有限元素法進行3度空間之電流密度模擬分析。發現於鋁導線與銲錫接點交接處，銲錫接點局部電流密度可高達1.24×105 A/㎝2。此外亦針對不同通電平均電流密度1×104 A/㎝2及5×103 A/㎝2 狀況，實施銲錫接點之溫度與溫度梯度量測。當通電平均電流密度為1×104 A/㎝2時，銲錫接點因焦耳溫升效應溫升值量可高達至54.5℃，而對應溫度梯度亦高達365 ℃/cm。焦耳溫升效應在電遷移破壞機制亦扮演一個重要的角色。以下將針對不同的失效模式，提出對應的電遷移破壞機制。|
Electromigration behavior of lead-free SnAg3.5 solder joints was investigated under the average current densities of 1 □ 104 A/cm2 and 5 □ 103 A/cm2 at 150℃. Different failure modes were observed for the above two stressing conditions. When stressed at 1 □ 104 A/cm2, damage occurred in both anode/chip side and cathode/chip sides. However, failure happened only in the cathode/chip under the stressing of 5 □ 103 A/cm2. Three-dimensional simulation of current density distribution by finite element method was performed to provide better understanding of current crowding behavior in the solder joint. The local maximum current density of flip chip solder joint was as high as 1.24×105 A/㎝2 under the stressing of 1 □ 104 A/cm2. And the location of the maximum current density occurred in the vicinity of the Al entrance into the solder joint. In addition, both temperature increases and thermal gradients were measured during the two stressing conditions. The measured temperature increase due to Joule heating was as high as 54.5 ℃, and the thermal gradients reached 365 ℃/cm when stressed by 1 □ 104 A/cm2. Joule heating plays an important role in the failure mechanism during higher current stressing. Possible mechanisms responsible for the different failure modes are proposed.
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