覆晶封裝銲錫接點的電遷移與應力遷移行為之研究

Abstract

覆晶封裝銲錫接點的電遷移是一個重要的可靠度議題，是故了解覆晶封裝銲錫接點內的電流密度及溫度分佈相當重要。另外，本研究第一個觀測到銲錫內有應力遷移現象，並導致大裂縫生成。我們利用實驗觀測，配合有限元素分析法與理論計算模型研究覆晶封裝銲錫接點的電遷移與應力遷移行為。 本研究探討了銅柱金屬墊層對於覆晶封裝銲錫接點內電遷移的電流聚集效應與焦耳熱效應的影響。三維電流密度分佈的模擬結果顯示銅柱金屬墊層改變了電流聚集效應在銲錫接點內的位置以及嚴重程度，進而改變了兩種不同銲錫接點結構的電遷移破壞模式。經由紅外線熱像儀的觀察，銅柱金屬墊層在降低銲錫接點內電流聚集效應的同時也減少了銲錫接點在通電時所產生的焦耳熱。因此，銅柱金屬墊層有助於提升銲錫接點的電遷移壽命。另外，孔洞生成與金屬墊層消耗兩種電遷移破壞模式皆出現在無溫度梯度的錫銀銲錫接點中。藉由理論模型計算錫原子在介金屬化合物與銲錫界面上電遷移通量和化學勢能通量的差異，若通往陽極端的電遷移通量較大，破壞模式為孔洞生成；而當通往陰極端的化學勢能通量較大，則破壞模式為金屬墊層消耗。且此理論計算模型與實驗結果吻合。 另一方面，應力遷移亦會導致銲錫接點的破壞。錫鉛複合銲錫接點在經過500週期的溫度循環測試後，應力梯度驅使錫原子移動，導致非等向性長條狀的錫晶粒粗化並聚集在銲錫接點的頸縮處。接著，由熱應力引起的裂縫便沿著長條狀的錫晶粒與富鉛相界面傳播。在經過14410週期的溫度循環測試後，裂縫會擴大並延伸穿過整個銲錫接點。應力模擬結果顯示銲錫接點的頸縮處是張應力區域，且裂縫的生成起始於錫鉛的界面上。

Electromigration (EM) has been an important reliability issue in flip-chip solder joints. Thus, to investigate the current density and temperature distribution in a solder joint is of great significance. In addition, we first reported stress migration in solder joints and it induced large cracks. In this study, we studied the EM and stress migration experimentally and adopted finite element analysis and theoretical analysis to provide further understanding of the two issues. Influence of Cu column under-bump-metallizations (UBMs) on current crowding and Joule heating effects of electromigration in flip-chip solder joints have been investigated. A three-dimensional simulation of the current density distribution was performed to provide a better understanding of the current crowding behavior, which was found to account for the different failure modes for the two kinds of solder bumps. One more important finding is, as confirmed by infrared (IR) microscopy, that the alleviation of current crowding by Cu column UBMs also helped decrease Joule heating effect in solder bumps during current stressing. Therefore, the measured failure time for the solder joints with Cu column UBMs appears to be much longer than that of the ones with the 2-μm Ni UBMs. In addition, void formation and UBM consumption failure mechanisms occurred respectively at the stressing conditions both without a thermal gradient in the SnAg solder joints with 5-μm-Cu/3-μm-Ni UBM measured by IR microscopy. We proposed a model considering the flux divergence at the intermetallic compound/solder interface to calculate the Sn EM fluxes toward the anode side and the chemical potential-driven fluxes toward the cathode side. UBM consumption is responsible for the failure when the Sn chemical potential flux surpasses the EM flux. Yet, voids formed at the interface when the trend reverses. This model successfully explains the experimental results. On the other hand, stress migration results in large cracks in solder joints. After 500 cycles of temperature cycling tests (TCTs) between -55 and 125 °C in SnPb composite solder joints, the Sn grains coarsened and developed anisotropic stripes close to the necking site in the solder joint because of stress-induced atomic migration. Then, cracks triggered by thermal stress were observed to propagate along the Sn stripe interfaces. After a prolonged 14410 cycles of TCT, the cracks expanded across the entire solder joint.