覆晶銲錫之電遷移研究: 溫度對破壞機制的影響與電阻曲線之分析

Abstract

本論文研究在不同溫度造成的電遷移破壞模式，主要著重在Cu under-bump-metallization （UBM） 與銲錫界面的行為，實驗條件的溫度為126°C、136°C、158°C、172°C以及185°C。在較低溫126°C和136°C時，電遷移的破壞模式主要為void formation，Voids生成的位置為Cu6Sn5 intermetallic compounds（IMCs）與銲錫層界面，然而，將通電溫度提升至158°C以上之後，破壞模式會轉變為IMC formation為主，主要觀察到有大量的Cu UBM溶解和大量的Cu6Sn5生成，只有些許voids殘存在界面，對於破壞模式因為溫度變化而轉換的機制，提出一個考慮Cu6Sn5 IMC與銲錫界面電遷移flux模型來做解釋，由模型分析結果可以知道在低於131°C，離開此界面的電遷移flux會大於進入的flux，因此，Voids生成在界面處，然而，當溫度高於131°C，進入界面的flux反而會大於離開的flux，因此成功的利用此模型來解釋實驗上看到的結果。 另外，發現在160°C通以1.2 × 104 A/cm2 Sn-rich phase會生成在Cu-Sn-Ni IMCs內部，在通電之後Sn-rich phases生成在含有Cu UBM的銲錫接點陰極端，此現象的機制以Cu6Sn5 IMCs的分解來做解釋，在通電期間當Cu6Sn5 IMCs相變化成Cu3Sn時，Sn原子將被釋放出來，此時如果Cu的供應又受到限制，Sn-rich phases將會累積在Cu-Sn-Ni IMCs內部。 電阻曲線在偵測銲錫接點電遷移破壞中扮演非常重要的角色，一般來說，電遷移電阻曲線的行為在前中期階段通常呈現緩慢上升，在後期階段才會急遽上升，其曲線行為的表現模式為凹向上，然而，近年來的研究發現到銲錫接點會呈現凹向下的電阻曲線行為，對於這樣的行為已往還未有深入的解釋與了解，此篇研究中，實驗上發現當銲錫接點單以IMC formation為主要的破壞模式時，其電阻曲線會呈現凹向下，相對的，如果曲線呈現凹向上的趨勢增加，則在實驗中會看到是以void formation為主要的破壞模式，為了了解兩種曲線行為的成因，使用有限元素分析模擬void formation與IMC formation對電阻曲線行為的影響，依照模擬結果來解釋void formation造成電阻曲線凹向上的行為是由於後期階段銲錫接點導電的截面積縮小導致電流集中在部份的位置，因此電阻在後期會急遽升高，對於造成凹向下曲線行為的主因是由於較高電阻的Cu6Sn5 IMCs快速生長在電流密度集中的位置，又因為Cu6Sn5 IMCs的電阻率大Cu約9倍之多，因此，當電流密度集中區域的Cu與Sn反應生成Cu6Sn5 IMCs時，會造成電阻急遽上升，又因為在高電流密度區域的Cu與Sn反應成Cu6Sn5 IMCs對電阻造成的上升比低電流密度要來的大，因此IMC formation會形成凹向下的電阻曲線。 Cu3Sn是在銲錫接點通電之後常常發現的Sn-Cu compounds，在本篇的研究中，發現Cu3Sn會根據通電時銲錫接點在液態下或是固態下形成兩種不同的型態，在170°C下通以1.30 × 104 A/cm2，銲錫層可以轉成Cu3Sn的接點，當溫度提升到222°C通以2.27 × 104 A/cm2，銲錫接點會轉變成porous Cu3Sn結構，對於形成porous Cu3Sn，成功的使用相變化與side wetting的機制來解釋。

Temperature-dependent electromigration failure was investigated in solder joints with Cu metallization at 126°C, 136°C, 158°C, 172°C, and 185°C. At 126°C and 136°C, voids formed at the interface of Cu6Sn5 intermetallic compounds and the solder layer. However, at temperatures equal 158°C or greater than, extensive Cu dissolution and thickening of Cu6Sn5 occurred, and few voids were observed. We proposed a model considering the flux divergence at the interface. At temperatures below 131°C, the electromigration flux leaving the interface is larger than the in-coming flux. Therefore, voids formed at the interface. Yet, the in-coming Cu electromigration flux surpasses the out-going flux at temperatures above 131°C. This model successfully explains the experimental results. This study also examines the formation of Sn-rich phases in the matrix of Cu-Sn-Ni intermetallic compounds (IMCs) after current stressing of 1.2 × 104 A/cm2 at 160°C. The Sn-rich phases were formed at the cathode end of the solder joints with Cu metallization, and this formation was attributed to the decomposition of Cu6Sn5 IMCs. When the Cu6Sn5 IMCs were transformed into Cu3Sn during current stressing, Sn atoms were released. Due to the insufficient supply of Cu atoms, Sn atoms accumulated to form Sn-rich phases among the Cu-Sn-Ni IMCs. Resistance curves play a crucial role in detecting damage of solder joints during electromigration. In general, resistance increases slowly in the beginning, and then rises abruptly in the very late stage; i.e., the resistance curve behaves concave-up. However, several recent studies have reported concave-down resistance curves in solder joints with no satisfactory explanation for the discrepancy. In this study, electromigration failure mode in Sn2.5Ag solder joints was experimentally investigated. The bump resistance curve exhibited concave-down behavior due to formation of IMCs. In contrast, the curve was concave-up when void formation dominated the failure mechanism. Finite element simulation was carried out to simulate resistance curves due to formation of IMCs and voids, respectively. The simulation results indicated that the main reason causing the concave-down curve is rapid formation of resistive Cu6Sn5 IMCs in the current-crowding region, where resistivity is nine times larger than that of Cu. Therefore, when Cu reacted with Sn to form Cu6Sn5 IMCs, the resistance increased abruptly, resulting in the concave-down resistance curve. Cu3Sn was constantly found in the solder joint after current stressing. In this study, two different types of Cu3Sn formed according to the stressing temperature of solder joints. The solder joint was under 1.30 × 104 A/cm2 current stressing test at 170°C, the solder joint could transform to layer Cu3Sn joints. However, when the stressing temperature increased to 222°C and the current density was 2.27 × 104 A/cm2, an interesting porous Cu3Sn formed at the solder joint. The formation mechanism of porous Cu3Sn, Could be explained by the phase transformation and side wall wetting phenomenon.