利用高速擺錘衝擊與摔落測試評估單邊與雙邊接合銲料接面之界面反應、元素分佈、晶向生長結構與 機械性質可靠度

Evaluation of Interfacial Reaction, Elemental Distribution, Grain Orientation and Mechanical Reliability via High Speed Impact and Drop Vehicle with the Systematic Attachment and Assembly Solder Joints

林修民(Hsiu-Min Lin)

指導教授 ： 杜正恭

國立清華大學/工學院/材料科學工程學系/博士(2014年)

摘要

藉由高速擺錘衝擊與摔落測試等不同的機械性質評估，嘗試鑑別銲料接面之機械性質與可靠度準則，利用一系列具不同鈀含量之單邊與雙邊銲料接面試片進行，不同的機械性質表現可利用統計歸納的方式進行比較。在高速衝擊測試下，含0.06 μm鈀層厚度的SAC305/ENEPIG銲料接面表現較佳，歸因於僅單一Cu6Sn5相生成在界面當中。然而，在利用摔落測試比較測量不同的雙邊接合試片時，含0.1 μm鈀層厚度的ENEPIG/SAC305/immersion Sn銲料接面呈現較佳的機械性質表現，可歸因於其較薄且層狀結構之介金屬化合物生長在界面。可預期的是，透過比較單邊及雙邊接合試片，並分別利用高速衝擊與摔落機械測試方式，可嘗試建立機械性質可靠度測試之準則。此外，在單邊與雙邊接合試片中，亦對ENEPIG表面處理的最理想鈀層厚度沉積進行驗證與確認。
由於機械應力累積的關係，經由摔落測試後的銲料接面大部分的失效區域都會靠近PCB(銅)板端的界面。其中裂縫與鈀添加後微量元素的分佈情形之關聯性，可藉由電子微探儀(FE-EPMA)的元素分佈圖進行確認。當ENIG與ENEPIG的雙邊接面試片在摔落測試下，裂縫會朝向最弱的界面進行延伸，即在Cu6Sn5和Cu3Sn兩相之間。介金屬化合物的形貌與厚度會主導裂縫的初始位置與持續延伸的狀況。在經過熱處理後，ENEPIG (Pd 0.1)/SAC305/immersion Sn的雙邊接合銲料接面呈現出層狀與較薄的介金屬化合物生長，使得裂縫延伸至較弱的界面有較長的距離，因此，其摔落測試呈現較佳的機械強度表現。
另外，銲錫接點中的晶粒結構與方向性是目前相當重要的可靠度議題，因此本研究利用背向電子繞射儀(EBSD)觀察介金屬化合物與銲錫，其晶粒結構的變化和晶粒的生長方向性將是研究的重點。在本研究中發現，介金屬化合物在ENEPIG(鎳)端及浸錫(銅)端的生長方式及優選方向有所不同，此一差異可能肇因於雙邊接合試片中兩端不同元素的交互擴散影響所至。為證實熱與機械應力是否會影響銲錫的晶粒結構與優選方向，分別觀察ENIG與ENEPIG的雙邊接面試片中經過熱處理與摔落測試後之銲錫。結果得知，鈀元素會穩定銲錫的晶粒結構與方向，在承受熱與機械應力的衝擊下，抑制減緩晶粒細化、再結晶、與其變形。特別在剛完成迴焊過程後，銲錫含鈀的晶粒形成單晶的晶粒結構，且銲錫之c軸大多呈現平行於基板的狀態。最後，針對雙邊接合試片中影響介金屬化合物生長方向的原因進行探討，並與所觀察到之晶粒結構與方向差異相關連，提出可能的反應機制。

關鍵字

高速衝擊試驗 ； 摔落測試 ； 元素再分佈 ； 破斷面統計 ； 裂縫起始與延伸 ； 晶向生長與結構

並列摘要

The criteria of mechanical reliability in solder joints can be identified and described by comparative evaluation via drop test and high speed pendulum impact test. Systematic samples of assembly and attachment joints with various Pd additions were employed and investigated in this study. The statistical values of mechanical performances were calculated and compared. Better high speed impact performance of SAC305/ENEPIG attachment joints with 0.06 μm Pd layers was confirmed owing to the single Cu6Sn5 phase growth. However, the comparative measurement of the better performance on drop testing exhibited in ENEPIG/SAC305/immersion Sn assembly joints with 0.1 μm Pd layers deposit resulted from the thinner and layer-type IMC growth. It is expected that through comparison between impact and drop test in mechanical reliability, a criterion of joints reliability can be established. Besides, the optimal Pd layer deposit for the ENEPIG surface finish in the attachment and assembly solder joints was demonstrated and confirmed.

Most failure regions existed at interface near PCB (Cu) side during mechanical drop testing due to the stress accumulation. The correlation between the cracks generation and Pd addition was established on the basis of the elemental X-ray color mapping via Field-Emission Electron Probe Microanalyzer (FE-EPMA). In the ENIG and ENEPIG assembly joints, cracks propagation might travel toward the weakest interface, which located at interface between Cu6Sn5 and Cu3Sn phases during drop testing. The IMC morphology and thickness might dominate the cracks initiation and propagation. Joints with layer-type and thinner IMC growth after thermal treatment revealed the longer crack propagation in ENEPIG (Pd 0.1)/SAC305/immersion Sn assembly joints. The better drop performance were confirmed in the ENEPIG (Pd 0.1) assembly joints.

Besides, the crystallographic orientation of IMCs and β-Sn in the ENIG and ENEPIG assembly joints was investigated. With the aid of EBSD analysis, various grain structures and preferred growth orientation of IMCs and β-Sn were observed. The distinctive growth behaviors of intermetallic compound on the ENEPIG UBM and immersion Sn substrates were associated with the cross-interaction of minor Cu, Ni and Pd elements. To verify that the thermal and mechanical influence would affect the β-Sn grain variation, the ENIG and ENEPIG assembly joints were employed. It is noted that Pd elements might stabilize the grain orientation of β-Sn, inhibiting the grain refinements, recrystallization and deformation during the thermal and mechanical stressing impact. Especially the single β-Sn grain texture with parallel c-axis to the substrate at as-reflowed stage was probed and demonstrated. Finally, the correlation between microstructure variation and grain orientation was investigated and discussed. The possible mechanism was also proposed.

並列關鍵字

High speed impact test ； Drop test ； Elemental redistribution ； Statistic of fracture surface ； Cracks initiation and propagation ； Grain orientation

參考文獻

132. Y. Tian, L. Niu, and C. Wang, “Analysis of Cu6Sn5 grain orientations in Sn3.0Ag0.5Cu lead-free solder joints”, 2011 International Conference on Electronic Packaging Technology & High Density Packaging Proceeding, p.353.

1. L.F. Miller, “Controlled collapse reflow chip joining”, IBM J. Res. Develop. 13 (1969) 239.

4. D.G. Kim, J.W. Kim, J.G. Lee, H. Mori, D.J. Quesnal, and S.B. Jung, “Solid state interfacial reaction and joint strength of Sn–37Pb solder with Ni–P under bump metallization in flip chip application”, J. Alloys Compd. 395 (2005) 80.

5. C.H. Wang, H.T. Shen, and W.H. Lai, “Effective suppression of electromigration-induced Cu dissolution by using Ag as a barrier layer in lead-free solder joints”, J. Alloys Compd. 564 (2013) 35.

6. C.A. Palesko and E.J. Vardaman, “Cost Comparison for Flip Chip, Gold Wire Bond, and Copper Wire Bond Packaging”, Electronic Components and Technology Conference (2010) 10.