錫銀銅銲料與無電鍍鎳(磷)/鈀/金底層金屬之微觀結構、可靠度測試及結構方向性

Microstructure Evolution, Reliability Test and Grain Orientation of SnAgCu Solder Joint with Electroless Ni(P)/Pd/Au Under Bump Metallurgy

曾建富(Chien-Fu Tseng)

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(2013年)

https://doi.org/10.6843/NTHU.2013.00028

摘要

近年來，無電鍍鎳鈀浸金(ENEPIG)已被廣泛應於用電子封裝中的金屬銲墊之表面處理。ENEPIG的盛行可歸因於其眾多優勢，其中，最重要的莫過於ENEPIG可抑制在無電鍍鎳浸金(ENIG)發生的”黑墊”(Black pad)問題。添加一層鈀於無電鍍鎳層與置換金層中，可有效抑制無電鍍鎳層於置換金溶液中發生的過度腐蝕現象。然而，鮮少有研究對於此兩類表面處理的界面反應比較及微結構差異進行詳細探究。因此，本研究將針對ENIG和ENEPIG這兩種表面處理進行研究，指出鈀元素造成的微結構差異與機械性質影響，並提出合理之生成機制。

與錫銀銅銲料接合並熱處理後，ENEPIG接點中的介金屬化合物(IMC) (Cu,Ni,Pd)6Sn5的生長明顯地被抑制。此外，Cu6Sn5轉變為Ni3Sn4的相轉變亦被抑制，此一相轉變的抑制可進一步減緩Ni3P層中孔洞的生成。為了進一步探究(Cu,Ni,Pd)6Sn5的生長動力學，ENIG與ENEPIG接點皆在銲料液態下進行反應，發現鈀在液態反應初期可扮演異質成核點，進而降低(Cu,Ni,Pd)6Sn5生長的活化能。較低的活化能可進一步確保接點界面無相轉變的發生。另外，將針對鈀元素對於介金屬化合物的生長影響進行探討，以及其對於Ni3Sn4相抑制之機制。

為了進一步驗證微結構差異是否直接到影響界面強度，本研究藉由高速衝擊試驗來評估銲錫接點的強度。從結果可知，ENEPIG接點強度隨著迴焊次數增加而下降的程度遠比ENIG接點少，此優異的機械性質表現與破裂面的改變可歸因於針狀形貌的(Cu,Ni,Pd)6Sn5，並提供了interlocking的效果。針對機械性質的結果，將結合前面所提出的鈀元素對微結構的影響，提出詳細的機制探討

另外，銲錫接點的晶粒大小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ENIG/SnAgCu/Cu和ENEPIG/SnAgCu/Cu雙邊接合試片中的界面IMC。在本研究中可發現，IMC在銅端及鎳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。最後，將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，並與所觀察到之微結構差異相關連，提出可行之反應機制。

關鍵字

無鉛銲料 ； 電子封裝 ； 鎳鈀金表面處理 ； 介金屬化合物 ； 微結構觀察 ； 可靠度測試 ； 結構方向性 ； 背向電子繞射儀

並列摘要

Electroless Ni(P)/electroless Pd/immersion Au (ENEPIG) is widely used as surface finish for metal bond pad in the electronic packaging industries. The widespread adoption of ENEPIG is attributed to its many advantages, and the most important one, it resolves the so-called “black pad” reliability problem in the electroless Ni(P)/immersion Au (ENIG) surface finish. The insertion of Pd layer is believed to relieve the corrosion of underneath Ni(P) layer from immersion Au plating solution. However, the complete interfacial reaction and comparison between these two surface finishes are still lacking in literature. Therefore, this study aims to probe the microstructure variation induced by the Pd addition and to discuss the possible mechanism.

The interfacial reactions of Sn-3.0Ag-0.5Cu solder jointed with ENIG and ENEPIG were first to investigated. (Cu,Ni,Pd)6Sn5 grew rather slower in the ENEPIG samples among all aging condition as compared with ENIG. It was demonstrated that ENEPIG could inhibit the formation of Ni3Sn4, which then decreased the growth of columnar Kirkendall voids inside the Ni3P layer. In order to further explore the growth kinetics of (Cu,Ni,Pd)6Sn5, the liquid state reaction was thus investigated. Pd may act as heterogeneous nucleation sites in the initial soldering and lower the activation energy of (Cu,Ni,Pd)6Sn5, as compared to (Cu,Ni)6Sn5. The lower activation energy of (Cu,Ni,Pd)6Sn5 growth ensured that no phase transformation occurred in the SAC305/ENEPIG joints, which may explain why the phase transformation was inhibited in the ENEPIG joints. The detailed impacts of Pd on the growth kinetics of IMC formation was investigated and discussed as well as the mechanism of Ni3Sn4 suppression.

To verify that the microstructure variation would affect the interfacial strength, the high speed impact test was utilized. The impact energy of ENEPIG joints declined slower than that without Pd-doped after prolonged reflow. The enhanced impact strength and the transition of failure mode in the ENEPIG joints was attributed to the needle-like morphology of (Cu,Ni,Pd)6Sn5. The detailed mechanism of improved mechanical strength for solder joints with Pd dissolved was deliberately addressed and discussed regarding the distinct microstructural evolution in the ENEPIG joint.

Besides, the crystallographic orientation of ENIG/SnAgCu/Cu and ENEPIG/SnAgCu/Cu assembled solder joints was investigated. With the aid of EBSD analysis, various grain structures and preferred growth orientation of IMC on the Cu and Ni(P) substrates were observed. The distinctive growth behaviors of intermetallic compound on the Cu and Ni(P) substrates were associated with the cross-interaction of minor Cu, Ni and Pd elements. Finally, the correlation between microstructure variation and grain orientation was probed and discussed. The possible mechanism was also proposed.

並列關鍵字

Pb-free solder ； Electronic packaging ； ENEPIG surface finish ； Intermetallic compound ； Microstructure Evolution ； Reliability Test ； Grain Orientation ； EBSD

參考文獻

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

4. M.E. Loomans, S. Vaynman, G. Ghosh and M.E. Fine, “Investigation of multi-component lead-free solders.” J. Electron. Mater. 23 (1994) 741.

5. A.A. Liu, H.K. Kim, K.N. Tu and P.A. Totta, “Spalling of Cu6Sn5 spheroids in the soldering reaction of eutectic SnPb on Cr/Cu/Au thin films.” J. Appl. Phys. 80 (1996) 2774.

6. H.K. Kim, K.N. Tu and P.A. Totta, “Ripening-assisted asymmetric spalling of Cu-Sn compound spheroids in solder joint on Si wafer.” Appl. Phys. Lett. 68 (1996) 2204.

7. S.K. Kang, R.S. Rai and S. Purrshothaman, “Interfacial reactions during soldering with lead-tin eutectic and lead (Pb)-free, tin-rich solders.” J. Electron. Mater. 25 (1996) 1113.