添加微量鈀對錫銀銅鈀/銅銲接點的微結構、界面反應與衝擊可靠度之影響

Effect of Pd Doping on Microstructure, Interfacial Reaction and Impact Reliability in Sn3.0Ag0.5Cu-xPd/Cu Solder Joints

莊琇琄(Hsiu-Chuan Chuang)

指導教授 ： 杜正恭

國立清華大學/工學院/材料科學工程學系/碩士(2012年)

摘要

以銅基板為主的銲接點最常遇到的問題為界面過量的介金屬化合物生成，以及因元素擴散係數不同而導致孔洞的出現，這兩點嚴重影響到銲接點對機械性質的表現，為了提升銲接點的可靠度，目前有許多文獻利用添加微量第四元素的方式進一步抑制介金屬化合物和孔洞的生成。以鎳基板為主的銲料凸塊之表面處理技術中，鈀層的導入能明顯的改善無電鍍鎳浸金(ENIG)中的”黑墊(black pad)”問題，因此無電鍍鎳鈀浸金(ENEPIG)已被廣泛用於以鎳基板為主的表面處理，然而鈀如何影響以銅基板為主的銲接點之介金屬化合物形成以及銲接點的強度仍是未知的。本研究將鈀添加至錫銀銅銲料中，並將銲料與銅基板進行迴焊(reflow)，於不同溫度下進行熱處理(aging)，觀察其生成的介金屬化合物厚度與成分的變化。此外，利用高速撞擊測試(high-speed impact test)去評估微量鈀摻雜對接點可靠度的影響。
銅基板與錫銀銅鈀銲料接合後，鈀原子於一次迴焊後擴散至界面，並生成(Cu,Pd)6Sn5，同時細化界面(Cu,Pd)6Sn5晶粒，鈀的添加能抑制介金屬化合物生成。然而在經過三次迴銲後，參雜鈀之銲接點其界面介金屬化合物的厚度與未參雜之銲接點相當，雖然如此，參雜鈀之銲接點在高速撞擊測試中仍有優異的表現。在長時間熱處理環境下，鈀參雜於銲料中對於抑制介金屬化合物生成之行為趨於顯著，同時也抑制孔洞的生成。從熱力學與動力學的觀點提出鈀影響界面反應之可能原因，可更進一步與場發射電子微探儀的分析結果相互印證。此外，經由高速衝擊測試後之斷裂面分析得知，界面介金屬化合物之機械性質會直接影響到銲接點對於機械測試的表現，再者由奈米壓痕測試，發現鈀的參雜能提升介金屬化合物之斷裂韌性，可有效阻擋裂痕延伸。
分析高速衝擊測試的結果，發現參雜鈀在迴銲後對於銲接點衝擊可靠度之提升最為顯著。基於業界成本考量，在降低鈀參雜量的同時保有鈀添加在銲料中對銲接點之可靠度表現的優異性，進而將鈀參雜在銲料薄片中，並與銅基板先進行一次迴銲後，錫銀銅銲料再經由第二次迴銲與銅板接合。結果發現參雜鈀之銲接點在衝擊測試的表現較未參雜之銲接點優異。根據這些研究結果可以得知錫銀銅鈀合金是個具有潛力，且適合應用於未來銲料設計的材料。

關鍵字

錫銀銅銲料 ； 鈀參雜 ； 介面反應 ； 衝擊測試 ； 銅基板

並列摘要

Rapid growth of intermetallic compounds (IMCs) and the formation of Kirkendall voids during solid reaction are critical issues for reliability of Cu-based solder joints. Adding minor 4th element into solder joints is proposed to improve the mechanical reliability of joints. Pd is another element of interest due to higher mechanical reliability of joints with Electroless Nickel/Electroless Palladium/Immersion Gold (ENEPIG) as compared to that with Electroless Nickel/Immersion Gold (ENIG). However, for Cu-based solder joint, the Pd distribution and detailed mechanism how Pd influences the interfacial reaction and reliability of joints are not yet proposed. In this study, Sn3.0Ag0.5Cu (SAC305) solder doped with 0~0.5 wt.% Pd was used to reflow with Cu pad. For liquid-state reaction, Pd tended to dissolve into interfacial Cu6Sn5 and refine the grain size of Cu6Sn5. After multiple reflows, the IMC growth rate was enhanced by Pd addition due to the higher number of diffusion channels between Cu6Sn5 scallops. Nevertheless, the impact test showed that the Pd doping would increase the bonding strength of the Pd-doped joints.
In solid-state reaction, the growth of Cu3Sn and Kirkendall voids were suppressed by the Pd doping into joints. X-ray elemental mapping and quantitative analysis demonstrated that Pd atoms were accumulated at the lower part of (Cu,Pd)6Sn5 layer. Based on thermodynamic calculation, the Gibbs free energy of (Cu,Pd)6Sn5 is more negative than that of Cu6Sn5, indicating that (Cu,Pd)6Sn5 is thermodynamically stable than Cu6Sn5. Therefore, the Pd-enriched (Cu,Pd)6Sn5 layer is regarded as a barrier to suppress the growth of Cu3Sn and the formation of Kirkendall voids. The detailed mechanism of Pd influence on interfacial reaction in solder joints was probed and discussed. In addition, the impact test was also employed to evaluate the Pd effect on the mechanical reliability. By the suppression of the Cu3Sn growth and the prevention of Kirkendall voids formation, the Pd-doped joints exhibits higher impact force than non-doped joints.
The crack propagation via interfacial IMC indicated that the mechanical properties of Cu6Sn5 and (Cu,Pd)6Sn5 might dominate the performance of solder joints in impact testing. Furthermore, the indentation data showed that (Cu,Pd)6Sn5 exhibited higher fracture toughness and lower Young’s modulus than Cu6Sn5. It is argued that the difference in mechanical properties between Cu6Sn5 and (Cu,Pd)6Sn5 was believed to be the main cause of higher impact reliability in the Pd-doped solder joints.
To amplify the advantage of the Pd doping during soldering with reducing amount of Pd addition into solder balls, the Pd-doped solder-on-pad (SAC-Pd SOP) surface finish was developed. The primary Cu6Sn5 and (Cu,Pd)6Sn5 formed at the interface of SAC SOP and SAC-Pd SOP system after 1st reflow, respectively. With the attachment of SAC305 solders during 2nd reflow, the IMC thickness decreased in SAC/SAC SOP joints owing to the increasing volume of solder matrix. However, the IMC thickness increased in the Pd-contained joints, resulting from thinner initial intermetallic thickness and stabilization of Pd-dissolved Cu6Sn5. Nevertheless, the IMC thickness in the Pd-contained joints was smaller than that in SAC/SAC SOP joints. In addition, the correlation between microstructure and related impact reliability in the SAC/SAC-xPd SOP joints was addressed and proposed. Finally, this study aimed to evaluate the potential application of novel Pd-doped lead-free solders for future solder designs.

並列關鍵字

SnAgCu solder ； Pd doping ； Interfacial reaction ； Impact test ； Cu pad

參考文獻

67.T.B. Massalski; H. Okamoto, P.R. Subramanian and L. Kacprzak, “Binary alloy phase diagrams”, ASM Int., Materials Park, Ohio, (1990) 1481.

1.J. Liu, T. Shi, Q. Xia, G. Liao, “Flip chip solder bump inspection using vibration analysis”, Microsyst. Technol. 18 (2012) 303.

4.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.

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.