전해도금 Cu와 Sn-3.5Ag 솔더 접합부의 Kirkendall void 형성과 충격 신뢰성에 관한 연구
A study of Kirkendall Void Formation and Impact Reliability at the Electroplated Cu/Sn-3.5Ag Solder Joint
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초록
Kirkendall void는 전해도금 Cu/Sn-Ag 솔더 접합부에서 형성되었으며 Cu 도금욕에 함유되는 첨가제에 의존한다. 첨가제로 사용된 SPS의 함량의 증가와 함께 150oC에서 열처리 후 많은 양의 Kirkendall void가 Cu/Cu3Sn 계면에 존재하였다. AES 분석은 void 표면에 S가 편석되어 있음을 보여주 었다. Cu/Cu3Sn 계면을 따라 파괴된 시편에서 Cu, Sn, S peak만 검출되었고 AES 깊이 프로파일에서 S 는 급격하게 감소하였다. Cu/Cu3Sn 계면에서 S 편석은 계면에너지를 낮추고 Kirkendall void 핵생성을 위한 에너지장벽을 감소시킨다. 낙하충격시험은 SPS를 사용하여 도금된 Cu의 경우 Kirkendall void가 형 성된 Cu/Cu3Sn 계면에서 파괴가 진행되고 급격하게 신뢰성이 감소됨을 보였다.
Abstract
A noticeable amount of Kirkendall voids formed at the Sn-3.5Ag solder joint with electroplated Cu, and that became even more significant when an additive was added to Cu electroplating bath. With SPS, a large amount of voids formed at the Cu/Cu3Sn interface of the solder joint during thermal aging at 150C. The in-situ AES analysis of fractured joints revealed S segregation on the void surface. Only Cu, Sn, and S peaks were detected at the fractured Cu/Cu3Sn interfaces, and the S peak decreased rapidly with AES depth profiling. The segregation of S at the Cu/Cu3Sn interface lowered interface energy and thereby reduced the free energy barrier for the Kirkendall void nucleation. The drop impact test revealed that the electrodeposited Cu film with SPS degraded drastically with aging time. Fracture occurred at the Cu/Cu3Sn interface where a lot of voids existed. Therefore, voids occupied at the Cu/Cu3Sn interface are shown to seriously degrade drop reliability of solder joints.
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