Trends of electronic packaging have been focused on the development of flip chip bonding technology but wire bonding still remains an important chip interconnection technology because of its cost effectiveness. Besides, fine wires with diameters less than 1 mil. have recently been developed to meet the requirement of fine pitch application. One important issue is the interfacial reaction between Au wire and Al pads. Due to the reduction of ball bonding area, bond reliability becomes more susceptible to the Au/Al IMCs (Intermetallic compounds) formation. One of main purposes of the addition of alloying elements to Au wire is to improve bond reliability but the role of alloying elements during Au-Al interfacial reactions has not been fully understood.
The objectives of this study are 1) to compare the effects of addition of alloying elements on interfacial reactions and 2) to investigate which alloying element improves bond reliability more effectively. In this study, seven types of Au wire - 4N Au wire, Au-0.05wt.%Cu wire, Au-0.5wt.%Cu wire, Au-1wt.% Cu wire, Au-0.05wt.%Pd wire, Au-0.5wt.%Pd wire, and Au-1wt.%Pd wire were bonded on 1 μm-thick Al pads. These wire-bonded samples were thermally aged at $175^\circ C$ and $200^\circ C$ to 1000 hours. Scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), Electron-Micro Probe Analysis (EPMA), and ion milling were performed to identify the phase of intermetallic compound (IMC) and to observe differences of interfacial morphology caused by the difference of alloying elements. Wire pull test (WPT) was performed to investigate effects of Cu and Pd addition on bond reliability.
According to the results, Au-Al interfacial reactions were affected by Cu and Pd addition and different interfacial morphologies were observed after thermal aging except for Au-0.05wt.%Cu wire and Au-0.05wt.%Pd wire. Cu and Pd addition slowed down Au-Al interfacial reactions and crack propagation. Especially, crack propagation in Au-0.5wt.%Cu wire and Au-1wt.%Cu wire was slowest. Au-0.05wt.%Cu wire and Au-0.05wt.%Pd wire showed similar IMCs growth behavior to 4N Au wire.
WPT results showed that Cu and Pd improved bond reliability. Especially, it was proved that Cu was more effective element to improve bond reliability than Pd except for 0.05wt.% addition of alloying elements. These results were well corresponded with results of interfacial phenomena such as crack propagation.
I/O수의 증가 및 I/O간의 피치의 미세화로 wire bonding 기술 또한 fine wire에 초점을 두고 있다. 그러나 wire의 직경의 감소가 Au-Al 본딩 계면에서의 신뢰성을 저하시키므로 이러한 문제의 해결 방안 중의 하나로 합금원소를 첨가시키는 방법이 사용되고 있다. 본 연구에서는 4N Au wire를 기본으로 하여, Cu과 Pd의 합금원소를 첨가한 Au-0.05wt.%Cu wire, Au-0.5wt.%Cu wire, Au-1wt.%Cu wire, Au-0.05wt.%Pd wire, Au-0.5wt.%Pd wire, Au-1wt.%Pd wire를 175도와 200도에서 1000시간까지 고온 aging처리를 하여 이들 wire와 Al pad간의 계면 반응 관찰 및 접합 신뢰성을 측정하였다. 본 연구를 통해 Cu와 Pd의 첨가가 Cu-rich layer와 Pd-rich layer를 Au와 금속간화합물의 계면에 형성시켜 Au-Al 계면 반응을 느리게 하였으며, crack 형성 또한 느리게 함을 알 수 있었다. 또한, Cu와 Pd을 첨가한 합금 Au wire가 비합금 Au wire에 비해 접합신뢰성이 향상되었으며, Cu가 Pd에 비해 접합신뢰성 향상에 더 효과적인 합금원소임이 밝혀졌다.
