The investigation of the alternative Pb-free solders to replace commonly used Pb-Sn solders has been initiated by environmental concerns over the use of lead in microelectronic packaging applications. In general, high-tin alloys are available as lead-free solder alloys, but there are few, if any, that replaces high-Pb solders which have high melting temperatures. One strong candidate is a eutectic 80Au-20Sn solder by weight (71Au-29Sn by molar content) that has a very attractive melting point of 280℃. It has been successfully used for highly reliable die attach and fluxless soldering in optoelectronic packaging. It has high thermal and electrical conductivity, corrosion resistance, no thermal fatigue and the possibility of fluxless soldering. Traditionally, the Ti/Pt under bump metallurgy (UBM) system has been used for the eutectic Au-Sn soldering. In this case, the interaction between Pt and Sn creates an excessive intermetallic compound (IMC) buildup such as PtSn and $PtSn_4$, which leads to a shift in the
eutectic Au-Sn ratio of the solder due to consumption of Sn near the Pt interface. As a result, a non-uniform joint structure is formed and it leads to degradation of the bonded chip performance. Therefore, several researches have been undertaken to replace the Pt-based UBM. Nickel is one of attractive candidates to replace the diffusion barrier metal because the Au-Ni phase diagram suggests that the solubility of Ni in Au is very limited at temperatures of
soldering and, Au and Ni are not dissolved into each other below 100℃. Moreover, Ni, as opposed to Pt, dissolves into a Sn-based solder at a very low rate and it has higher thermal and electrical conductivities than other barrier metals of Cr and Pt. The present work is undertaken to clarify the microstructural evolution of the eutectic Au-Sn solder joint interface.
Three kinds of experiments will be conducted. Firstly, the microstructural evolution of IMC between the eutectic Au-Sn and the pure Ni substrate (no phosphorus containing) will be examined with different soldering time and temperatures. Secondly, the reaction of the eutectic Au-Sn solder with the sputtered UBM (Au/Ni(V)/Al) will be studied with time at 300℃.
Finally, the reaction of the eutectic Au-Sn solder with the electroless Ni-P UBM (Au/Ni(P)/Al) will be studied with reflow times at 300℃.
In the case of the Ni substrate, two types of the intermetallic compound (IMC) were observed at the joint, $(Au,Ni)_3Sn_2$ and $(Au,Ni)_3Sn$. The shape of $(Au,Ni)_3Sn_2$ grains generally changed from a thin and long rod-type to a thick and short one with increased soldering temperatures.
The degree of the interfacial IMC buildup was nearly similar up to 32 min even if soldering was conducted at three different temperatures between 300℃ and 400℃.
Secondly, the reaction of the eutectic Au-Sn solder with the sputtered Ni UBM (Au/Ni(V)/Al) was studied at 300℃. By 20 sec soldering, the protective Au layer was dissolved away and the Ni(V) layer partially started to be dissolved into the solder and thus some of the Au reacted with the Al underlayer to form the $Au_8Al_3$ phase, which accompanied volume expansion at the joint. Thereby, the $(Au,Ni)_3Sn_2$ layer was lifted up and several interlocked $(Au,Ni)_3Sn_2$ grains were broken and separated at weak points along the joint interface.
Finally, the reaction of the eutectic Au-Sn solder with the electroless Ni-P UBM
(Au/Ni(P)/Al) will be studied with the number of reflow times at 300℃. After first reflow, there is no problem in solder joint. But the liquid solder which flew through the gap between Ni-P and $SiO_2$ at the end of UBM reacted with Al to form the $Au_8Al_3$ phase. $Au_8Al_3$ phase causes volume expansion at the solder joint. As a result, an UBM was spalled into the solder.
Unlike previous UBM structure, the failure at the solder joint was retarded in a modified Ni-P UBM structure.
공정 80Au-20Sn 솔더합금을 솔더링 시간과 온도를 달리하여 Ni 위에서 솔더링 하였다. 주사전자현미경(SEM)을 사용하여 계면에 생성된 IMC 의 조성, 상, 모양에 대해 조사하였다. 계면에는 $(Au,Ni)_3Sn_2$ 와 $(Au,Ni)_3Sn$ 의 두가지 IMC 가 생성되었다. 그 중 첫번째 생성된 IMC 인 $(Au,Ni)_3Sn_2$ 상은 솔더링 온도에 따라 모양의 변화가 관찰되었다. 두번째 실험으로 80Au-20Sn 솔더와 sputtered Ni UBM 의 반응, 세번째로 80Au-20Sn 솔더와 electroless Ni UBM 의 반응에 대해 살펴보았다.
