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Design of highly reliable three-dimensional network structure joint based on Cu@Ni@Sn double-shell microparticles for high temperature interconnection
Keywords: three-dimensional network, high-remelting temperature, transient liquid phase bonding
The aim of this paper is to fabricate a stable joint which can reliably work under high temperature up to 400℃, but only need low heat reflow temperature (<260℃) by using a novel transient liquid phase bonding (TLPB) process. The joint can be applied to bond SiC MOSFET die and double side copper (DBC) substrate to replace current high lead soldering process which soldered at relatively high temperature affecting the component properties and might be forbidden by ROHS in the near future because of the poisoned Pb. Typically, the joint was realized by designing a new three-dimensional (3D) network structure bondline with Cu metal particles embedded in the matrix of (Cu,Ni)6Sn5/(Cu,Ni)3Sn intermetallics (IMCs) which have high-remelting temperature (for example, Cu6Sn5 415℃, Cu3Sn 676℃). This 3D network structure was formed based on Cu@Ni@Sn double-shell microparticles with following press to preform and a TLPB heat reflow process, Sn was completely consumed to form IMCs network skelection surrounding Cu particles during TLPB process. Cu@Ni@Sn double-shell structure particles with different thickness of Sn layer and Ni layer were fabricated using electroless plating methods and compressed as preform. Influence of the thickness of Sn and Ni shell on 3D network joint microstructure and properties were investigated, the joints with different IMCs thickness made by different thickness of Sn shell were achieved by using TLPB bonding process. The preferred thickness of Sn shell that made the good mechanical properties was decided. The microstructure evolution and phase transformation mechanism were studied under different reflow temperature and isotherm solidification time. The results showed that Sn coating layer was completely consumed to form the corresponding (Cu,Ni)6Sn5/(Cu,Ni)3Sn IMCs. A minimum thickness of Ni coating layer is required to inhibit Cu atom diffusing towards (Cu,Ni)6Sn5 to form (Cu,Ni)3Sn. Further studies on shearing strength and hardness of the joint showed Ni coating layer on microparticles can strengthen IMCs, and the result revealed that stronger and stiffer IMCs were achieved with the increase of Ni content. In order to reduce the thermal stress caused by mismatch of coefficient of thermal expansion (CTE) between SiC die and DBC substrates, the thermal stress distribution of the joint was numerical calculated and imitated, and the influence of joint thickness on joint thermal stress was studied, the results revealed that the peel stress mostly concentrated on the corners of the joint, the peel stress decreased with increased joint thickness, the peel stress could be negligible when joint thickness was greater than 50μm, compared to the high- lead alloy system and sinter nano-silver system, the three-dimensional network structure bondline based on Cu@Ni@Sn and Al@Ag composite particles system possessed the minimum tensile stress (3MPa) and compress stress (15MPa). The affinity of Cu-Sn is much greater than that of Cu-(Cu,Ni)6Sn5, it is inevitable that few (Cu,Ni)3Sn phase generated accompanied by the (Cu,Ni)6Sn5 formed during TLPS process, due to the density of (Cu,Ni)3Sn is smaller than that of (Cu,Ni)6Sn5, there was no longer accompanying volume shrinkage upon process completion and working on high- temperature >400℃. So, when Sn layer was completely consumed, Cu6Sn5 was inhibited to transformed to Cu3Sn by barrier layer of (Cu,Ni)6Sn5, the interconnects exhibit excellent reliability under thermal shock cycling from -50 to 200℃. The microstructure and morphology before and after thermal treatment at 400℃ for 500h was compared and analyzed, results indicated that there was no obvious crack or large voids on the cross section of the interconnection after thermal treatment. A large shearing strength of 25MPa can be achieved with the high- strength (Cu,Ni)-Sn IMCS in the interconnections at 400℃.
Hongyan Xu, Ju Xu, Docter, Senior engineer
Institute of Electrical Engineering, Chinese Academay of Sciences
Beijing, Beijing

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