Development of Die Attachment Technology for Power IC Module by Introducing Indium into Sintered Nano-Silver Joint

Abstract

Wide band gap semiconductors have becomevery attractive for power electronics because of their excellentproperties at high power and high junction temperaturesabove 300°C. However, the maximum operation temperaturesof conventional packaging materials, like tin-based solders oreven tin-lead solders, are limited to around 220°C. Thus, a newpackaging material with a higher melting temperature must bedeveloped. Sintered silver paste die attachment has beenregarded as one of the most promising interconnectiontechnologies for high temperature and high power applicationsbecause of its excellent electrical connection and thermaldissipation abilities and the low processing temperature andhigh operating temperatures up to 700°C. Nevertheless, somecritical problems still exist in this technology, for example highporosity, poor wettability at interfaces and severe oxidationproblem when aging at temperatures above 200°C in air. Theseserious issues cannot be neglected before this promisingtechnology steps to practical application. Our research aims atsolving these problems of sintered nano-silver die attachmentby introducing indium into the sintered nano-silver joint. In our study, we put a slice of indium foil on nano-silverpaste, which had already been stencil-printed with a certainthickness on substrate. Indium foil would begin to melt at thebeginning of the bonding process and then permeated andfilled into the pores between silver nanoparticles. Indiumreacted quickly with silver and transformed the whole jointinto a silver-indium intermetallic, which has a high meltingtemperature above 600°C. Then, a much lower porosity jointwas produced and the wettability between sintered joint andsubstrates also became better, which showed a betterconnection at interface. To compare the high temperaturereliability between pure sintered nano-silver joint and silverindiumjoint, we conducted high temperature storage test at 300°C in air. The results showed that after thermal aging at 300°C in air for 100h, the shear strength of pure silver jointsdramatically reduced because of the oxidation of coppersubstrates. In contrast, no copper oxide could be observedinside the silver-indium joints, and even had twice higher shearstrengths than the as-bonded ones. Therefore, we suggest thatby introducing indium the high temperature reliability of thesintered silver joint can be significantly improved. Furthermore, the silver-indium joint also passed hightemperature storage test at 300°C in air for 1000h, and wasproven to have anti-tarnishing property by other researchers. To sum up, we provide a simple method and an idealpackaging material that can successfully overcome someserious problems of the sintered silver paste die attachmenttechnology and bring this technology much closer to practicaluse for high temperature and high power applications.