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High Melting Lead-Free Mixed BiAgX Solder Paste System And Its Joint Formation Mechanism
Keywords: High Temperature lead free solder, Die attach, BiAgX
In the current work, a mixed powder BiAgX solder paste system with the melting temperature above 260oC and the comparable reliability to the high lead solders has been studied and proposed as an alternative high temperature lead free solution for die attach application. The metal powders in the paste system are composed of a high melting first alloy powder as majority and the additive powder as minority. The additive is designed to react aggressively with various metallization surface finishes before or together with the melting of the majority solder to form a controllable IMC layer. The formation of the controlled IMC layer in BiAgX system improves not only the wettability but also the associated bonding strength comparing to BiAg. The IMC layer thickness of the mixed powder system is controllable with the quantity of the additives and insensitive toward thermal aging at 175oC and thermal cycling from -55oC to 125oC while the high lead ones do show a considerable increase. Microstructure investigation shows a unique interface structure, where the “fishbone” shape IMC layer interlocks with both Cu substrate and some micron size Ag-rich particles at the solder side. Also, it is observed that numerous submicron size Ag precipitations in the joint form the network walls to isolate the brittle Bi phase colonies. Under thermal aging and thermal cycling, the interface structure is relatively stable and no obvious structure change has been observed. The same time, the fine Ag network walls isolating Bi colonies are still discernible although the Ag particle coarsening is seen. The unique interface structure together with the network walls formed by submicron size Ag particles are attributed for the relatively high reliability in the mixed BiAgX solder paste system.
HongWen Zhang,
Indium Corporation of America
Clinton, NY

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