Novel TIM Solution with Chain Network Solder Composite

Runsheng Mao, Sihai Chen, Elaina Zito, David Bedner,
and Ning-Cheng Lee
Indium Corporation
Clinton, NY

A novel epoxy SAC solder paste TIM system has been developed with the use of non-volatile epoxy flux. Cu filler was added to the solder paste, with Cu volume % of metal ranged from 17 to 60 volume % of metal. Formation of semi-continuous high melting Cu chain network was achieved, with the use of CuSn IMC bridges between Cu particles. This chain network, at sufficient concentration, serves as skeleton and maintains the shape of the sandwiched solder paste layer, thus prevented further spread out and outgassing upon subsequent SMT reflow process, and also allowed formation of TIM joint even in the absence of solderable metallization on flip chip and packaging housing. Presence of significant amount of ductile solder within TIM joint promises high resistance against brittle cracking under stress. The Cu content could be further optimized between 17 and 33 volume % of metal to avoid flux bleeding and maintain good epoxy adhesion between TIM phase and parts. The 20C thermal conductivity achieved was 6.1 W/mK and could be up to about 13 W/mK with further epoxy flux optimization.

A novel epoxy SAC solder paste TIM system has been developed with the use of non-volatile epoxy flux. Cu filler was added to the solder paste, with Cu volume % of metal ranged from 17 to 60 volume % of metal. Formation of semi-continuous high melting Cu chain network was achieved, with the use of CuSn IMC bridges between Cu particles. This chain network, at sufficient concentration, serves as skeleton and maintains the shape of the sandwiched solder paste layer, thus prevented further spread out and outgassing upon subsequent SMT reflow process, and also allowed formation of TIM joint even in the absence of solderable metallization on flip chip and packaging housing. Presence of significant amount of ductile solder within TIM joint promises high resistance against brittle cracking under stress. The Cu content could be further optimized between 17 and 33 volume % of metal to avoid flux bleeding and maintain good epoxy adhesion between TIM phase and parts. The 20C thermal conductivity achieved was 6.1 W/mK, and could be up to about 13 W/mK with further epoxy flux optimization.