Fluxless and virtually voidless soldering for semiconductor chips

A novel soldering technique using a solder washer preform has been developed. A conventional vacuum-release process is used in the soldering procedure. That is, after curing of the assembly in vacuum ( approximately 1 torr) the fused washer preform wets the specimens (e.g. chip and heat sink) to seal up the through-hole area of the washer preform with approximately 1 torr. Subsequently, the vacuum is released to make the fluid pure solder flow into the through-hole area in such a manner that most of the oxide film of the washer preform is destroyed. It is demonstrated that the procedure results in an ideal bond with pure solder in a vacuum, without using flux. It also demonstrated that the above technique can be used to reduce voids to less than 5% on average in solder bonds with 1-cm/sup 2/ chips or 3-in wafers. Improved solder bond strength due to the new technique is also confirmed.<>