A method and structure for a
solder interconnection, using solder balls for making a low temperature
chip attachment directly to any of the higher levels of packaging substrate is disclosed. After a
solder ball has been formed using
standard methods it is reflowed to give the
solder ball a
smooth surface. A layer of
low melting point metal, such as,
bismuth,
indium or
tin, preferably, pure
tin, is deposited on the top of the solder balls. This structure results in localizing of the eutectic
alloy, formed upon subsequent low temperature joining cycle, to the top of the high melting
solder ball even after multiple low temperature reflow cycles. This method does not need
tinning of the substrate to which the
chip is to be joined, which makes this method economical. It has also been noticed that whenever temperature is raised slightly above the eutectic temperature, the structure always forms a liquid fillet around the joint with
copper wires. This liquid fillet formation results in substantial
thermal fatigue life improvement for reduced stress at interface; and secondly, provides an easy means to remove
chip for the purpose of chip burn-in, replacement or field repairs.