It is found by studying the factors governing the loop profile in Au bonding wires that the thermal history in forming the ball caused the observed V-shaped hardness distribution in the heat affected zone. It is proved by combining calculation and simulation that the minimum hardness portion is a dominant factor governing the loop height. As recent LSI packaging is becoming thinner and thinner, a low profile wire is required for the bonding wire, which can be developed by utilizing this approach.
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