Development of 2 stage bump forming method using wire bonding technique

Among the features of SBB (Stud Bump Bonding), which is a flip-chip mounting method, there is a “2 stage bump formed by wire bonding technique”. In order to form the 2 stage bump the second bonding process must be added to the first, which is necessary for conventional wire bonding. To stabilize the second bonding, techniques are needed: to assure the Au-Au junction between a bump base formed by the first bonding and gold wire; and to stabilize the gold wire break point after the second bonding. The authors make clear the influence on these techniques of the capillary profile and heat influenced area, or recrystallization area, which appears when a ball is formed at the top of the gold wire. They confirm that, by properly selecting such conditions, the second bonding can be stable and a 2 stage bump of high quality is formed in actual mass production