Surface Mount International Conference Proceedings


Author: Yasuhisa Kaga
Company: The Furukawa Electric
Date Published: 4/28/1997   Conference: Surface Mount International

Abstract: Recently industry is expressing deep interest in technologies for forming bumps (Au stud bumps) on IC chip pads using Au ball bonding. Because the ball bonding method is used, Au stud bumps can be formed on the Al pads without any special metallization. When mounting chips with Au stud bumps on the board, it is common to supply solder to the board pads and then mount the chips with reflow (referred to as stud bump FCA here). Stud bump FCA offers significant merits from the aspect of volume productivity. The Au stud FCA structure, however, has three major problems: (1) special solder is required for Au soldering, (2) Kirkendall voids occur at the interface with the Al, degrading bond strength, and (3) chip cracking may occur during formation 1’2). To resolve these problems, the authors developed a stud bump FCA using Cu. When compared with Au stud bump, the new Cu stud bump FCA offers a number of advantages: 1) Wide range of solder selection, supporting both standard eutectic solder and lead-free solder. 2) Diffusion into Cu is generally low, and solder connect stable. 3) Kirkendall voids are uncommon at the Al interface, providing stable bonding with Al. Cu stud bumps also suffer from a disadvantage in that chips are susceptible to cracking during bump formation, because Cu is harder than Au. Preventing this cracking is a key factor in the practical application of Cu stud bumping. This paper presents results of an investigation into a method of forming impact cushioned stud bumps, to prevent cracking during the Cu stud bump forming process, and an evaluation of chip damage. The impact cushioned stud bump formation method consists of placing Al foil as a cushion between the ball and the chip, thereby preventing cracking. Testing consisted of (1) investigation of cracking prevention effect, and (2) measurement of bump bonding strength through shear testing. Simulation was performed for stress generated during two stud bump processes (attach and ultrasonic loading), using finite element method (FEM) analysis.

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