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Free Air Ball Consistency of Palladium Coated Copper Wire
Keywords: palladium coated copper, free air ball, microhardness
The formation of free air ball (FAB) with palladium coated copper wire depends on many factors including the flow rate of oxidation protection gas, the Electric Flame Off (EFO) current, firing time and gap between the tip of wire and EFO wand. These factors can individually or, in combination of which, affect how good a ball can be formed prior to bonding onto the bond pad of a device. It is therefore important to find an optimum condition in forming good FAB so as to avoid poor bonded ball quality, or worse, non-uniform ball hardness that could lead to damage of the bond pad upon impact. This paper focused on understanding the FAB characteristics of different diameters of the wire processed under different protection gas and EFO current settings. The very first requirement for a FAB to be good for bonding is the shape of the ball. The ball has to be spherical without surface defects and consistently achieve this in every firing cycle. Hence, FAB consistency was carried out to determine which conditions will render the quality of the balls be acceptable. As palladium has a much higher melting temperature compared with copper, during the FAB formation under EFO firing condition, the Pd at the surface of the wire does not necessarily form a complete solid solution with copper during the split-second melting process of the coated wire. It was found that the ball consists of large grains of copper with palladium or palladium rich phase scattered around the ball. The location of the Pd phase may reside within or at the surface of the ball, depends on the EFO parameter settings and perhaps Pd thickness. Microhardness measurement at different locations within the ball revealed non-uniform hardness which renders great challenges in avoiding damage to pad in ball bonding
Johnny Yeung, R&D Director
Heraeus Materials Singapore Pte Ltd.
Singapore 569871,
Singapore


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