Silver alloy wire bonding and optimization using robust development approach

Abstract

Copper wire and palladium coated copper wires used for interconnections, widely in mass production today. Many forecasts are showing an accelerating trend that the market is moving more into copper wire business due to cost. Though copper wire technology maturity has increased more and more over these past decades, there are still a few limitations, intrinsic to the material itself, which leads to possible bonding process issues. Copper wire hardness is the current limitation for Front End technologies with delicate pad metal technology [1]; it can be very difficult to define a process window with zero defects. Silver wire, on the other hand, is striking the middle lane between hardness and bondability, making it a suitable candidate for the copper non-convertible products [2]. In this study, Ag alloy wire is proposed as an alternative to and Cu wire to lower down packaging cost without compromising on quality. The methodology applied to develop and validate the wire bonding process window is meeting the highest automotive standards and has passed AEC Q100 grade 1 reliability. Highlights have been put on how to optimize wire bonding workability and reliability of using Ag alloy wire in a QFP package. Design of Experiment is discussed and optimization to eliminate defects during reliability test to meet automotive requirements. An alternative to check the intermetallics coverage is also discussed. Furthermore, reliability results over four different pad metal technologies will be shared and discussed. Lastly, additional experiments are also done to check the effects of sulfurized wire during staging and ball formation due to poor forming gas, which usually affect copper wire reliability results.