Production challenges of TSOP Copper wire bonding

摘要

With the Copper wire bonding already occupying majority of production lines, there are many significant factors continue to surface out. The Copper wire bonding has also becomes very susceptible to production variability of small leaded packages such as Thin Small Outline Package (TSOP). The TSOP utilizes thin and flimsy leadframes, causing a lot challenges at 2nd bond (stitch). The stitch bonding shows high occurrences of pre-mature failures or wire breaks, while struggling to maintain good stitch bonding strength. Furthermore, the inconsistent stitch bonding can also cause inconsistent tail formation, subsequently affecting the formation of Free Air Ball (FAB) and 1st bond quality. With many DOEs (Design of Experiments), several improvements were executed to address the stitch forming issues. Some of these major actions include optimizing several parameters such as the 2nd bond force, wire looping speed, wire loop mode change and wire sequencing. However, even with the implementation of these improvement actions, the Copper wire bonding performance was only improved by 50%. More comprehensive mapping was extended into possible impact of leadframe location and design. Surprisingly, die attach process was found to be the confounding with the leadframe design. It looks like wire stitch formation at ground lead is not consistent, mostly affected by the die attach epoxy bleed and outgas. In order to obtain more robust Copper wire bonding performance, the die attach process was optimized accordingly. Without losing any die attach quality, optimized process provided much larger operating margin for Copper wire bonding. All the combined improvements managed to eliminate wire break occurrence, while maintaining the good bonding strength of 1st and 2nd bonds. The lessons learnt were very important and already embedded to all new Copper wire bonding projects in order to realize 1st pass success into production mode.