Multi-Tier Wire Bonding

Advanced Bonding Application


Multi-Tier wire bonding offers an alternative to increase the number of I/O beyond 1000 wires without having to reduce the pad pitch, especially for those chips used in graphic cards and chipsets. Multi-tier bonding is not new in the industry. Staggered wire bonding for fine-pitch and ultra-fine pitch has been around for a long time. Currently, volume production for multi-tier wire bonding was limited to 60um bond pad pitch (BPP) for tri-tier wire bonding and 70µm BPP for quad-tier wire bonding.

 Staggered Bonding
 Tri-tier Bonding
Quad-tier Bonding

The introduction of quad-tier wire bonding requires that capillary designs be reassessed. Increased capillary bottleneck height is required to prevent contact with adjacent wires. However, this change has the potential to degrade capillary reliability as well as bond quality, due to a reduction in the efficiency of ultrasonic energy transfer from the bonding tool to the bond interface. Through physical modeling of the capillary using customized software, the different capillary profiles were evaluated in the context of their interaction with the looping profile that would be encountered during actual bonding based on the quad-tier configuration.

Finite element analysis (FEA) was also used to analyze the effect of different bottleneck height values. This is a potential significant issue, because it could increase the risk of capillary breakage during production run. FEA was used specifically to study the stress level at the tip and the transition profile for different designs, as these are the locations where breakage is most likely to occur.

From the various evaluations and analysis conducted, the PI Capillary design showed the best attributes in terms of efficiency of ultrasonic energy transfer and better bond integrity.


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