Wire Bonders' Speak

The final stronghold for wedge bonding is with RF Packaging, where wire interconnects experience parasitic losses due to inductance with adjacent wire bonds, resulting in signal disturbance. In effect, the wire bonds act as discrete inductors. The effect of the wire bonds can be significant in RF compared to digital and should be accounted for in the design of the device.

To achieve the benefits of wedge bonding RF devices, while simultaneously maximizing bonding speed, reliability and reduced "keep out" space of a ball bonder, CHAIN BONDING was developed.

An RF Test Study 

Palomar engineers documented the results from and RF test vehicle where the wedge interconnects were replaced using a ball-chain bonding technique. The purpose of the test was to evaluate ball-chain bonding using a mature package design. The part selected for this test was high frequency transistor assembly where all package interconnects had been wedge. Due to wedge bond design guidlines and tool clearances, the wedge transistor assembly (WTA) package required three different wire sizes (1.0 mil, 1.3 mil, and 1.5 mil). Pre-test analysis suggested that all wire could be replaces using 1.5 mil ball-chain interconnects, thus reducing several process steps. During capillary testing, low destructive pull-test failures occured. In each case the low pull value was defined as a "crescent break". It was determined that a standard capillary design might not be optimal for chain bonding. This was evident through SEM inspection that showed stress where the wire exited at the crescent. 

Based on these results, an alternative capillary design design was used that would yield good chain interconnect, but at the same time tear off consistently for the terminating bond. 

This photo below shows the typical chain interconnect bonded with the improved tool design, performed on the Model 8000 Wire Bonder.