Electronics Assembly Knowledge, Vision & Wisdom
Overcome Nodules and Scratches on Wire Bondable Plating on  PCBs
Overcome Nodules and Scratches on Wire Bondable Plating on PCBs
Manufacturing was improved to minimize nodules and scratches. Wire bonds were evaluated to determine the allowable size of nodules and scratches.
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Authored By:
Young K. Song and Vanja Bukva, Teledyne DALSA Inc., Waterloo, ON, Canada
Ryan Wong, FTG Circuits, Toronto, ON, Canada

Summary
Initially adopted internal specifications for acceptance of printed circuit boards (PCBs) used for wire bonding was that there were no nodules or scratches allowed on the wire bond pads when inspected under 20X magnification. The nodules and scratches were not defined by measurable dimensions and were considered to be unacceptable if there was any sign of a visual blemish on wire bondable features. Analysis of the yield at a PCB manufacturer monitored monthly for over two years indicated that the target yield could not be achieved, and the main reasons for yield loss was due to nodules and scratches on the wire bonding pads.

The PCB manufacturer attempted to eliminate nodules and scratches. First, a light scrubbing step was added after electroless copper plating to remove any co-deposited fine particles that acted as a seed for nodules at the time of copper plating. Then the electrolytic copper plating tank was emptied, fully cleaned, and filtered to eliminate the possibility of co-deposited particles in the electroplating process. Both actions greatly reduced the density of the nodules but did not fully eliminate them. Even though there was only one nodule on any wire bonding pad, the board was still considered a reject. In order to reduce scratches on wire bonding pads, the PCB manufacturer utilized foam trays after routing the boards so that they did not make direct contact with other boards. This action significantly reduced the scratches on wire bonding pads, even though some isolated scratches still appeared from time to time, which caused the boards to be rejected. Even with these significant improvements, the target yield remained unachievable.

Another approach was then taken to consider if wire bonding could be successfully performed over nodules and scratches, and if there was a dimensional threshold where wire bonding could be successful. A gold ball bonding process called either stand-off-stitch bonding (SSB) or ball-stitch-on-ball bonding (BSOB) was used to find out the effects of nodules and scratches on wire bonds. The dimension of nodules including height, and the size of scratches including width were measured prior to wire bonding. Wire bonding was then performed directly on various sizes of nodules and scratches on the bonding pad, and the evaluation of wire bonds were conducted using wire pull tests before and after reliability test. Based on the results of the wire bonding evaluation, the internal specification for nodules and scratches for wire bondable PCBs was modified to allow nodules and scratches with a certain height and a width limitation, compared to initially adopted internal specifications of no nodules and no scratches. Such an approach resulted in improved yield at the PCB manufacturer.

Conclusions
In order to resolve significant yield loss during PCB manufacturing caused by nodules and scratches on wire bondable features, the PCB manufacturing process was improved to minimize the occurrence of nodules and scratches and wire bonds were evaluated to determine the allowable size of nodules and scratches on bonding pads for the gold wire bonding process, called SSB. Based on the wire bonding test results, the initially adopted specification, which did not allow for any nodules and scratches on any wire bondable surface, was changed to accept certain nodules and scratches, based on size and the effect on the surface finish.

Initially Published in the SMTA Proceedings

Comments
I am familiar with the ball-bonding process using .001 gold wire to gold PCB pads. Scratches and other defects didn't matter due to the thermosonic scrubbing action of the bonding process. Are the SSB and BSOB a thermosonic scrubbing action type process to create the bond? I used a heated stage that brought my substrate temp to 60c prior to bonding, which may have helped.
Erron Pender, Indium Corporation
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