SMT007 Magazine

SMT007-July2019

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96 SMT007 MAGAZINE I JULY 2019 Feature by Young K. Song and Vanja Bukva, TELEDYNE DALSA INC., and Ryan Wong, FTG CIRCUITS Abstract 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 magni- fication. 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 fea- tures. Analysis of the yield at a PCB manufac- turer monitored monthly for over two years indicated that the target yield could not be achieved, and the main reasons for yield loss were due to nodules and scratches on the wire- bonding pads. The PCB manufacturer attempted to elimi- nate nodules and scratches. First, a light-scrub- bing step was added after electroless copper plating to remove any co-deposited fine parti- cles that acted as a seed for nodules at the time of copper plating. Then, the electrolytic cop- per plating tank was emptied, fully cleaned, and filtered to eliminate the possibility of co- deposited particles in the electroplating pro- cess. 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 con- sidered a reject. 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 iso- lated 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 determine the effects of nodules and scratches on

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