Technical Library | 1999-05-07 11:28:39.0
There are many things that can go wrong when soldering to gold plate over nickel surfaces. First of all, we know that gold and solder are not good friends, as any time solder comes into contact with gold, something seems to go wrong. Either the solder bonds to the gold and eventually pulls off as the tin and gold cross-migrate, leaving voids; or the solder completely removes the gold and is expected to bond to the metal which was under the gold.
Technical Library | 2023-08-04 15:27:30.0
A designed experiment evaluated the influence of several variables on appearance and strength of Pb-free solder joints. Components, with leads finished with nickel-palladium-gold (NiPdAu), were used from Texas Instruments (TI) and two other integrated circuit suppliers. Pb-free solder paste used was tin-silver-copper (SnAgCu) alloy. Variables were printed wiring board (PWB) pad size/stencil aperture (the pad finish was consistent; electrolysis Ni/immersion Au), reflow atmosphere, reflow temperature, Pd thickness in the NiPdAu finish, and thermal aging. Height of solder wetting to component lead sides was measured for both ceramic plate and PWB soldering. A third response was solder joint strength; a "lead pull" test determined the maximum force needed to pull the component lead from the PWB. This paper presents a statistical analysis of the designed experiment. Reflow atmosphere and pad size/stencil aperture have the greatest contribution to the height of lead side wetting. Reflow temperature, palladium thickness, and preconditioning had very little impact on side-wetting height. For lead pull, variance in the data was relatively small and the factors tested had little impact.
Technical Library | 2020-08-27 01:22:45.0
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 wirebond 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 were due to nodules and scratches on the wirebonding 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. To reduce scratches on wirebonding 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 determine the effects of nodules and scratches on wire bonds. The dimension of nodules, including height, and the size of scratches, including width, were measured before 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 was conducted using wire pull tests before and after reliability testing. Based on the results of the wire-bonding evaluation, the internal specification for nodules and scratches for wirebondable 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.
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COT specializes in high quality SMT nozzles and consumables for pick and place machines. We provide special engineering design service of custom nozzles for those unique and odd components.
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