Friday, August 21, 2009

Board-by-Board Optimization

Just as each board type enjoys its own thermal profile in your surface mount oven, each board type also enjoys its own board-wave parameters in your wave machineThe optimal dwell time for that board was found to be 3.6 seconds, in contrast to 2.8 seconds for the first board. As you can see, the “dwell time profiles” of the two boards are different. This process resulted in dramatically lower defect rates for the second board studied (which had also been previously run at 1.0 seconds), although never quite as low as the new baseline which was attained for the first board. This strongly indicates the presence of sources of defects unrelated to dwell time, for example non-optimal immersion depth or design problems.Immersion DepthChanging your immersion depth changes your contact length and dwell time. This makes the direct and accurate measurement of immersion depth critical. Your pump speed produces a wave height (although this can diminish as your solder pot empties of solder), but the actual immersion depth of your boards depends on several factors, including solder pot height, how they sit in the fingers, if your fingers are bent, broken or crooked, the angle of your conveyor and whether or not pallets are used.Yet controlling your immersion depth - measuring it and keeping it consistent - is only one piece of the puzzle. Another is: At which immersion depth is your board quality optimized? This point is illustrated by figure 4. See that the defect rate of the board represented by the blue bars is optimized over a different range (48 mil, or even 36 to 60 mil) than the board represented by the yellow bars (at 24 to 36 mil). So, different board types benefit most from different immersion depths.Quantifying Cost BenefitsPrior to this study, yield loss was tracked on a monthly basis as a measure of the cost of production failures. Production volume for the board studied was 11,000 per month.1.) With the implementation of optimal dwell time procedures using the described device, yield loss went from 3.0% (330 boards) to 1.6% (176 boards) in the first month of daily use.2.) This meant a reduction in yield loss of 154 boards per month. For a 30 day month, this means 5.13 boards per day.3.) At 0 for the cost of each board, cost reduction based on improved yield loss alone was ,200 per month, which annualizes to 4,400.4.) That means that the return on investment on the device used in the study was less than five days.Conclusion: A seemingly small daily improvement in wave solder quality meant fast, very large, measurable monthly and annual cost savings. These figures do not even account for the valuable savings from benefits like reduced rework and field failures, less downtime and increased throughput, each of which in its own right can be more valuable than the rapid savings on yield loss.

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