I work for an organization that repairs high end (read that very expensive) printed circuit boards. We do final inspection and test on 100% all part numbers untill we acheive a 13 week average failure rate of LT 1% for at least 4 weeks. At that time we go to a sampling plan for the PCB.
Our current charts are strictly "run" charts with a 13 week rolling average calcualted to show the average as well as weekly board count and failure count. I have tried a number of other charts but I have two issues to overcome. First - the reason for the "13 week average" is that weekly unit counts can be very sporadic. We repair PCB's as the customer forecasts their need. So I may get one board per week of a given part number or I may get 10 or I may get up to 100 per week. Worse than that is that I often get 10 followed by 1 followed by none for a couple weks followed by 100. The 13 week averaging technique sort of works, but it gives some strange average lines as varying volumns and failures come on and fall off the 13 week window. The data I collect is strictly pass / fail at my final QA test station. It is the final decison to ship or rework and retest. For those who are not familiar with this kind of operation - repairing complex boards that come to you "broken" is a difficult task. Due to troubele shooting techniques used by field engineers the actual status of a PCB is not guaranteed to be bad. Many boards do not fail our tests, no matter how long they are tested. They may have been replaced as a group and once the customer machine is running - all the boards come to us. My second need is for a simple to do and simple to understand chart. We collect chart data weekly and keep over 200 different charts. So obviousely I need charts that are easy to produce. They also need to be understood by a diverse workforce to be of any value to our operation. We are ISO certified to the 9002:1994 standard - upgrading to the 9001:2001 standard. In my early study of the new standard I think I see a larger requirement for SPC.
Sorry for the long post but I think all this needs to be known by any of you out there that might be able to help. Thanks in advance for any input.
Ron