process defects ppm

PPM? Or RTY?
Can you explain the situation better? Is each cell making a completely different product or are they just performing different steps for he same product? Does every unique product go thru the all of the cells?

 

RTY is Rolled Throughput Yield

If you have three sequential operations for the same product, and each operation has a 90% (.90) yield, the RTY = .90 x .90 x .90 = .729 (72.9%) RTY. If the products are different, RTY does not apply.

 

So the easiest way to calculate the defect rate (ppm is not advised unless you are making at least a million units a week; other wise use a percentage for the defect rate) is to add all defects in a group of parts divided by the number of parts started. It’s not mathematically perfect but as long as the # of lost parts are not too large it will be close enough to be insightful. If the number of lost parts is large you can calculate the fraction of defects by started parts at each work station then combine using the proper fraction mathematics. OR just use RTY as Miner described above - it is the preferred way due to mathematical correctness and ease.

One clarifying thing about MIner’s response is that YIELD = #parts - #DEFECTs. So if you have 100 parts and there were 10 defects at any station then the yield = 100-10 = 90%