SMT First Pass Yield and Rolled Throughput Yield Calculator
First pass yield at one stage looks fine, 97 or 98 percent, but the yields multiply. This calculator shows the rolled throughput yield across paste print, placement, reflow and test, and what that costs in rework per lot.
Your numbers
Your results
Rolled throughput yield assumes independent stages and counts any rework as a first pass failure. Boards that are reworked and later pass are not counted as good in RTY, though they ship. Use the actual rework cost from your own labor and material rates.
RTY vs test yield
Sweeps ICT / functional test first pass yield from 80 to 100, holding your other inputs fixed.
Compute yield from your MES and ERP data
Optional: discuss automated stage yield reporting without manual entry.
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How to use the calculator
Enter the number of boards started and the first pass yield at each stage. First pass yield means boards that pass the stage the first time with no rework, divided by boards entering the stage. The calculator multiplies the four stages to give the rolled throughput yield, then estimates how many boards need rework or scrap and what that costs. Drag the chart assumption to see how much the final yield depends on the weakest stage.
- Use data from the same period and the same assembly family.
- Count rework at any stage as a first pass failure.
- Do not include boards that were scrapped before the stage.
- Use your own average rework cost, including labor and retest.
The formula
Rolled throughput yield is the product of the first pass yields of every stage: RTY = FPY1 x FPY2 x FPY3 x FPY4. First pass yield for one stage is units passing without rework divided by units entering. RTY tells you the chance that a board goes through the entire process first time, which is always lower than the lowest stage yield. Four stages at 98.5, 99.2, 97.8 and 96.5 percent give about 92.2 percent.
Reading the result
With the defaults, 1,000 boards produce about 922 first time good boards, so about 78 boards need rework or are scrapped. At 6 dollars per rework, that is about 470 dollars for the lot, or about 51 cents per good board. The lowest stage yield identifies where to look first, but also check the stage with the largest volume of rework; a small yield change at a high cost stage like functional test can matter more.
Common mistakes
Do not use final yield after rework as FPY, which hides process losses. Do not average stage yields; multiply them. Do not compare RTY between lines with different test coverage, since a line with more inspection steps shows more first pass failures. Also separate false calls from true defects so the numbers reflect real process issues.
Frequently Asked Questions
What is the difference between FPY and RTY?
FPY is the first pass yield at one process step. RTY multiplies FPY across all steps to give the probability that a unit passes every step first time. RTY is always lower than any single step FPY.
What is a good SMT first pass yield?
It depends on the product. Simple boards often exceed 99 percent at each stage, while complex boards with fine pitch, BGAs and dense layouts may run lower. Compare against your own history and customer targets.
Should test failures from component faults count?
Yes for RTY, since the board did not pass first time, but categorize the cause. A component fault is a supplier issue, not an assembly process issue, and should be handled differently in corrective action.
Does this include scrap loss?
The value at risk output shows the board cost if every failed board were scrapped. In practice many failed boards are reworked successfully, so use the rework cost for the realistic estimate.
If you want yield by stage computed from MES and ERP data instead of keyed in, we can build it on your own servers and keep the data private.
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