I would like to address several issues. First, your reflow is 150 PPM for the same exact assemblies that have lower PPM's? or are you giving a 150 PPM rating for all products produced through the reflow process in a specific time period?
Reflow is a benign process i.e. the only thing added is time and heat. How do you derive that the reflow process is causing more defects than print, pick and place? The few things that come to mind that can cause reflow defects include an out of balance machine (jerky chain/belt, profile not optimized, convection blowers are blasting parts around, etc.) Otherwise, the root cause of defects must be attributable to the prior processes. Second- What are your defined attributes to calculate PPM? Do you include: quantity of assemblies produced x((total component count + total placements (components - PCB) + total terminations)/ 1,000,000 ? My observation regarding 50 PPM target- easily reproduced with an assembly with total component counts greater than 800. (motherboards are a good example) In this range, it takes about 10 solder defects to be equal to 1 misplaced component, affecting your PPM by a value of 1 (going from 50 PPM up to 51 PPM) (OK y'all - don't challenge the actuals here- this is just a generalized comment)
On the other hand, assemblies with low total component counts (and terminations) tend to trend much higher (200 PPM and greater). Why? A single defect with a low parts and termination count has an effectively higher influence to the PPM. 2 solder defects can cause the PPM to rise 30-40 points.
Finally - what can you do? 1. Stabilize your process ( get rid of all the obvious mistakes). 2. Accept whatever PPM (per assembly) you get as a baseline reference. 3. Apply 6 sigma rules to the daily PPM results. A big hint - there's no such thing as a Lower Control Limit when Process Improvement efforts are driving the PPM rate down. Just make sure that you react to upward trends. Periodically adjust the target value down (with good statistical practices employed)
Compare apples to apples - monitor and track PPM rates per each individual assembly! To do otherwise would be creating an average that smooths out the high and lows, giving you false PPM values.
I fail to find the significance of the dollar value of the assembly to be a determining factor as to why one would be proactive in reducing defects, therefore reducing costs.
I could produce 10 very expensive boards and have a lower rate of return on my efforts than to fix process problems for a cheap assembly that is produced in the hundreds/thousands quantities on a daily basis.
A defect is a defect, regardless of where found, all causing greater cost in rework and lost time/materials.
Good luck
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