Hi, Cathy,
No, the acceptable percentage of missed faults is zero, but with visual
inspection, it's accepted as a fact of life that only 80% of faults are
noticed at any time - partly because "pass" and "fail" conditions are
subjective up to a point, and partly because the human mind tends to
wander, and thoughts may be elsewhere at the time the eyes are passing over
a fault, so the fault doesn't register in the inspector's mind. It's
accepted, but not acceptable. The concern is the question of how serious
the faults are that aren't found. You can reasonably assume that major
defects will be spotted, but after 20+ years in this industry, I promise
you it's not just the minor faults that escape detection.
Unfortunately, we have no means of automating this sort of inspection work,
so we have to live with the imperfect yield of fault detection. We can
improve it a bit with training and awareness sessions for inspectors, but
even so, about 20% of faults are said not to be found. This does have an
effect of product life in the field, and field defects cost - money,
hassle, time, loss of customer confidence, resources to fix, etc.
I take it you have a yield problem? Or different inspectors find/miss
different faults?
Good luck with your bug hunt - I hope you find ways to reduce their
numbers.
Regards
Peter Duncan
Cathy Killen
<ckillen@SMTE To: [log in to unmask]
K.CO.UK> cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST
Sent by: Aero/ST Group)
TechNet Subject: [TN] Solder Joint Defects
<[log in to unmask]
ORG>
10/18/01
07:33 PM
Please
respond to
"TechNet
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Forum."
Could anyone help with this.
When visually inspecting PCBs to class 3, is there an acceptable percentage
of faults missed? Also is there an acceptable defect level considering PPM
with regards to soldered joints.
Cathy Killen
Training Instructor
Smtek Europe Ltd.
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