I would suggest that the sampling plan should be applied to each of the
sub-lots, since each of the sub-lots had different processing. Sampling
plans assume that you are drawing your sample from a homogenous
population.
The sample size is in panels not coupons; so no matter how many
slugs(min of 2 per panel) are taken from a panel it shouldn't change the
sample size. In this case unless you had over 125 panels in a sub-lot
you would sample all the panels for micro-sectioning.
Just my 2 cents
Elaine
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dave Schaefer
Sent: Thursday, August 14, 2008 2:44 PM
To: [log in to unmask]
Subject: [TN] IPC6012 Sample Frequency / Method
A product I am working on recently experienced a higher than normal drop
out
both internally and externally due to open traces. Microsection analysis
of
several of the failing PCBs found barrel cracks, plating folds, and
insufficient
wall thickness.
Our fabrication specification requires "Copper Plating, Annular Ring,
Conductor
Spacing, and Solder Mask requirements shall be in accordance with
IPC-6012,
Class 3". PCB is 6 layers or IPC Type 3.
I applied IPC-6012 tables 4-2 Sampling Plan and 4-3 Acceptance Testing
and
Frequency to give the following:
- Table 4-3 specifies sample frequencies of 2.5 for Plating / Coating
Thickness
and 0.1 for Hole Plating Thickness (structural integrity after stress),
both per
panel.
- Our lot size was approximately 2000 PCBs, yield is 6 PCBs from an 18"
x 24"
fabrication panel, therefore at least 330 fabrication panels would have
been
processed.
- From Table 4-2, my assumption is that lot size should be 330 panels,
and
the corresponding sample sizes for 0.1 is 125 and 2.5 is 16 (please
correct me
if I'm interpreting this incorrectly !).
Some questions:
1. What governs how the fabricator applies the specification to their
process.
In other words, does a vendor have freedom to perform (in our case) 125
samples / 16 samples from a single fabrication panel and then pass the
other
329 based upon those results, or is there some other specification which
which the vendor must follow for sample identification ?
2. If so, how do I properly convey this to the vendor ?
We have since determined the cause was 1 of 7 sub lots at our vendor was
improperly processed through desmear. It's my feeling that our Class 3
requirement should have caught the failures at the fabricator if proper
sampling / testing was performed - comments ?
Thanks,
Dave
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