Dave,

In table 4-2 for class 3 a minimum of 125 coupons would have been collected
(although it does show 2.5 or 16 for structural integrity, typically the
same coupons are used for multiple attributes). Typically, following these
guidelines you could expect almost 1/2 of the lot to be samples (330 panels,
125 coupons). Most companies would not collect for sampling multiple coupons
off the same panel, although some do, there is no restriction to this in
6012 other than the 2 coupons per panel requirements (a minimum perhaps?).

The easiest solution to ensuring your product is adequately evaluated is to
simply specify in the purchase documents that each panel will be evaluated,
and if you choose have them provide you with a set of coupons for these,
including data.

That approach is not uncommon and most shops performing military work
collect samples from each panel regardless.

As for proper sampling catching all defects, well sampling has always been a
risk to the consumer, and always will be. There are multiple variables which
would result in 1 in 7 sub lots, when processing 330 panels, being
defective. A lot was left in etch too long, not long enough, someone was on
break, current fluctuation, worn drills, debris in the holes, between
backup/entry, carelessness, etc etc etc.

You need to work with the supplier, see where they stand on performing and
providing data for all panels in the lot, you might be surprised how willing
they are to perform this for you.

When any supplier, including yourself, is made to comply with a standard, it
is often applied as direct as possible, if you wish for them to exceed the
standard, just ask them and see where it takes you.

Franklin



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dave Schaefer
Sent: Thursday, August 14, 2008 1:44 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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