Note: Such percentages have been in 6012 for many years without complaint.

Mike

-----Original Message-----
From: IPC-600-6012 [mailto:[log in to unmask]]On Behalf Of Chris
Mahanna
Sent: Wednesday, April 20, 2005 10:01 AM
To: [log in to unmask]
Subject: Re: [IPC-600-6012] Proposed IPC-6012B Change for SnPb in SMOBC


Strictly from a metrology perspective, the following (well intended)
requirement will have very little meaning and would be rather counter
productive to those assessing conformance.

1% of the conductor surfaces for Class 3 and 5%
of the conductor surfaces for Class 1 and 2.

I would guesstimate the total uncertainty of this metric to be AT LEAST
+/-5% of conductive surfaces.  Therefore, if you are near the above
pass/fail thresholds, you have literally no confidence in the go/no go
decision.  We have this same problem with the 95% solderability requirement.
Luckily, with solderability, it doesn't usually come into play because
typically the results are <<5% or >>5%.

If my guesstimate is off-base let me know.
For a very nice treatment of how requirements like this put us in a bad
position see:
ILAC-G8:1996 "Guidelines on Assessment and Reporting of Compliance with a
Specification (based on measurements made in a laboratory)"

Chris Mahanna
Quality Manager
Robisan Laboratory Inc.
6502 E. 21st Street
Indianapolis, Indiana 46219
317.353.6249 phone
317.917.2379 fax