We support any change that allows both 6012 and 6013 to have
commonality. As it is, we produce boards IAW MIL-PRF-55110, MIL-P-50884,
MIL-PRF-31032, IPC-6012, and IPC-6013. Finding commonality in these
diverse specifications is difficult at best.
 
From a board fabrication perspective, the selective solder strip process
is difficult to complete with a perfect soldermask/copper/surface finish
interface. One possible result of minor variations in the developing
process is a small amount of surface area at the mask interface where
the mask material may not be completely bonded to the copper. This
provides a pathway for final finish seepage under the mask. 
 
From a reliability perspective, what is the true issue? Is there a
definitive difference in reliability between 1% and 5% of exposed copper
at the interface? Is there a reduction in reliability if every interface
has 0.005 or 0.010 inches of exposed copper? 
 
It would seem to me that the real issue here is a loss of adhesion of
the soldermask that allows excessive amounts of exposed copper on the
conductive pattern. If the final surface finish has leaked under the
mask at the interface and the mask peels revealing final surface finish,
does this produce a reliability concern? Perhaps from a FOD position.
 
From my viewpoint:
 
1.                Exposed copper is already covered in paragraph 3.7.2.
If needed this paragraph should be modified (any help in making
soldermask adhesion requirements in 6013 agree with those in 6012 would
be helpful).
2.                Assuming small amounts of exposed copper do not
represent a reliability risk, 0.005 - 0.010 of exposed copper at the
final finish interface should be permitted.
3.                Encroachment of final finish under the mask at the
interface should be acceptable provided mask adhesion is not
compromised.
 
Alan Exley
Director of Quality
Cosmotronic
949-660-0740
 
 
-----Original Message-----
From: John Perry [mailto:[log in to unmask]] 
Sent: Tuesday, April 19, 2005 9:05 AM
To: [log in to unmask]
Subject: [IPC-600-6012] Proposed IPC-6012B Change for SnPb in SMOBC
 
Colleagues,
 
The IPC D-33a Rigid Board Performance Task Group is beginning the
development of an Amendment 1 to IPC-6012 Revision B.  Relative to this
is a request to modify and append text in section 3.5.4.7, Final Finish
Coverage (Areas not to be soldered).
 
Background on change request:
 
A printed board was found to have small amounts of Tin Lead under the
solder mask with reflow/SMOBC finish.  The part has been fabricated
using the selective solder strip process.  The customer rejected the
parts for small amounts of tin lead found on bare copper and under the
solder mask, claiming with Tin Lead, the part no longer was Solder Mask
over Bare Copper.  There is currently no IPC specification that
prohibits such Tin Lead on the bare copper.  However, at some point it
becomes a workmanship issue per IPC 6012B paragraph 3.3.9.
 
Rationale for change request:
 
This new accept/reject criteria provides a check and balance for the tin
lead strip process (i.e., the process is not capable of absolute ZERO
tin lead as there is always trace amounts on some circuits) and at the
same time we don't want to be throwing away printed boards that are
functionally fine.
 
Proposed Change within 3.5.4.7 of IPC-6012B:
 
3.5.4.7 Final Finish Coverage
Final finish shall meet the solderability requirements of J-STD-003.
 
3.5.4.7.1 Exposed Copper (Areas not to be soldered) Exposed copper on
areas not to be soldered is permitted on 1% of the conductor surfaces
for Class 3 and 5% of the conductor surfaces for Class 1 and Class 2.
Coverage does not apply to vertical conductor edges.
 
3.5.4.7.2 Tin-Lead under SMOBC
Tin or Tin Lead under SMOBC on areas not to be soldered is permitted on
1% of the conductor surfaces for Class 3 and 5% of the conductor
surfaces for Class 1 and 2.
 
If you approve the proposed change without comment, please send your
approval, by May 3rd, to [log in to unmask]  If there is a need to comment
on and discuss this within the task group, please respond through this
e-mail forum.
 
Thanks,
 
John Perry
Technical Project Manager
IPC
3000 Lakeside Drive # 309S
Bannockburn, IL 60015
[log in to unmask]
1-847-597-2818 (Phone)
1-847-615-7105 (Fax)
1-847-615-7100 (Main)