All

Before we contemplate a change lets step back and understand the problem.

From John Perry's background statement it seems to me that the customer didn't understand the selective solder process; and that with this process there must be an overlap of soldermask and tin/lead to avoid a bare copper demarcation line.

In practice when the job is tooled the solder will intentionally be extended .005 to .010" under the soldermask.  In this case the starting point is at the maximum suggested in this thread.  This would allow for no registration issues at all.  I would also be hard pressed to come up with a realalistic maximum number. .015? .020?  You may also get some flow of the solder after reflow.

Rather than assigning a number (which in my opinion would have to be arbitrary) I would suggest a statement in 6012 that explains the selective solderstrip process and the overlap requiured. Simutaneously there could be a change to 2221 with design guidelines for selective solder.  I don't have the magic words but I would suggest something like:

3.5.4.7.2 Tin-Lead under SMOBC (Selective Solderstrip)
Designs where solder is selectively removed from the board to allow soldermask to adhere to bare copper will have areas of tin/lead encroaching under the soldermask.

(Period. No numbers - it tells the customer that this is not a workmanship issue it is a design issue.)

We could also add a fit form function statement:
In no case should the solder be allowed to flow under the soldermask allowing the solder to flow from one pad to another.

We may also allow the soldermask to lift from the this reflowed area as we did when soldermask on tin lead was the rule rather than the exception.



Just my 2 cents worth,

Pete Menuez

-----Original Message-----
From: John Perry [mailto:[log in to unmask]]
Sent: Tuesday, April 19, 2005 12:05 PM
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
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1-847-597-2818 (Phone)
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1-847-615-7100 (Main)