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Solder Mask Specification

John S

#17985

Solder Mask Specification | 24 October, 2001

Does anyone have any guidelines or IPC standard they can suggest for specifying the type of solder mask used? We've started having problems with poor masking, and when I asked our designers what they use as a specification, they can't give an answer. Is this normally left up to the board shop as long as it meets the general guidelines in IPC-A-610?

Thanks John S.

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Hussman

#17988

Solder Mask Specification | 24 October, 2001

It's usually up to the board house. They use their lines to make a variety of other customers boards and never want to be locked into using a special mask. I'd task your QC department to catch and return these boards. That usually straightens out most board houses.

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Michael Parker

#17989

Solder Mask Specification | 24 October, 2001

If you let your design team leave things up to chance, chances are something will go wrong. You are right to want to take control rather than be lead down an undesirable path by the fab house.

Various masks serve certain desirable purposes. You need to determine what the overall masking goal would be for each assembly. This includes considering your processes through SMT and Wave. What technologies are you using? Is it fully SMT only? Mixed? Through hole only? What pad finishes are spec'd? HASL? Entek?

Dry Film mask can give a flat surface, which is desirable for Stencil to Fab gasketing during paste print. But do you need a glossy or matte finish with that dry film?

LPI mask may be more common to get because it's easier for the fab house to use.

You didn't say what specific problems you are experiencing. Is the masking a poor finish, with bubbling, chipping, poor registration to pads, inclusions in the surface? Are you experiencing specific processing problems? Bridging? Solder balls, webbing? Pad surface height lower or higher than the mask?

Please be more specific.

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#17991

Solder Mask Specification | 24 October, 2001

Allowing the board fabricator to define the board you expect to receive is fine, as long as they do a good job. Regardless of the existence of a clear product definition or not, it�s reasonable for you to argue that your receipts must meet the requirements of A-600 - Acceptability of Printed Boards. A-600 is, as you suggest, the board fabricators� analog of the assemblers� A-610.

In your procurement specification, consider defining: * Feature clearance * Solder resist coverage * Performance requirements of the mask * Cure and adhesion * Resist thickness

These specifications will vary with the type of resist you use [ie, wet film, dry film, Liquid Photo-Imageable]. So, be careful that you don�t get too specific about too many things and get wrapped around the axial. Your supplier should able to help you be specific, without being restrictive.

Ask your designers if may be they should spring for: * IPC-2221 - Generic Standard On Printed Board Design * IPC-2222 - Sectional Design Standard For Printed Board For Organic Printed Boards * IPC-6011 - Generic Performance Specification For Printed Boards * IPC-6012A - Qualification & Performance For Rigid Printed Boards

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