On Fri, 26 Jul 1996 14:51:00 -0700 
[log in to unmask] wrote:

> From: [log in to unmask]> 
Date: Fri, 26 Jul 1996 14:51:00 -0700
> Subject: PWB stretch
> To: [log in to unmask]
> 
> 
> TO:[log in to unmask]
> 
> PWB stretch or shrinkage seems to be the limiting factor for stencil 
> printing of larger PWB's with fine pitch pad's. We use laser cut stencils 
> and stencil printers with visual alignment but still have problems with 
> larger PWB's.
> 
> What is reasonable to expect from PWB suppliers?
> Does anybody know about a specification for PWB stretch? I would like to ask 
> for a stretch / shrinkage tolerance depending on PWB size and smallest pad's 
> on the PWB. What can be expected on FR4 boards without paying a premium? 
> What can be supplied at higher cost?
> 
> Thanks in advance for any help on this
> 
> Klaus Giese
> Siemens Energy & Automation
> Johnson City, TN 37601
> (423) 461 2459
> [log in to unmask]
> 
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> 
Bill,
        I hope the following information meets your need:

FROM MIL-S-13949/4D (16 August 1993):
MILITARY SPECIFICATION SHEET FOR GF BASE MATERIAL (I.E. FR4)

Dimensional Stability (for base material thicknesses less than .020 inch (0.51mm)).

        After bake:  Class A, 0.00075 inch/inch (0.00075 mm/mm) maximum;
                    Class B, 0.00050 inch/inch (0.0005 mm/mm) maximum

        Same values are given for the 'after stress' condition.

Typical (nominal) values given in Laminate Suppliers' Product Data Sheets are:

< .0030 in/in (for thin cores <= .028"
< .0035 in/in (for thicker cores between .028" and .062")



Fred J. Paul, Sr. Process Engineer
PCB Operations, FLUKE CORPORATION
direct:  206 356-5734  fax:  206 356-6070
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