First, if you want the lowest cost possible, spec a quality material, not a cheap material.  And make sure you spec exactly what you need.  IPC slash sheets are a decent generic starting point, but if you have a specific requirement, (like Dk), add it to your notes.  Leaving most of the material spec up to the fabricator will likely result ni expensive scrap at some point.

Here's our material spec notes:

FABRICATE BOARDS IN ACCORDANCE WITH IPC-6011 AND IPC-6012, CLASS 2. FINISHED BOARDS SHALL MEET THE QUALIFICATION AND PERFORMANCE SPECIFICATIONS DEFINED THEREIN. UNLESS OTHERWISE SPECIFIED, BOARDS SHALL MEET THE ACCEPTANCE CRITERIA OF IPC-A-600, CLASS 2.

MATERIAL SHALL BE IN ACCORDANCE WITH IPC-4101, EPOXY-GLASS LAMINATE, NON-DICYANDIAMIDE, NEMA GRADE FR-4, FLAMMABILITY RATING: UL 94V-0.
         MINIMUM Tg (DSC) 170
         MINIMUM Td (TGA) 330
         MAXIMUM CTE (Z-AXIS) 25C-260C 4%
         COPPER THICKNESS:  0.5 OZ. EXTERNAL LAYERS PLATED  TO 1 OZ. MINIMUM,  1 OZ. INTERNAL LAYERS

This data is modified as needed.  For RF boards that we've qualified based on a specific Dk, we add a note.  For calculatyed or controlled impedance, it gets noted.  We also have an environmmental specification to cover RoHS compliance.  This CTE spec is not a common method, but it gives us a little more leeway in allowing multiple materials that will still suit our needs.

We also have a short list of approved fabrication vendors.  They supply data sheets for the material they intend to use.  Putting the fabs out to bid loses that trust, and you have no way of knowing what material they use.  Many low price fab shops have no qualms about ignoring your specs and substituting whatever laminate they can get cheap.

I am currently researching updating these notes based on our experience with RoHS materials and processes.  A big help is Werner Engelmaier's white paper.   http://www.engelmaier.com/white-paper.html  I'd call it a must read if you spec PCBs.

Pete

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