I have seen a general move in IPC documents to avoid defining default values for various design requirements or test parameters.  This applies to specific design parameters, as well as in the various qualification
and quality conformance testing that may be performed.  In the world of military specs, when we specified to build something to a mil spec (e.g. MIL-STD-275) we always had some established minimum requirements in
the military specification that were guaranteed (assuming the manufacturers met the specs which was a big if) simply by stating that PWB design must conform to MIL-STD-275.  The same for testing, when we invoked
MIL-P-55110, we got a minimum set of tests.  If needed, on a case basis, we would supplement or otherwise tailoir MIL-STD-275 and MIL-P-55110 to suit a unique design application.  With some IPC standards, we leave
too much to the customer to specify.  Most new DOD contracts today are placed via performance based requirements.  That is, we want an airplane to fly at this minimum speed, or we want a submarine to be able to dive
to this maximum depth.  Since the military is moving away from specifity we need Industry to write specifications that at least guarantee some baseline set of requirements.  You guys are the experts.  You should
know what minimum things need to be done to provide acceptable product.  What I am afraid of is that if we leave it up to the customer/designers to exercise ordering options by providing specificity, we likely will
wind up with contracts that do not address what details are required.  In other words, in the real world, the customers will place contracts via performance based requirements without recognizing that they need to
specify the minutia in order to select the options covered in the IPC documents.

What I recommend is that the IPC policy makers mandate some minimum default conditions in all documents they generate or revise.

Rene Martinez wrote:

> I also think a default is in order.  Especially because the designers need to know the limitations of how tight of a tolerance can be called out when you add inherent tolerances of copper,laminate, plating, etc.
>
> -----Original Message-----
> From: Jih Yuan [mailto:[log in to unmask]]
> Sent: Friday, September 22, 2000 5:47 AM
> To: [log in to unmask]
> Subject: Re: [IPC-600-6012] End Product Requirements for Board Thickness
> Tolerances
>
> John, From an OEM perspective, I think it's always good to have a default
> value. That's what the industry standard is about.  Jih
>
> -----Original Message-----
> From: IPC-600-6012 Mail Forum [mailto:[log in to unmask]]On Behalf Of
> John Perry
> Sent: Wednesday, September 20, 2000 5:37 PM
> To: [log in to unmask]
> Subject: [IPC-600-6012] End Product Requirements for Board Thickness
> Tolerances
>
> Hello Everyone,
>
> IPCWorks 2000 has come and gone.  Ready for Expo in Anaheim?  Sheez, are you
> kidding?
>
> As participants in the IPC-6012 and IPC-A-600 groups, many of you have also
> been involved with the IPC-2220 design series, and many of you may recall
> the ancient IPC-D-300G Printed Board Dimensions and Tolerances document.
> This old dinosaur was recently replaced by the IPC-2615, thankfully without
> the outdated end-product requirements such as bow and twist and annular
> ring, all of which have been updated through the years where they belong -
> in the design and performance specs.
>
> One, however, hasn't been transferred to the appropriate requirements
> documents, and I wanted to make sure that it was an intentional exclusion.
> IPC-D-300G used to provide tolerances for board thicknesses.  We don't have
> any applicable tolerances for this in any of the other spec.  Sure, IPC-4101
> does provide tolerances for laminate materials (Table 7) , and IPC-6012 and
> IPC-A-600 provide thicknesses for foil plus plating (under review now as we
> speak!), but did we intentional avoid the transfer of nominal thickness
> tolerances for finished rigid boards to the design and performance series of
> documents that we currently use for boards?  Should we point people to the
> use of IPC-4101 in conjunction with the IPC-6012?  As always, your input is
> much appreciated.
>
> Best Regards,
>
> John Perry
> Technical Project Manager
> IPC
> 2215 Sanders Road
> Northbrook, Il 60062
> 1-847-790-5318 (P)
> 1-847-509-9798 (F)
> [log in to unmask]