First of all, none of what I have written was intended as a personal
attack, you stupid idiots!
***JOKE!!!*** No one writing here is stupid, though sometimes some of us
could use a little coffee.
At 05:41 PM 7/9/01 -0400, Michael Reagan wrote:
>Abdul and Brad,
>If your design is critical to a particular thickness, then yes you must
>specify it. no doubt, however if it aint, specify the min acceptable and
>give your design house the latitude to build up.
Brad's point, and mine, is that one may not think the thickness critical,
but thickness *will* vary the high-speed performance of the board (and a
few other things which are not so likely to cause trouble). If the board is
all low frequency analog, no problem. But if the board has even one or two
digital nets with fast transitions, thickness variations will cause
differing amounts of ringing, crosstalk, and radiated noise.
So a board that works from one vendor might not work with another. It would
be a shame to discover that after one has tested a board with prototypes
from one vendor and maybe a small production run, and then, for a large
batch where the economics are forceful, has bought the boards elsewhere.
*I have seen this happen.*
Now, I have actually followed Mr. Reagan's described practice. The vast
majority of my designs, over the years, have not specified many details of
board fabrication but have instead relied upon the fabricator's standard
production characteristics. So I have not followed the advice I am now
giving. I should.
> Funny we are all on this
>subject, I plan to submit a paper to PCD magazine, about board specification
>and fab notes later this month or early next month.
Therefore it is a great time to review the subject and to consider new ideas.
In another post, Mr. Reagan wrote:
>to clarify IPC states"
> Class C is a fully documented procurement package. Documentation is to the
>extent that the information is self sufficient and be sent to multiple
>vendors, with each producing the identical product. This documentation
>package requires that all the full manufacturing allowances are disclosed
>and documented. This is an IPC spec and a very important one. I write
>specifications all the time, I did aprox 80 designs last year ranging in
>various sizes and speeds to 2.7 Ghz. ( fast enough) Any specification
>should be written with the minimum requirements without tying your vendors
>hands.
IPC 2221 4.2.1.2 notes that some laminate or prepreg characteristics
*shall* not be included on the master drawing. I do not specify the
thickness of the material, but of the finished product. How the fabricator
gets there is his business; but if the board is a multilayer board, the
finished internal dimensions are normally a critical attribute. Normally,
I've seen layer distances specified as nominal value without stated
tolerances, unless the board is controlled impedance, in which case
impedance may be specified, with tolerance, rather than dielectric gap.
Again, the fabricator can figure out how to get there.
The advantage of specifying impedance is that it can be measured without
slicing up the board.
So, unless it is critical, fully specifying dielectric thickness can be
omitted, but the nominal values should *not* be omitted, because it would
then be possible for a fabricator to wildly vary the thickness, making it
more likely that a significant change will take place in board performance.
I don't have a copy of D-325, which is the PWB documentation standard, but
I would read the certification guide's restatement of it on this point as
requiring nominal dimensions on the master drawing for Class B
documentation (5.1, p. 132). Class C documentation, essentially, is fully
defined including tolerances, Class B is less formal. Class A is "usually
used for internal use."
What is the difference, by the way, between "usually used for internal
use," "usually for internal use," or "normally for internal use," the
latter two being good writing? But for the IPC publications, this is not
bad, at least it is reasonably clear!
Mike, why not submit your article to a few designers for review before it
is finalized for publication? Many heads, almost always, think better than
one; you'd still be the organiser and writer!
The best list for discussion of this topic is probably the Designer's
Council list.... Most Protel people are electronics engineers less involved
with formal specifications and more likely to do a Class A-documented
design. There are exceptions, of course.
[EMAIL PROTECTED]
Abdulrahman Lomax
P.O. Box 690
El Verano, CA 95433
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