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Subject:
From:
Clive Ffitch <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 24 Jul 1998 10:42:12 PST
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     Ed,

     I came to the design of surface mount pads rather the other way
     around, first being involved with hybrids and ceramic substrates -
     Copper on Ceramic - with soldered LCCs, chip R's/C's etc, which was a
     perfect match for CTE (we refer to it as TCE!). I then moved on to
     being involved with PCB substrates and the design of pads for the more
     "conventional" surface mount world. We always took into account the
     size of packages and associated tolerances, and if anything, ended up
     with some pad designs slightly on the larger side. As our surface
     mount pad designs have evolved, we have not differentiated between
     substrates in terms of pad dimensions, and if we were to do a ceramic
     board now, we would use the same rules.

     The primary difference however, was that with ceramic/thick film, the
     construction of the substrate was essentially sequential, layer over
     layer, with blind and buried vias constructed as you go. This enabled
     integral vias to be designed in the pads for SMT reflow, whereas the
     conventional (then) pth of PCBs would not. Now of course, we're
     looking at micro-vias in PCB, and we can do integral vias again!

     So in answer to your question, I believe the guidelines should be the
     same, for pad dimensions, on PCB or ceramic. Would you have different
     guidelines for any other constrained substrates, eg Aramid/Epoxy? No.

     This only of course goes for the substrate itself, what happens above
     it - stress in the solder joint, component lead style, component
     style/material, stand-off height, TCE matching, soldering process -
     thats something else. Although I should point out we have different
     size pads for wave solder SMT, and even that upsets some people!

     Having said that, I have assumed that the component pads you speak of
     in your hybrid are based on reflowed solder joints. Are they perhaps
     conductive epoxy conections which can easily be the case in a hybrid?
     This may put a different light on the pads that concern you, and could
     even make two components sharing the same pad easier to swallow. Funny
     things can happen inside hybrids, especially the chip and wire ones,
     as by their very nature they are a "hybrid" of technologies, and it
     can get pretty crowded inside!

     Hope the above is of some small help, but I fear you may have a fight
     on your hands if space is a premium, and its somebody else's hybrid
     design/manufacturing rules.

     Clive ffitch
     MBUK, England



______________________________ Reply Separator _________________________________
Subject: [TN] SMD pads on hybrids
Author:  "TechNet E-Mail Forum." <[log in to unmask]>, [log in to unmask] at
INTERNET
Date:    23-07-98 13:55


Fellow professionals (yes, you know who you are!)

For the past few years, I have been a manufacturing engineer involved with
the entire circuit board assembly process.  I have spent considerable
amount of time working with the PCB designers with SMT pad design,
component spacing, solder thieves, etc. to design a board that can be
manufactured repeatably to a high quality level.  I have used pad designs
and suggestions taken from IPC, SMT Plus, and numerous others in the field.
Things have gone great thus far, with process and quality improvements,
etc.

For the past few months, I have slowly become involved in a hybrid, or
thick film project within my company.  Much of the design work has been
done outside, and I have only become involved as problems have developed.
The SMT pad designs used by these companies are similiar to what I have
used in the past, but not as robust as those that I have helped develop for
past circuit board designs.  It is as if the SMT pad design clock had been
turned back about 10 years.  These companies are using smaller pads,not
using pads that will allow for component size variation or placement
variation, or having two components share pads.  In my opinion, pad designs
that are not robust, and are not pads that I would accept for boards I have
helped design.  Space is a limiting factor on this design.  My question is,
is there a different set of guidelines for SMT pads on a ceramic substrate?
  Can components share the same pad (end to end), and not be a problem due
to the different expansion characteristics, etc.

I appreciate any comments and suggestions

Ed Holton
Hella Electronics

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