Hi, Clive,
For hybrid circuit, the pad design for solder paste and conductive
epoxy should be very much different. For example, the pad design for
0402 capacitor using conductive epoxy should be smaller than that for
solder paste. The pad design is different for stencil printing and
dispensing, if conductive epoxy is used for 0402 attachment.
Is there any pad design standard for conductive epoxy?
Eden
______________________________ Reply Separator _________________________________
Subject: Re: [TN] SMD pads on hybrids
Author: Non-HP-clive.ffitch ([log in to unmask]) at
HP-Singapore,shargw51
Date: 7/25/98 2:42 AM
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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