The aspect ratio is an easy way to predict what your MINIMUM hole size
should be, and it is only a guideline.
There is nothing wrong with having holes that are LARGER than what you
need if the customer requests it.

We have been asked to do this very same thing for ribbon cables with lots
of pins because it is very difficult to manually wiggle them into place.
We oversize them just to help people, it doesn't affect the soldering.

(p.s. don't assume the "common aspect ratio of 6:1" is DESIRABLE, in other
words. It should probably be a WORST CASE, try to provide a BETTER aspect
ration than that if you can)

Jack





Eric Bonatti <[log in to unmask]>
Sent by: DesignerCouncil <[log in to unmask]>
01/10/2006 03:11 PM
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Subject
[DC] through hole component hole question




Caterpillar: Confidential Green Retain Until: 02/09/2006        Retention
Category:  G90 - General Matters/Administration


Hi all,
I have a question:

One of our clients has designed the circuit, and specified a component
through hole (finished) of 0.032" for a round pin size of 0.018" dia. This
is for a flex approximately 0.011" thick.  This makes the aspect ratio of
almost 1:3 (instead of a common aspect ratio for a rigid board of say 6:1
for example).

This is a press-fit pin in another assembly that is being soldered to the
flex later.  The pin manufacturer recommends a size for the press-fit
side, but not for the solder tail side.

We are trying to explain to the customer that a 0.032" finished hole is
too wide to solder a 0.018" mil pin, but they are not very receptive to
our suggestions and are wondering what the problem is.  Keep in mind that
the original size they spec'ed was 0.047" !!  They reduced to 0.032", but
are now wondering what is wrong with us.

Is there a IPC standard that defines the best design practice of a
component through hole size for a specific pin size, maybe a chart or a
formula?  I looked in IPC-2221 and could not find this.  I would like to
show them a clear recommendation from IPC in Black&White.

Thank you all in advance for any help.

Eric Bonatti, C.I.D.
StrataFLEX  Corporation
Tel.: +1.416.752.2224 xt.2232
Fax: +1.416.752.6719
Toll Free: +1.866.771.7765
E-mail: [log in to unmask]
Web: www.strataflex.com

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