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February 2002

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Mon, 4 Feb 2002 08:25:50 +0800
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Hi, Lorraine,

It's obvious that your designer has no mechanical sympathy whatever!

Your question connects with a previous thread concerning removal or not of
'non-functional' internal pads during board fab. If you have a 3 thou
interference on the finished hole diameter, the corner contact points of
your pins to the hole wall are going to cut through the barrel plating and
into the hole wall material itself. I doubt that you have more than 1 oz
plated copper inside the holes = 1.4 thou (mils) per side, and you may even
have 1/2 oz, which will make the situation worse, as the pins will be
cutting through less copper and more substrate material.

Until the holes are filled with solder, pre-heating and other thermal
excursions will make the hole diameter contract, causing the pin corners to
dig even further into the substrate material, which will damage it, though
by how much is hard to say. There will be some local delamination as the
material displaces to make room for the pins, and this delamination may
spread further in time.

The major damage, as I see it, will be to the hole plating. Apart from
cutting through it, the shear stresses imposed on the interconnection
between the hole plating and the different layers, caused by forcing the
pins into the holes, stands a good chance of severing the connection. This
may happen straightaway, or later if your boards experience resin
recession, and it will be worse if non-functional pads have been removed
from the internal layers, as there will be less mechanical support to
resist the shear stresses. You might even find the combination of
delamination and shear stresses causes surface pads to lift.

I strongly urge you to take some cross sections of boards assembled in the
manner you describe. You will see then what damage has been caused and can
then present your designer with the findings.

Peter




                    "Reid, Lorraine"
                    <Lorraine.Reid@TRI        To:     [log in to unmask]
                    VIRIX.COM>                cc:     (bcc: DUNCAN Peter/Asst Prin Engr/ST
                    Sent by: TechNet          Aero/ST Group)
                    <[log in to unmask]>         Subject:     [TN] connectors and hole diameters


                    02/01/02 06:45 PM
                    Please respond to
                    "TechNet E-Mail
                    Forum."; Please
                    respond to "Reid,
                    Lorraine"






Can anyone point me in the direction of any investigations into the
potential for long term failures caused by inserting a thru-hole mounted
connector into holes which are slightly too small to fit?

We have a situation where a connector, with square pins, diagonal
dimension 25 thou, is being inserted into plated holes, finished size 22
thou.  The connector is then soldered.   The connector pins can be
forced in, but I am concerned about the potential for damage that may
not appear immediately obvious.  The board is 4 layer, so obviously
there are connections from the holes to the inner layers.  The obvious
solution is to enlarge the holes (not possible for this batch), but the
designer is unwilling to make changes unless we can prove that there is
a genuine risk of failure of the interconnects due to forcing the
connector into the board.

Thanks in advance

Lorraine Reid

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