At 12:20 PM 10/9/98 -0700, Martin wrote:
>The Lead engineer here has asked me to reduce the hole sizes for a few
>components on our through hole designs. The component leads in question have
>rectangular cross sections, and he has reasoned that if the hole is a bit
>small we will get a "pressure fit". These components have been known to
>"float", but up till now we just lived with it. I should mention that we
>switched to new assemblers who use hand loading instead of auto insertion
>for their through hole jobs.

I overlooked, at first, that these were square or rectangular pins, a very
different animal from round pins. Pressure fit on round pins for hand
insertion, forget it, as another noted, unless these are non-plated holes
very precisely drilled, and the pins are similarly precise.

But one can push the tolerance on a *diagonal* dimension. The normal
minimum hole oversize is 0.006 [inch] over the maximum diameter. The rule
of thumb I have always used is to make the hole 0.008 to 0.020 over the
nominal lead diameter. However, I routinely use an 0.040 hole with 0.025
nominal square pins, that is, with a nominal 0.035 diagonal. Yes, if the
pin is maximum and the hole is minimum, it will be pressure fit. But all
that will mean is that the corners of the pin are cutting into the soft
plating. At least that's the theory. It seems to work.

Going too far with this, however, will lead to far worse headaches than a
few components which don't lay down neatly.

If one cares to test it, I would drill a series of holes of varying
dimensions into PCB material and attempt to press a carefully measured
rectangular pin into them. When the oversize arc (the chord defined by the
hole circle and the intersecting hole) is zero, obviously the pin can be
inserted easily. When it approaches 90 degrees, it will be impossible.
Where the dividing line is might be interesting. And then one would need to
take into account the various tolerances, on the pins, on the holes as
drilled, and on the plating, else one batch of boards might assemble fine
and the next, when you get that big order that needs to be out next week,
won't work at all....

I would also worry a bit about tearing up the hole wall plating, expecially
if this is a multilayer board. It *might* be just fine after the hole fills
with solder, but, then again, the more that this is pressure-fit, the more
likely it is that the hole will not fill. I've managed to avoid
pressure-fit holes that needed to be soldered through, so far, so I don't
know how far one could go with this.

[log in to unmask]
Abdulrahman Lomax
P.O. Box 423
Sonoma, CA 95476

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