From: [log in to unmask]
Date: Wed, 16 Oct 1996 12:47:32 -0500 (EST)
Subject: HARDWARE LOOSENING AFTER WAVE SOLDER
Hi Tom, to Jerry's comments I'd like to pass on the following additional
comments.
>We have been having a problem with assembled PCB's having improperly
>torqued hardware and making it out of the factory and think the
>problem is that the hardware loosens after wave solder. We have
>done some simple experiments and confirmed that this happens. The
>hardware is in common applications such as stiffener bar attachment,
>voltage regulator attachment and connector attachment.
>- Do any of you have experience with this?
Yes at my previous employer it was a problem, due to flow (wave) soldering
and due to power cycling (use).
>- Do some types of hardware work better than others?
Yes, if you're going to torque prior to soldering you need some form of
hardware that will maintain the desired tension on the fastener system. In
many of our applications we used "spring washers" to maintain the desired
tension on the fastener(s).
>- Is there a good guideline for proper torque specs for certain
>types of hardware on PCB's?
>Any information you could share would be appreciated.
>Thanks
In several experiments we determined printed board base material will
plastically flow when compressed by mechanical fasteners and subjected to
heat. If you repeat the re-torque and re-heat cycle several times, you will
eventally force out (plastic flow) the resin until you eventally have
"solid" glass between the clamped hardware. As mentioned, the plastic flow
of the resin will be relatively quick due to flow soldering or other
"thermal shock" like environments (such as SMT reflow), or will be over a
much longer time due to power cycling of the assembly/components.
As design rules, we did not allow for any electrical power, grounding, or
grounded electrical terminations to be "clamped" between any form of plastic
materials. The only exception was some of our design engineers wanted there
power semiconductors mounted in the printed board - so we needed a reliable
method for forming the simultaneous mechanical and electrical connection.
We did this through the use of spring washers(Bellvill - I'm not about
spelling, at least phonetically it's close).
We had two methods for assembly: the preferred method was to use "tempory"
mounting fasterers of corrosion resistant (CRES) (tradenamed "stainless
steel") just to hold the parts in place, then replace the mounting hardware
prior to final inspection; the second method was to do all the mounting in
"one-shot". For "flow" soldering, we placed the screw head and flat washer
on the secondary-(solder) side of the printed board assembly, then the
printed board, then we placed another flat washer, spring washer, and nut on
the primary- (component) side of the assembly. In most cases, we used CRES
mounting hardware. The diameters of the lands on the printed boards were
slightly larger than the diameter of the flat washers, and the lands should
NOT be solder coated for currents much over a few amperes.
If you don't use a fastener system that maintains the required tensile load
on the fastener, your compression electrical connection will eventually
"loosen" up, the interconnection resistance will increase, and at higher
electrical currents they will start to arc.
Hope this helps,
Ralph Hersey
Ralph Hersey & Associates
Phone/FAX 510.454.9805
e-mail: [log in to unmask]
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