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Avoiding PCB stress from connectors ?

Started by Dave Nadler July 31, 2013
I'm sure there are some standard techniques to
minimize this ?

Product has a number of connectors on a PCB,
mounted in a box. Because the external cabling
and connectors will be subjected to stress,
there are (for example) hex nuts fastened through
the sturdy box through DB connectors on the PCB.
Then when the external cable is yanked, it doesn't
break the PCB.

So, the PCB is fastened to the box not just by
normal mounting screws, but at the connectors.

If all the connectors are perfectly sized and
aligned, and the box is perfect, everything
fits exactly... If anything isn't perfect, the
screws through the box are pushing/puling on
the DB connectors and the PCB.

How do folks deal with this, such that there's
a nice sturdy mount for the external connector,
but no stress on the PCB, given normal
manufacturing tolerances ?

Thanks in advance,
Best Regards, Dave

PS: This problem is much worse if the box is
manufactured with a tile saw, which seems to
have happened in a recent thread ;-)
On Wed, 31 Jul 2013 13:53:08 -0700, Dave Nadler wrote:

> I'm sure there are some standard techniques to minimize this ? > > Product has a number of connectors on a PCB, mounted in a box. Because > the external cabling and connectors will be subjected to stress, > there are (for example) hex nuts fastened through the sturdy box through > DB connectors on the PCB. Then when the external cable is yanked, it > doesn't break the PCB. > > So, the PCB is fastened to the box not just by normal mounting screws, > but at the connectors. > > If all the connectors are perfectly sized and aligned, and the box is > perfect, everything fits exactly... If anything isn't perfect, the > screws through the box are pushing/puling on the DB connectors and the > PCB. > > How do folks deal with this, such that there's a nice sturdy mount for > the external connector, but no stress on the PCB, given normal > manufacturing tolerances ? > > Thanks in advance, > Best Regards, Dave > > PS: This problem is much worse if the box is manufactured with a tile > saw, which seems to have happened in a recent thread ;-)
One thing to do is to make sure that where the PCB is supported by the connectors, it is _not_ supported by screws. This keeps the connectors and the screws from getting into a fight. Another is to attach the connectors to a jig that holds them in the correct relationship to one another (and possibly the screw holes on the PCB), and then solder them to the board. Obviously this will drive up the manufacturing price. I don't know what else to suggest, unless you want to experiment with mounting the board into the case using some thick epoxy putty as a spacer between the connectors and case: that would nail down the correct relationship between case and connector, at the cost of forever marrying each case and board. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
On 7/31/2013 1:53 PM, Dave Nadler wrote:
> Product has a number of connectors on a PCB, > mounted in a box. Because the external cabling > and connectors will be subjected to stress, > there are (for example) hex nuts fastened through > the sturdy box through DB connectors on the PCB. > Then when the external cable is yanked, it doesn't > break the PCB. > > So, the PCB is fastened to the box not just by > normal mounting screws, but at the connectors. > > If all the connectors are perfectly sized and > aligned, and the box is perfect, everything > fits exactly... If anything isn't perfect, the > screws through the box are pushing/puling on > the DB connectors and the PCB. > > How do folks deal with this, such that there's > a nice sturdy mount for the external connector, > but no stress on the PCB, given normal > manufacturing tolerances ?
Fasten connectors to case. Use oversized/slotted holes (depending on which direction(s) you need "slop") in the PCB. Fasten these points after all connector mounting hardware. [You run the risk that someone (repair depot/end user) takes the device apart and fails to follow the same order of reassembly] Of course, if the connectors are in the wrong place on the Z axis, you've got another problem to deal with... Alternatively, mount connectors on service loops (adds cost, decreases reliability -- all else being equal -- but gives you lots of leeway in what you put where)
On 31.07.2013 22:53, Dave Nadler wrote:
> I'm sure there are some standard techniques to > minimize this ?
Yes. The ultimate solution of course being "don't even go there", i.e. don't put connecters onto the PCB in the first place. A reasonably short piece of cable can makes your electrical connection while maintaining mechanical isolation. Another approach the PC ATX form factor one: have the connectors mounted _only_ to the PCB, and feed them through slightly tolerant openings in the housing from there. Or you make the connectors the only fasteners of the PCB in its housing. This would be particularly applicable to PCBs that are effectively dominated by connectors already. Yet another approach is to use connectors that have the equivalent of above-mentioned short cables built-in, e.g. angled connectors with somewhat long-ish legs that go from the connector's body down to the PCB.
Dave Nadler wrote:
> I'm sure there are some standard techniques to > minimize this ? > > Product has a number of connectors on a PCB, > mounted in a box. Because the external cabling > and connectors will be subjected to stress, > there are (for example) hex nuts fastened through > the sturdy box through DB connectors on the PCB. > Then when the external cable is yanked, it doesn't > break the PCB. > > So, the PCB is fastened to the box not just by > normal mounting screws, but at the connectors. > > If all the connectors are perfectly sized and > aligned, and the box is perfect, everything > fits exactly... If anything isn't perfect, the > screws through the box are pushing/puling on > the DB connectors and the PCB. > > How do folks deal with this, such that there's > a nice sturdy mount for the external connector, > but no stress on the PCB, given normal > manufacturing tolerances ? > > Thanks in advance, > Best Regards, Dave > > PS: This problem is much worse if the box is > manufactured with a tile saw, which seems to > have happened in a recent thread ;-)
About the only thing you can do is review the tolerances and dimensions on both the PCB and the panel. *IF* they are correct, tight as possible, and some common sense is used by your assemblers, it's generally not a problem.