PCB mounting fasteners

PCBs cold-flow under pressure. A star washer will have much higher pressure where it contacts the board as opposed to a flat washer.

If vibration is a serious issue, I'd use a wavy washer or a Belleville, for extra compliance, as a hedge against board flow.

We usually use split-rings and screws, and haven't had any loosening problems. Vibration and high temperature might justify fancier stuff.

(Oh, *never* use a screw connection to conduct substantial current into a pc board. It causes cold-flow runaway.)

John

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Mobos leave tinned metal where the screws go. Any explanation for this?

"John Larkin"

** Crikey - I was just talking about EXACTLY that on the phone to a colleague not an hour ago !!

Two large, screw terminal filter electros in a popular 1990s mosfet amplifier ( Jands M600 ) were attached to a small, single sided PCB of their own, butting up against the insulating side and held by bolts and washers passed through holes from the copper side.

After a couple or three years age, M600s started turning up with massive humming noise in both channels caused by one or other electro having isolated itself from the PCB pads - by burning the copper away ALL ROUND the bolt head !!

Similar damage was evident all four bolt heads and all were only finger tight.

The damage was a real PITA to fix too and the bolt head was so burnt it could not be re-used.

One repair was to solder a suitable brass washer over the damaged copper area and hope it spread the pressure enough to prevent a repeat any time soon. Then a new bolt and star washer to hold it tight.

None ever returned to me for the same trouble.

..... Phil

For sedentary applications, that just have to hold together during shipping, use a screw with integral lockwasher (split-ring or star) and nothing else, IMHO.

Paint/glue/goop is handy if you need to know if the customer tampered with the works. Or in industrial applications where vibration is 24/7.

Depends a bit on the consequences of a screw getting loose, of course.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

Grounding, of course.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

On a sunny day (Mon, 28 Dec 2009 21:30:39 -0800 (PST)) it happened " snipped-for-privacy@sushi.com" wrote in :

Yes, no current

This is in equipment that will be transported a lot (weekly) on a combination of highway and bumpy back roads.

If the screws come loose it will be a big problem most of the time.

hmmm, Well that sent a cold shiver down my back.

I have a isotop MOSFET with the four terminal screws going through plated (MAT TINNED) through holes (TOP and BOTTOM). The included screws (M4 screw) have a captive split washer on them.

Think this will have a similar problem? I'm hoping (hows that for design practice :( ) the spit washer maintains contact pressure.

The current through the terminals is 2A max.

Wait, what? Those things are frickin' expensive, why would you waste one on only 2A?

For that matter, why would you waste one on a hundred amps? They just aren't worth it... I honestly don't know who buys them, a couple TO-247's comes out 2-3 times cheaper every time.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

That's probably OK. The current density is low. Of course, if the fet gets hot, it will warm the board and increase the flow rate, which could loosen the screws. If the fet power cycles a lot, that will make things worse, gradually loosening the screws. Maybe, but not likely.

Check some nut torques after some hours of operation.

John

"John Larkin" wrote in message news: snipped-for-privacy@4ax.com...

What would be the best solution for this, Swaged standoffs soldered in? keystone Screw terminals? I guess those big-ass IGBT's are only meant for bussbar connections.

Cheers

mook johnson Inscribed thus:

Lock washer every time ! Minimum parts count. Minimum cost.

--
Best Regards:
                     Baron.

mook johnson Inscribed thus:

In that case use slotted head and solder a retaining wire across the slot.

--
Best Regards:
                     Baron.

And the problem with snap-in insulated standoffs like everybody else uses is....?

--
Many thanks,

Don Lancaster                          voice phone: (928)428-4073
Synergetics   3860 West First Street   Box 809 Thatcher, AZ 85552
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Please visit my GURU's LAIR web site at http://www.tinaja.com

I'm not sure what a splice lock washer is. If it's a split washer then I'd also use a flat washer. If it's a 'star' lock washer then not.

BTW I recently found seastrom washers in the US

The Be-Cu wave washers solved my grayhill switch grounding problem. They make a large variety of wahsers.

George H.

That was meant to be internal sline washer.

John,

Can you elaborate on what you consider "substantial current" to be? I have an application where we are bolting a 30A terminal block to an FR4 backplane for power entry. On one side of the backplane, we are using M5 bolts, with a star washer under the head of the screw. The base of the terminal is on the opposite side of the backplane. I'm trying to figure out if we are in trouble. We have not had any products fail at customer sites thus far, but I want to know if I should be expecting issues.

Thanks, Miles

The PCB may cold-flow and reduce the tension on the washer. And the star washer teeth will dig in, forming a few very high-pressure, high-resistance points. Any heating at the contact points will increase the pcb temperature locally and increase the flow rate. This can run away. 30 amps is definitely in the danger zone.

It could be a time bomb. I'd suggest that you pull some older units and check the screw torque to see if things are loosening up.

Soldering is best. After that, Belleville or wavy washers, preferably beryllium copper, would be a better bet than stars or split rings. Both have more contact area and more compliance.

John

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how about a flat washer underneat the star washer? will still get the spring, but the teeth will dig into washer and bolt not pcb.

-Lasse