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July 2014

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Mon, 7 Jul 2014 08:09:26 -0600
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TechNet E-Mail Forum <[log in to unmask]>, Steve Gregory <[log in to unmask]>
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Steve Gregory <[log in to unmask]>
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Hi Phil,

It's to isolate the three of TO220 packages from the bracket/heatsink that they are screwed to, the fourth TO220 device that is screwed to the bracket/heatsink  is the  full pack type where it doesn't need a washer for isolation. All these devices sit on top of a custom cut Berquist SIL pad, and the bracket/ heatsink is designed to capture the board next to it when it is broken out of the panel and folded over. The two boards are connected by a flex circuit which is soldered in during wave solder. So it winds up being a rigid flex assembly without really being a rigid flex PCB, I guess to keep costs down. There are two sets of these boards in a panel, and you can't get to the screws easily on one set of boards without them being broken out of the panel. They don't get broken out of the panel until they've been tested and conformal coated, which we don't do, the customer does that. 

So you can see the quandary we're in. Probably going to recommend going to a different material washer, along with some sort of chemical threadlocker.

Steve 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Nutting, Phil
Sent: Monday, July 7, 2014 7:31 AM
To: [log in to unmask]
Subject: Re: [TN] Screws losing torque by themselves

Steve,

First of all, why do you need the Nylon washer?  Nylon/wave solder heat... bad combination... expect the Nylon to change shape.  Why it didn't come loose before might be explained by someone substituting cheaper Nylon 6/6 for a glass filled Nylon.

My choice to mount the devices, if isolation is required,  Would be to use T-Gard by Laird Technologies and an Aavid 118300F00000G metal screw down clip with a square cone stainless screw using 10 inLb of torque. Or to eliminate the screw parts and operation simply use an AAVID CLP212G clip, set it and forget it.

But this is just me opinion backed be 4 decades of experience.

Phil

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Victor Hernandez
Sent: Monday, July 07, 2014 7:20 AM
To: [log in to unmask]
Subject: Re: [TN] Screws losing torque by themselves

Is it possible that the screw can be torqued post solder instead of pre solder?

Victor,

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Monday, July 07, 2014 6:01 AM
To: [log in to unmask]
Subject: Re: [TN] Screws losing torque by themselves

It is a good idea to double check the torque after soldering as part of the final inspection.
I would suggest after the final inspection, that the nut be captured with Glyptal. The nylon washer will continue to cold flow over time, as my good friend Mr. Bogden explained, and the torque could continue to relax somewhat. The Glyptal will at least prevent the nut from loosening.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of stephen gregory
Sent: Sunday, July 06, 2014 5:24 PM
To: [log in to unmask]
Subject: Re: [TN] Screws losing torque by themselves

Hi Wayne,

Thanks, and thanks to everyone that responded. I guess I didn't think about things clearly when I posted my question. The reason was that that we've been building these assemblies for a while and just lately the problem has become an issue with our customer, which leads me to believe that for some reason they've been recently checking torque values on this hardware after we've shipped the assemblies to them.

I can surmise a lot of things about why they might have been checking them, but I don't know for sure. But I do know one thing now, those screws and nuts will lose torque during our normal build processes.

Steve


________________________________
From: Wayne Thayer
To: [log in to unmask]
Sent: Sunday, July 6, 2014 6:26 AM
Subject: Re: [TN] Screws losing torque by themselves


Hi Steve-

I love the video. I've been trying to tell the mechanical engineers I've worked with about the problems with the helical washers (sometimes stupidly referred to as "lock" washers!). This video assists with developing an intuitive feel for what's going on. The helical washers do have their place, but not in a high vibration environment, and virtually always in addition to a thread locking system of some type (double nut or chemical adhesive).

The root of your problem is in the understanding of what Glass Transition Temperature means. It means that above that temperature, the material will deform PERMANENTLY if placed under a load. Nylon has an extremely low Tg, but even most PC board materials top out at about 170C.

So you're asking us why, when you put a clamp on some plastic material and take it way above its Tg, it seems to deform, thus reducing the clamp pressure?

Wayne
________________________________________
From: TechNet [[log in to unmask]] on behalf of Yuan-chia Joyce Koo [[log in to unmask]]
Sent: Thursday, July 03, 2014 5:28 PM
To: [log in to unmask]
Subject: Re: [TN] Screws losing torque by themselves

Nylon washer might made of nylon 66, that would be lost of mechanical strength during your wave temperature.  see attached.  IMO.  (double check your datasheet for the washer for thermal properties.  fyi.
http://www.plastic-products.com/spec1.htm
    joyce
On Jul 3, 2014, at 4:19 PM, Steve Gregory wrote:

> Hi all,
>
>
>
> Thought I might try to catch you before you left for your 4th of July 
> holiday. We have a strange little issue that we're trying to figure 
> out that's been happening with some torqued hardware on a board that 
> we build.
>
>
>
> The assembly has this heat-sink/bracket deal with four TO-220 devices 
> put together before we stuff the board and wave solder it.
> The bracket is aluminum and we first put down a SIL pad, then the
> TO-220 devices, three of them get nylon shoulder washers, then a split 
> lock washer, then a nut. The three devices with the nylon shoulder 
> washers get torqued to 3.4 in/lb., and the one without the shoulder 
> washer gets torqued to 6 in/lb. We tighten and torque from the screw 
> side, and hold the 3/16ths nut stationary.
>
>
>
> Then the whole bracket assembly is stuffed into the board along with 
> the rest of the parts, then the board is wave-soldered and cleaned. 
> Any touch-up is done, then there are certain parts and points on the 
> board that is RTV'ed and cured, then the board is shipped.
>
>
>
> Our customer has been finding that when they get the boards and they 
> check that the hardware is torqued, they can get ¼ to sometimes a ½ a 
> turn on the screws with the shoulder washers before the torque driver 
> clicks, and of course we're getting zinged for not torqueing the 
> hardware. The ones without the shoulder washers are not near as bad, 
> but they tell us that they have been finding a few that they can turn 
> a little before the torque driver clicks.
> We've got new torque drivers and they're set correctly. We even sent 
> our drivers to our customer so they could check them out and compare 
> them with the drivers that they have. They were pretty much identical. 
> We use Lindstrom torque drivers, so they're not some cheap knock-off 
> brand.
>
>
>
> Here's a picture of the bracket and screws we're having most of the 
> trouble with:
>
>
>
> http://stevezeva.homestead.com/Bracket-Heatsink.jpg
>
>
>
> I've checked them here, and I find the same thing. I know they were 
> torqued properly when the brackets were assembled, but when I check 
> the boards that have been wave soldered and cleaned, I find I can get 
> ¼ turn one the screws with the nylon shoulder washers before the 
> torque driver clicks.
>
>
>
> I've always thought that when you used a screw and nut with a split- 
> washer it would hold it's torque, but then I found this:
>
>
>
> http://www.boltscience.com/pages/helicalspringwashers.htm
>
>
>
> So now I'm not sure. Has anybody ever run into this issue? Is it 
> normal to have things "relax" like this after they've been torqued?
>
>
>
> Steve
>
>
>
>
> ______________________________________________________________________
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