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August 2018

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From:
"Stadem, Richard D" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D
Date:
Thu, 30 Aug 2018 18:13:13 +0000
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Yay! If it works, don't fix it again, right?

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Nutting, Phil
Sent: Thursday, August 30, 2018 10:51 AM
To: [log in to unmask]
Subject: Re: [TN] Mounting hole question.

We have had several opinions over the years of proper board configuration relative to hardware with "unsupported" (not plated thru) holes and plated thru holes for grounding.  Some folks get, shall we say passionate, when talking about the correct configuration.

Many years ago we manually assembled a split lock and flat washer onto the screw and then inserted the screw into the hole.  Adds a lot of labor.  Then we switch to external tooth SEM screws.  Boy do they dig up the copper "pad".   Well those looked ugly and if they were not tight would vibrate out and go looking for a circuit to short out.  Ok, add Loctite... more labor.  So now we are down to nylon patch hardware, no washers and either an unsupported hole or plated hole pad.  6-32 screws torqued to only 12 inch pounds.  Never ripped a pad off the board even after many screw insertions in the same hole.

Just my 2 cents.

Phil

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Decker, Scott UTAS
Sent: Thursday, August 30, 2018 11:34 AM
To: [log in to unmask]
Subject: Re: [TN] Mounting hole question.

Thanks Richard. I think this is the first time I've run across this issue... I can't say I've seen it before so I was just wondering and that's why I asked the group. Aint technology grand!

Scott Decker - Staff Engineer, PCB Design Services CID+ - Electronic Systems Center
UTC AEROSPACE SYSTEMS
3445 S. 5th Street, Suite 170, Phoenix, AZ 85040 U.S.A.
Tel: 602 308 5957  FAX: 602 243 2347
KE7MWT  AKA:PadMasterson
[log in to unmask]   www.utcaerospacesystems.com

CONFIDENTIALITY WARNING: This message may contain proprietary and/or privileged information of
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-----Original Message-----
From: Stadem, Richard D [mailto:[log in to unmask]] 
Sent: Thursday, August 30, 2018 8:28 AM
To: TechNet E-Mail Forum <[log in to unmask]>; Decker, Scott UTAS <[log in to unmask]>
Subject: [External] RE: Mounting hole question.

That is actually very common. If the screw/washer does not require a full plated copper ring under it for electrical purposes, then generally the screw is able to clamp the 270 to 360 degrees around the hole, giving it a good mechanical clamp. The copper around that part is going to lift the washer or screw head above the exposed FR-4 at least a few thousands, so it is usually OK if not overtightened.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Decker, Scott UTAS
Sent: Thursday, August 30, 2018 9:56 AM
To: [log in to unmask]
Subject: [TN] Mounting hole question.

Hi All,
   So to pad or not to pad, that's not really my question, but it's close. Over the many years I've been designing boards, I have always put a copper ring/pad/solid copper area around mounting holes. If you are going to put a screw and washer in the hole, I want to have something more than FR4 under the hardware. So now we have a situation where the mounting holes are so close to the edge of the board, if I put a copper ring large enough to ensure the hardware is on it, my pad is hanging over the edge of the board or within .010" of it. If our normal copper cut back from the edge is .050" or more, I won't have a ring any longer, but more of a "C" shape ring. That says that most of the washer/hardware is on the copper and the remaining part is on the FR4. I don't have a photo of this, but I think you get the picture... Anyone have any comments about this type of arraignment? I told the mechanical engineers that they need to move the holes inboard more to allow the full ring of copper, but in the current case, that's not an option. Going forward I would like to ensure the ring is at the .050" away, but to ME's that's space wasted and the holes don't have to be that far in... Ironically, the ME says we need to make sure we have a .050" cutback for copper from the board edge, (Yes, I know that, I like .075" or .100" even.) but yet, he's creating a situation where I can't have a full ring.

    Am I still in the dark ages and insisting copper be under all of the hardware or does it matter it's only partly covering? Any data to support having a full ring or not having a full ring?

Thanks in advance...


Scott Decker - Staff Engineer, PCB Design Services CID+ - Electronic Systems Center UTC AEROSPACE SYSTEMS
3445 S. 5th Street, Suite 170, Phoenix, AZ 85040 U.S.A.
Tel: 602 308 5957  FAX: 602 243 2347
KE7MWT  AKA:PadMasterson
[log in to unmask]<mailto:[log in to unmask]>   https://urldefense.proofpoint.com/v2/url?u=http-3A__www.utcaerospacesystems.com&d=DwIFAg&c=ilBQI1lupc9Y65XwNblLtw&r=Z5GKU9L_wgbp5uvPzUdN9HWtU4lRZiIaLoTaEwcy40w&m=Vfuu8-xr03XesbXPhm8cH_GrRDV87KmI6Gvn3pXBlWw&s=10zrx_vDioG1xIzbMRj35dOFA1mlcIxitD2cTg2tnNg&e=<https://urldefense.proofpoint.com/v2/url?u=http-3A__www.utcaerospacesystems.com_&d=DwIFAg&c=ilBQI1lupc9Y65XwNblLtw&r=Z5GKU9L_wgbp5uvPzUdN9HWtU4lRZiIaLoTaEwcy40w&m=Vfuu8-xr03XesbXPhm8cH_GrRDV87KmI6Gvn3pXBlWw&s=CkfdjjIBoOmwwZLcSpSMRu4N_FEgL4YqO5oGuRUqa7c&e=>

CONFIDENTIALITY WARNING: This message may contain proprietary and/or privileged information of UTC Aerospace Systems and its affiliated companies. If you are not the intended recipient please 1) do not disclose, copy, distribute or use this message or its contents, 2) advise the sender by return e-mail, and 3) delete all copies (including all
attachments) from your computer. Your cooperation is greatly appreciated.
This Document Does Not Contain Export Controlled Technology Or Technical Data.

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