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'[EE] Solder mask strength for traces underneath ba'
2011\10\13@093303 by Oli Glaser

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Hi all,

I have a small project that uses a couple of coin cell battery holders:
http://uk.farnell.com/keystone/3003/retainer-coin-cell-20mm-pk5/dp/908654

The negative contact is provided by a pad about a third the area of the battery on the PCB.
Space is quite tight, so I was thinking about running some traces underneath the battery near the negative contact.
My worry is about the solder mask breaking down and shorting the traces to the underside of the battery, especially with repeated battery changes (although this should be infrequent, perhaps once every six months)
I will probably just fiddle about and find a way to route around, but I would like to hear any thoughts as to whether this would be "safe" to do, and how resistant solder mask generally is.
Thanks.

2011\10\13@100415 by RussellMc

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> I have a small project that uses a couple of coin cell battery holders:
> http://uk.farnell.com/keystone/3003/retainer-coin-cell-20mm-pk5/dp/908654
>
> The negative contact is provided by a pad about a third the area of the
> battery on the PCB.
> Space is quite tight, so I was thinking about running some traces
> underneath the battery near the negative contact.

I have no exactly specific experience, and it would depend on the mask
material, and I'd expect an epoxy two pot mask to survive well BUT I'd still
be highly wary of doing this. It looks like being well within the range of
things that Murphy excels in being good at.

By putting an unrelated track under the contact area you are introducing a
masked track to compete with the unmasked contact. If the contact is using
immersion gold or something else very thin the track + mask could end up
higher than the contact and preferentially take the contact pressure.

An "obvious" [tm] solution seems to be to build up the contact height 'ever
so slightly". Either roller tin or some purposeful extra plating or even an
actual contact shim etc.

Two very thin wires say 75% or so of the contact width apart and terminated
somewhere convenient in vias/holes just outside the battery fotoprint would
allow the battery to "sit up on rails above the pcb surface
When rendered in Courier New the following makes an ugly diagram of the
above, but it's unlikely to be needed.

BBBBBBBBBB
BBBBBBBBBB   Battery
CCCCCCCC     Contact
 o    o        Wires
PPPPPPPPPP       PCB

Two or one strip of thin shim may be kinder to the cell. May be able to be
soldered at end nearest push in direction only. May.

Battery manufactuer have differing ideas of how wide the centre contact is.
Figures of 16, 17 and 17.7 mm can be found for a CR2032 cell.

Your holder    http://www.farnell.com/datasheets/15138.pdf

Wikipedia CR2032    http://en.wikipedia.org/wiki/CR2032_battery

Very nice links page to CR2032 from many manufacturers    http://cr2032.co/

Sony 17mm    http://cr2032.co/datasheets/sony-cr2032-datasheet.pdf

Duracell - don't know and draftsman was drunk (see 20mm dimension arrow)
http://cr2032.co/datasheets/duracell-cr2032-datasheet.pdf

Energiser 17.7mm  http://cr2032.co/datasheets/energizer-cr2032-datasheet.pdf

Varta - shoulder dimension <> contact width?
http://cr2032.co/datasheets/varta-cr2032-datasheet.pdf

Panasonic 16mm    http://cr2032.co/datasheets/panasonic-cr2032-datasheet.pdf

Maxell - go away    cr2032.co/datasheets/maxell-cr2032-datasheet.pdf
Some nice others specs.


   Russell McMahon
   Applied Technology ltd.












>

2011\10\13@111336 by Joe Wronski

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Use teflon tape under battery holder at assembly time?
Joe W

On 10/13/2011 9:32 AM, Oli Glaser wrote:
{Quote hidden}

>

2011\10\13@114004 by alan.b.pearce

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Personally I wouldn't put solder mask in the area that the battery lives, to ensure that the pad contact surface is that few thou clear of the fibreglass, but that is not what the OP is wanting ...



> {Original Message removed}

2011\10\13@115623 by RussellMc

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> Use teflon tape under battery holder at assembly time?

If you look at how the battery holder works, that's not viable, alas.

Battery outer is upwards and is spring finger  contacted.
Battery centre is downwards and made to slide on PCB contact. Teflon
tape would eithrr block contact r, if a hold was left, would still
separate battery from PCB.


Russel

2011\10\13@140759 by Oli Glaser

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On 13/10/2011 15:03, RussellMc wrote:
> I have no exactly specific experience, and it would depend on the mask
> material, and I'd expect an epoxy two pot mask to survive well BUT I'd still
> be highly wary of doing this. It looks like being well within the range of
> things that Murphy excels in being good at.

These were pretty much my thoughts too - I know solder mask is generally pretty tough (from experience scraping it off to probe traces etc) but Murphy is bound to ensure that a few percent have problems, for instance if someone uses a sharp implement to push the old battery out when changing batteries and accidentally scrapes a bit of the mask off.
I will ask my PCB house if they have any suitable coating that can be added in that area.

>
> By putting an unrelated track under the contact area you are introducing a
> masked track to compete with the unmasked contact. If the contact is using
> immersion gold or something else very thin the track + mask could end up
> higher than the contact and preferentially take the contact pressure.

This is an excellent point, will see what could be done about this also.

> An "obvious" [tm] solution seems to be to build up the contact height 'ever
> so slightly". Either roller tin or some purposeful extra plating or even an
> actual contact shim etc.

Yep, another question for PCB house.

> Two very thin wires say 75% or so of the contact width apart and terminated
> somewhere convenient in vias/holes just outside the battery fotoprint would
> allow the battery to "sit up on rails above the pcb surface

Interesting idea, another thing to consider, thanks.

2011\10\13@141151 by Oli Glaser

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Thanks to all who replied - reading them and having had a few hours to ponder, I think I will almost certainly go with the route around option for peace of mind.
For the "price" of a little more work and thought it seems to make good sense.
There are also other holders available that do things differently, although I picked this one as it was low profile and cheap (requirements for project)
I'll figure something out :-)

2011\10\13@165458 by Brent Brown

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part 1 1639 bytes content-type:text/plain; charset="us-ascii" (decoded quoted-printable)

On 13 Oct 2011 at 14:32, Oli Glaser wrote:

> Hi all,
> > I have a small project that uses a couple of coin cell battery holders:
> uk.farnell.com/keystone/3003/retainer-coin-cell-20mm-pk5/dp/90865
> 4
> > The negative contact is provided by a pad about a third the area of the
> battery on the PCB. Space is quite tight, so I was thinking about
> running some traces underneath the battery near the negative contact. My
> worry is about the solder mask breaking down and shorting the traces to
> the underside of the battery, especially with repeated battery changes
> (although this should be infrequent, perhaps once every six months) I
> will probably just fiddle about and find a way to route around, but I
> would like to hear any thoughts as to whether this would be "safe" to
> do, and how resistant solder mask generally is. Thanks.

FWIW, once upon a time I helped design a PCB that had an "embedded" transormer... the windings were spiral traces on the PCB, two piece
ferrite core through routed slots in PCB and a clip to hold it together.
The traces under the ferrite were covered by soldermask, plus we added
regions of top and bottom overlay fill to increase ruggedness, see
attached picture. Even then I think the idea was to glue the ferrite in
place so it didn't move around and wear through anything.

-- Brent Brown, Electronic Design Solutions
16 English Street, St Andrews,
Hamilton 3200, New Zealand
Ph: +64 7 849 0069
Fax: +64 7 849 0071
Cell: +64 27 433 4069
eMail:  spam_OUTbrent.brownTakeThisOuTspamclear.net.nz




part 2 8715 bytes content-type:image/jpeg; name="PCBTX.jpg" (decode)


part 3 181 bytes content-type:text/plain; name="ATT00001.txt"
(decoded base64)

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2011\10\14@123546 by M.L.

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On Thu, Oct 13, 2011 at 9:32 AM, Oli Glaser <.....oli.glaserKILLspamspam@spam@talktalk.net> wrote:
> My worry is about the solder mask breaking down and shorting the traces
> to the underside of the battery, especially with repeated battery
> changes (although this should be infrequent, perhaps once every six months)
> I will probably just fiddle about and find a way to route around, but I
> would like to hear any thoughts as to whether this would be "safe" to
> do, and how resistant solder mask generally is.
> Thanks.
>

I use tinned pads underneath the coin cell. The pads are a few
thousands thick after being tinned, and I otherwise have vias and
traces underneath the cell.
No problems with this method so far. A few hundred units have been made.

-- Martin K

2011\10\14@124639 by Oli Glaser

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On 13/10/2011 21:54, Brent Brown wrote:
> FWIW, once upon a time I helped design a PCB that had an "embedded"
> transormer... the windings were spiral traces on the PCB, two piece
> ferrite core through routed slots in PCB and a clip to hold it together.
> The traces under the ferrite were covered by soldermask, plus we added
> regions of top and bottom overlay fill to increase ruggedness, see
> attached picture. Even then I think the idea was to glue the ferrite in
> place so it didn't move around and wear through anything.

Interesting stuff, thanks.
Although I think I have found a suitably priced holder with a "proper" bottom contact, I think I will still order two proto boards (one with new holder and one using mask) mainly just to satisfy my curiosity about how well it might work. Not asked PCB house for their views yet, will be interested to see what they could do without adding too much cost.

2011\10\14@125415 by Oli Glaser

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On 14/10/2011 17:35, M.L. wrote:
> On Thu, Oct 13, 2011 at 9:32 AM, Oli Glaser<oli.glaserspamKILLspamtalktalk.net>  wrote:
>> My worry is about the solder mask breaking down and shorting the traces
>> to the underside of the battery, especially with repeated battery
>> changes (although this should be infrequent, perhaps once every six months)
>> I will probably just fiddle about and find a way to route around, but I
>> would like to hear any thoughts as to whether this would be "safe" to
>> do, and how resistant solder mask generally is.
>> Thanks.
>>
> I use tinned pads underneath the coin cell. The pads are a few
> thousands thick after being tinned, and I otherwise have vias and
> traces underneath the cell.
> No problems with this method so far. A few hundred units have been made.
>

Thanks, just the reply I was looking for.
As mentioned in my other mail, I will grab a few proto boards of each type and see how it goes. If we can achieve reliable results with this method (e.g. mask/tinned pads/simple holder) then it will be preferable, as plastic battery holders with both contacts are quite a bit more expensive than the basic metal Keystone ones.

2011\10\14@130445 by M.L.

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On Fri, Oct 14, 2011 at 12:53 PM, Oli Glaser <.....oli.glaserKILLspamspam.....talktalk.net> wrote:
> Thanks, just the reply I was looking for.
> As mentioned in my other mail, I will grab a few proto boards of each
> type and see how it goes. If we can achieve reliable results with this
> method (e.g. mask/tinned pads/simple holder) then it will be preferable,
> as plastic battery holders with both contacts are quite a bit more
> expensive than the basic metal Keystone ones.

I hope I was clear that the PCB tinning isn't enough, you'll have to
have the assembly people tin the pads manually with a soldering iron
(or automatic reflow with solder paste)

It might work with HAL but if you go gold or silver finish it will be
much too thin.

-- Martin K

2011\10\14@165646 by Oli Glaser

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On 14/10/2011 18:04, M.L. wrote:
> On Fri, Oct 14, 2011 at 12:53 PM, Oli Glaser<EraseMEoli.glaserspam_OUTspamTakeThisOuTtalktalk.net>  wrote:
>> Thanks, just the reply I was looking for.
>> As mentioned in my other mail, I will grab a few proto boards of each
>> type and see how it goes. If we can achieve reliable results with this
>> method (e.g. mask/tinned pads/simple holder) then it will be preferable,
>> as plastic battery holders with both contacts are quite a bit more
>> expensive than the basic metal Keystone ones.
> I hope I was clear that the PCB tinning isn't enough, you'll have to
> have the assembly people tin the pads manually with a soldering iron
> (or automatic reflow with solder paste)
>
> It might work with HAL but if you go gold or silver finish it will be
> much too thin.
>

Yes, I got the idea thanks. It will be sold as a kit so the pad could be tinned during construction if absolutely necessary. I intend to talk to the PCB house tomorrow and ask what they can do, as I would prefer to make things as "easy" as possible to put together by having it tinned/raised sufficiently in production.

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