VIA holes under coin cell battery, acceptable?

Thread Starter

Sam Matthews

Joined Jan 16, 2016
178
Good evening to you all, or whatever time it is where you are ;)

Just a quick question, I apologize if this doesn't fit here, what're your thoughts regarding VIA holes located underneath a coin cell on a PCB? On a through-hole battery holder there is sufficient insulation from the battery holder itself. However, on a 'clip' style coin cell holder there is no insulation between the battery GND and the PCB itself. Would you advise to not have any VIA's underneath it at all, filled vias or exposed?

I've attached an image of the style of holder I'm using.

Thank you,
Sam Matthews.
 

dl324

Joined Mar 30, 2015
9,577
If the vias are covered by a solder mask, they may be fine. If you want some added protection, add more insulation.
 

SLK001

Joined Nov 29, 2011
1,543
It depends on the consequences of a via shorting to the down side of the battery. A piece of Kapton tape over the vias (and under the battery) would probably be okay for a non-critical rout. You can't depend on solder mask alone to be an insulator.
 

paulktreg

Joined Jun 2, 2008
778
Because you are relying on the PCB for the negative connection on your battery and hence no solder mask it would be unwise to have any other connections under your battery?
 

Thread Starter

Sam Matthews

Joined Jan 16, 2016
178
Thank you for your responses. I'm really tight on space on this design so i thik for now i don't have much choice other than to route with VIAs under the batt and insulate them. They're input pins for an ATMega328p, more specifically SPI, TX, RX and a few others. I will go with some Kapton tape like suggested. I have a 'sort of' prototype here that i have wired up with VIAs under the batt and it is working, but i wanted to know the 'proper' answer that conforms to the rule book.
 

tindel

Joined Sep 16, 2012
701
I generally try to keep via's out from under parts - metal parts or otherwise. It's generally bad practice, but rules can be broken if the consequences are understood. In your case, since the battery can be removed and replaced, I would do anything I could to keep from putting vias under that battery. Kapton is fairly thick - I'm not sure your battery will go under that lip with kapton in place. Not to mention kapton isn't terribly adhesive so sliding a battery in and out will probably move the kapton a bit.

The most important thing is to NOT depend on the solder mask to provide insulation. I can't tell you how many times I've seen solder mask delaminated or missing from the board - especially when something is moving against the surface (i.e. a screw). I would encourage you to try different routing techniques until you find one that meets your routing requirements and keeps vias out from under the battery. Sometimes this can be accomplished by using a different gate of a part, or different via size, moving a part to another location, adding additional layer(s), using different trace width and spacing, etc. Some board manufactures will put insulators on top of the silkscreen that is much more robust and thinner than kapton for a nominal fee.

Having said that - almost every board I have designed has had a via under a part somewhere. Usually the parts are plastic or ceramic though - almost never metal. The times I have put vias under metal parts (high-rel FPGA's and oscillators), I have ALWAYS used a insulator layer placed by the manufacture.
 

djsfantasi

Joined Apr 11, 2010
5,923
It looks to me that that battery holder depends on pads underneath it to make the negative contact. Usually, a small solder blob on the pads is required to make mechanical contact with the battery. This reduces the room for any vias or traces.
 

RichardO

Joined May 4, 2013
2,271
I have used that type of batter holder many times and I would _never_ consider putting vias under the battery.

I can not think of any way of making a contact for the battery without it also making contact to the vias.

Actually, I can think of one way. That is to install a brass rivet as the negative battery connection (pad). I have done this for single sided PCB's that have the battery on one side of the board and SMT parts on the other side. I would not do this for a production assembly since there is still a chance of the battery contacting the vias on insertion or removal.
 

Thread Starter

Sam Matthews

Joined Jan 16, 2016
178
There is a centrally located pad under the battery for its negative connection. If I were to use Kapton tape I would put it only around the pad (leaving the pad available to the battery) and apply a little solder to the pad itself to increase its height off the board to allow for a connection. Now you come to inform me that its quite thick, and yet not that insulating, i'm going to really struggle with the routing of this. Its a very small board (has to fit behind a light switch) with an RF module attached to it where i have to keep the area underneath the antenna free from all traces possible.
 
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