Traces under CR2032 battery holder - how tough is solder mask? - Page 1
Forum section: electronics / projects

author: andersjm
author_rank: Contributor
timestamp: November 30, 2016, 05:44:36 am
content: I'm designing a small battery powered device.I'm using this battery holder:http://www.digikey.se/product-detail/en/mpd-memory-protection-devices/BK-5067/BK-5067-ND/1033686It uses just an exposed pad on the PCB for negative terminal.Since I'm space constrained, I'm tempted to route signals under the battery. To avoid a short circuit, I would then be relying on the solder mask.Will this work reliably?Or is there a risk that the solder mask will wear off, causing a short-circuit between the CR2032-battery negative terminal and the exposed traces on the PCB?What are your thoughts?Edit: The thread title says CR2032, but the holder is CR2016. I mistyped the title.

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 30, 2016, 05:47:35 am
content: I would NOT do that. It sounds like just setting yourself up for failure.Can you not route the traces on the other side of the board or something?At the very least, I would put a reasonable coating of solder on the battery terminal pad so that the entire bottom of the battery isn't constantly touching the board.

author: ataradov
author_rank: Super Contributor
timestamp: November 30, 2016, 06:29:57 am
content: Your biggest problem would be actual electrical contact. For this thing to work you need to have the whole area under the holder exposed. If you have only small circle exposed and the rest are traces covered with solder mask, then the thickness of the traces and solder mask will be bigger than the thickness of the exposed copper alone, so there will be no electrical contact, unless the battery is slightly convex.PS: But otherwise solder mask is pretty reliable. I personally would not worry about it wearing out for a typical remote control application with battery life of 1-2 years. Your buttons or other mechanical things will fail much faster.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 30, 2016, 06:47:32 am
content: Solder mask is not reliable enough when you have something like a battery cell which is inserted and possibly reinserted a few times. Sliding it in by the end user puts more stress on the solder mask, especially if there is some dirt inbetween.In some cases, it's ok to do that (and then, adding a silk screen fill over the critical place will help a bit more), but this doesn't sound like such a case.

author: RogerRowland
author_rank: Regular Contributor
timestamp: November 30, 2016, 06:52:01 am
content: While agreeing with all previous comments, in case you haven't seen it, the recommendation from the manufacturer says to use gold or nickel plating for the PCB pad, not tin or paste.http://media.digikey.com/pdf/Data%20Sheets/Memory%20Protection%20PDFs/BatteryRetainerGuide2011.pdf

author: AndyC_772
author_rank: Super Contributor
timestamp: November 30, 2016, 07:29:41 am
content: Never rely on solder mask to be an insulator.

author: andersjm
author_rank: Contributor
timestamp: December 03, 2016, 09:54:52 am
content: Quote from: ataradov on November 30, 2016, 06:29:57 amThank you!! I hadn't thought about this aspect at all!The exposed negative pad rises up above the etched part of the board, but I didn't think about the fact that so do regular traces! D'oh!I managed to reroute all traces, but now I just realized I should probably have added a keepout for the ground fill as well (for which I wasn't worried about short circuit).How much do pads rise above soldermasked ground fill?I already sent an order to the PCB fab for the first prototype. Maybe I should order a rev2 already now before even receiving rev1?

author: SeanB
author_rank: Super Contributor
timestamp: December 03, 2016, 11:02:09 am
content: There are plated negative contacts and a stick on insulator for CR cells and holders, very often used to do this and keep insulation. The metal plated tag solders into a hole in the middle of the holder area, and then you put the sticky label over it to provide the insulation. Your boards might be retrofittable using a small circular mylar sticker and a gold contact finger ( like in most edge connectors) to provide the slightly higher bump for the negative terminal.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 03, 2016, 11:03:04 am
content: With ENIG, pads are BELOW soldermasked ground fill, because the soldermask adds thickness whereas the ENIG coating is almost nonexistent in thickness.

author: ataradov
author_rank: Super Contributor
timestamp: December 03, 2016, 06:21:43 pm
content: Quote from: andersjm on December 03, 2016, 09:54:52 amJust consider rev1 a prototype and add a dab of solder to build up the thickness.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 04, 2016, 06:54:55 am
content: Quote from: andersjm on December 03, 2016, 09:54:52 amDon't do that, because the rev1 most likely has a few additional small issues you can easily fix manually while prototyping. Then you can fix all at once for rev2, and probably will only need 2 or 3 revisions total, not 10.

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 04, 2016, 07:48:47 am
content: You could put a blob of solder on the pad that contacts the battery, and then a piece of Kapton tape over the footprint of the battery, with a hole punched in the middle for the battery contact. That would be a guaranteed solution, but it would be practical only for small-scale hand-assembly. It would not be practical for a commercial product on a production line.

author: janekm
author_rank: Unknown
timestamp: December 04, 2016, 08:03:44 am
content: You can get little stamped metal contacts to use for the negative terminal, that avoids most of your issues and will give a reliable connection to the battery.

author: mariush
author_rank: Super Contributor
timestamp: December 04, 2016, 09:56:40 am
content: You could repurpose one of those metal dome buttons ... you solder/glue it in the center of the pcb pad / battery and the battery keeps a constant pressure on it, making good connection.Here's some links if it's not clear what I mean :http://www.snaptron.com/products/http://www.memcon.eu/products/metal-domes/So not use them as actual tactile switches, but as a way to make contact between pcb and battery and add a tiny bit of separation between them.Can't find them on Digikey or Farnell/Newark but they should be cheap if you buy hundreds/thousands of them.You could also get some adhesive tape and just cut a square in the center of the tape, enough to let these dome go through, while the rest of the pcb is insulated.

author: janekm
author_rank: Unknown
timestamp: December 04, 2016, 01:50:28 pm
content: Here's an example:http://www.battery-contacts.com/part.php?pn=BK-2982&original=Coin%20Cell&override=Coin%20CellBut there's a lot of variants of that available. I'm sure there's some on digikey too.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: December 04, 2016, 05:34:37 pm
content: If you're going to use a holder like that, I suggest the through-hole version. They will find a way to rip the holder off the board when changing batteries:http://www.digikey.se/product-detail/en/mpd-memory-protection-devices/BK-913/BK-913-ND/2329371Alternatively, the insulated one with contacts for both terminals lets you route under the battery holder:http://www.digikey.se/product-detail/en/mpd-memory-protection-devices/BC2032-E2/BC2032-E2-ND/2077836

author: technix
author_rank: Super Contributor
timestamp: December 04, 2016, 11:24:22 pm
content: Actually I would refrain from using soldered battery at all. A battery holder always include insulation between the cells and the board, making routing a bit easier.