PCB Footprint Design of a Coin Cell Holder

Question

I have a PCB that has issues with the coin cell bottom side making poor contact with the GND pad below it. Even with solder paste applied during the assembly, the pad is a bit inset of the soldermask. I know the image looks shiny, but there is conformal coat masking under the coin cell holder.

My initial reaction was to add more solder paste, but Ive read other posts (link below) that list issues arising from using solder as the contact point due to corrosion. My goal is to design something that will work reliably for 20 years so that is a bit of a concern to me.

Coin cell battery retainer, problems and best practices

Is the best option here to add a copper keepout under the coin cell (surrounding the pad)? What are typical best practices for coin cell holder footprints?

Answer 1

It looks like the other holder uses ENIG and has a very wide area to make contact with the coin cell. ENIG is more suitable for protection against corrosion and gold does better with this because it doesn't corrode or oxidize. Even with small mechanical variations area will do better with a wider area. So I'd use ENIG and a wider area for the PCB pad.

You can go with a solder bump, but then you have to work out galvanic action and see if the metals are dissimilar, water also wrecks havoc on metal interfaces like this (maybe you could also cut the humidity somehow). The best thing would be to use a battery holder with a spring loaded tab but I don't know if your cost structure supports that.

Answer 2

We had a similar problem with poor contact on the bottom side of an oval shaped battery when we used a Keystone Electronics 3039 battery holder. Under the holder we added a Keystone Electronics 2993 spring contact.

This fixed our electrical contact issue, but the spring contact had a tenancy to push one end of the battery up so it was no longer parallel to the PCB, and under vibration the battery would slide out. At first we tried adding nylon screws to act as a stop, but it wasn't reliable. In the end we made a plastic part to keep the battery from sliding out and keep it parallel to the PCB. I don't have a good picture of the plastic piece, but it essentially was a rectangle that we bolted to the PCB that had an oval cutout to grab the end of the battery.

I can't describe the application, but it was deployed on a military vehicle for more than a year without issue.

Answer 3

Only thing preventing shorting the battery in your example is solder mask. I would recommend moving the trace running across the battery.

With the same component I ended up making a larger area ENIG pad and left solder mask out from around it to prevent the battery from climbing onto it.

The design survived mechanical testing for "rough use" and has worked fine in the field.

PCBA manufacturers consider ENIG plating to be stable enough for soldering process for ~12-24 months. That's not the same as how long it remains conductive, but indicates that it too corrodes.

Hard gold will withstand the test of time and wear from battery insertions better, but it's an increased cost, especially middle of the board and therefore not practical.

Answer 4

You could perhaps use one or more of these:

They come in many different diameters, and are typically intended as targets for pogo pins to contact. This one is the largest and thinnest I could quickly find, 6 mm in diameter and only 0.6 mm thick. If you use a coin cell holder designed for two stacked cells, or designed for a cell the same diameter as your intended one but 0.6 mm thicker, you should be able to use one of them as the second contact point. You could also use an array of several smaller-diameter ones.

Answer 5

They make contacts for the other terminal - eg keystone 299x or MYOUNG MY-FJ-131-J

Answer 6

Most coin cells have recessed sides, so they won't short out when placed on a conductive surface. Assuming you have one of these, your simplest solution is to remove all solder mask for the area of the coin cell, and ideally a mm or so outside. That will give a perfect contact for the lower connection.

It's not going to help you with the current device, but it'll definitely improve things in future.

I'd also note that if you're doing this for a non-disposable piece of electronics, gold plating is best practise, because that connection pad is going to oxidize and the connection is going to get worse over time. Not a problem if your device isn't expected to last longer than 6 months; but it might be for something longer-lived.