Problem on connection with the negative 2450 coin cell PAD

i made my board with oshpark and in the back of the board i put a 2450 battery holder to put a rechargeable battery, the battery holder is this one :

and the datasheet is here :

as you can see the negative contact of the battery is connected directly to the PAD of the board. the negative PAD which is supposed to be in contact with my Coin cell battery is a square one with 6.07x6.07 mm as mentionned in the datasheet

after i soldered all my components, i tried to test it out the battery but it seems to have problem between the negative battery side and the negative PAD on the board. it works sometimes and sometimes not. if i touch little bit the battery i lost connection between the Coin and the PAD .

as you can see below an image of how i think the problem come from, but i’m not sure:

i don’t know if the negative pad plated with ENIG is higher than the solder mask or not.

what should i do in my next design to avoid this problem ??
should i add a solder mask clearance to the negative PAD ? or should i increase the size of the neagative PAD.

Any idea ?

Add a small blob of solder to the pad and spread it out. That will make it convex.

My thoughts exactly? You didn’t solder the pad?

He said it’s ENIG, so for all purposes pretty well protected against corrosion (I’d assume as reason for using ENIG)… putting a blob of lead there will solve the electrical contact problem, but kinda works against the corrosion protection that he had in mind (tin whiskers -

What kind of coin cell are you using?
Does the negative side protrude past the positive cap? Do you always use cells of that type?
Can you rule out shorts when you put it on a flat conductive surface?

If you need ENIG:
You could then increase the size of the negative pad as big as the coin cell (make it a circular SMT pad) and it will work as no solder mask will be protruding higher than the pad surface. Just forget about the datasheet recommendation for the pad in this case.

If you don’t need ENIG:
just do as @madworm said, might even have some solder paste (the pad then has got a paste layer as well) there to get you a blob started. And for corrsosion protection rub some silicone paste onto the coin cell before inserting… works better than nothing :wink:

Personally I’d increase the pad to the coin cell size plus a bit more (if you can rule out shorts there), get it circular and use ENIG if it’s needed for corrosion protection and use some silicone paste on top of the ENIG :sunglasses:

Maybe it’s ENIG because oshpark doesn’t offer any other finish?!

Oh haha… sorry, my mistake. Well then… no wonder they’re so expensive. I never dealt with them.

Thx for your answers.

@madworm i was thinking about this solution "Add a small blob of solder to the pad and spread it out. " but i’ll make as last resort if i didn’t find another way to fix that problem.

The reason why i avoid to put solder to the pad is that the solder will cause a corrosion with time to the soldered PAD. and may be causes also bad connectivity later. and this is not the case with the ENIG.

I think i’ll follow @Joan_Sparky idea and increase the Pad dimension to the maximum meaning 24 mm so no solder mask will stop the contact between PAD and coins cell.

@Joan_Sparky i use this one:

A bit old now - but maybe still an issue for the OP, or other readers . . .

I’m pretty sure the expected course to get the battery negative side to contact the pad is to build up the pad with a “blob-o-solder”. Don’t worry about corrosion between the ENIG surface and the solder blob, You might get some surface oxidation / schmutz on the air-side surface of the solder blob, but that should come off easily with a pencil eraser, or just with sliding the battery in and out a few times.

I searched on Google, and here is a link to a good picture of this at tronixstuff (go a few screens down the page).