We have a board that uses a solder pad for one of the contacts of a button battery and had no problems when we were washing the boards. We are now using a no-clean flux and find that on a bunch of boards there is a coating of flux covering the pad so the battery doesn�t make contact with the solder pad. (note: that the battery is not soldered, the battery slides into a holder) The boards are either Immersion Silver or ENIG, what do you guys think of not applying any paste to this pad?
Some no-cleans will pool flux on the top of the solder joint which acts like an insulator. I normally do not paste "contact" pads. You can have your board house HASL them if you need the extra bump on the pad.