Will soldermasking over large holes work? - Page 1
Forum section: electronics / projects

author: jolshefsky
author_rank: Regular Contributor
timestamp: April 10, 2018, 01:34:05 pm
content: I have a project where I have a switch that will mount on one side with a large metal backing. The knob for the switch has a ball bearing on a spring for indexing into holes. I'd like to try to detect the switch position using a conductor on that ball bearing, making contact when it seats into the hole.If the holes are exposed under the metal backing, it will sometimes or always short out. At first I thought I should just put an insulating spacer, but then I thought I could just cover soldermask over the holes where the metal of the switch sits.I know you can tent vias, but that's probably fine because they are tiny holes and the surface tension of the liquid soldermask prevents it from flowing through. My intuition says you should mask out large holes or else the soldermask liquid will just flow right through and make a mess.Am I wrong? Will it work to apply soldermasking over a large hole on one side?

author: TomS_
author_rank: Frequent Contributor
timestamp: April 11, 2018, 02:41:48 pm
content: I dont think soldermask would work, namely as you suggest because you wouldnt reliably get tenting of a large hole, and because I wouldnt expect it to be that durable. So in that case the insulator would be a better bet I think.Otherwise, can you make the holes smaller such that the ball bearing cannot protrude far enough to make contact with the metal backing on the other side, but still provide a positive detent for each switch position?Or could you potentially make contact with the metal backing work to your advantage, perhaps by using it as a source of ground or vcc to pull up/down your position sensing signals, with the ball bearing making contact with through plated holes and shorting them to the metal backing? I would think tolerances would be pretty tight for that though, making sure the ball bearing sits properly in the hole, making contact with the plating of the hole and the metal backing. At least on that front it should be easy enough to design some holes with pad only on one side, and/or solder mask on the side where the metal backing will be.

author: jolshefsky
author_rank: Regular Contributor
timestamp: April 11, 2018, 04:40:22 pm
content: Quote from: TomS_ on April 11, 2018, 02:41:48 pmI'll have to consider that for a future project—a flexible metal plate with holes large enough that the bearing contacts it and the hole's contact simultaneously. For this, though, it's not acceptable.The actual project is to replace the bulb and shutter in a movie projector with an electronically controlled LED. I picked an LED with a Vf of 22V and figured I could use the existing 30VAC supply for the incandescent bulb. I almost designed it like that then realized that 30V tap is probably an auto-transformer style tap since it comes from the motor itself—hence, mains referenced. (Talk about cost-cutting design!) The LED probably does not expect mains voltage (potentially) from its pins to its heatsink at the case (even though the case is, by design, electrically floating.) Also, all the electronic controls would come from that 30V supply and thus also be mains-referenced.I didn't like any of that so I went back and used a separate power supply instead.

author: capt bullshot
author_rank: Super Contributor
timestamp: April 12, 2018, 09:40:16 am
content: Your idea might work if you place the soldermask right to the edge of the hole.Anyway, from safety and regulation side, soldermask isn't an isolation means.From practice side, soldermask usually provides you with isolation, but you just can't rely on this.