Mechanical design question - Page 1
Forum section: electronics / beginners

author: Spike
author_rank: Regular Contributor
timestamp: August 17, 2014, 01:04:49 am
content: Hi, AllI'm still pretty new at electronics and I've been going through the basics to build my skills, including building the obligatory power supply; in my case a simple LM317 based adjustable breadboard supply. The circuit itself is very basic--almost straight out of the datasheet--but I've learned a good deal from building it up on a breadboard, experimenting with various modifications (none of them worthy of including in the final circuit), working through the thermal calculations, etc.Now I'm down to the final bits of designing the PCB in kicad to send off to be manufactured by one of the board houses, probably OSH Park. I'm amazed at all the fiddly details that must be dealt with in order to have a shot at a functioning board. Things like verifying footprints (thank goodness I did this), verification of physical dimensions and placement of connectors, mounting holes, heatsink, etc., etc.Now I have one final thing I'm not sure about.Aside from manufacturing cost, what are the advantages or disadvantages of plated vs. non-plated mechanical holes? (not necessarily mounting holes for screws)I made a footprint for this right-angle slide switch from C&K:datasheethereAs you can see, there are a couple of tabs (or non-electrical pins, or whatever they're called) near the slide actuator. I don't know if these are intended to be soldered to the board or simply inserted in an unplated hole to provide extra support for shear forces. If they are soldered and since they are non electrical, are there different requirements for the size of the annular ring or should they basically be the same as the holes for the electrical pins?Sorry for the basic questions, but for someone without the hard-won experience of designing lots of PCBs, these sorts of details drive me crazy.Thanks in advance for the help.

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 17, 2014, 01:16:39 am
content: It probably depends on who is making the boards and how. In some (many?) cases, non-plated holes invoke a separate, additional step which adds cost.Plated-thru mechanical holes will be inherently more secure than non-plated holes.And the larger the pad, the better the mechanical integrity.The mechanical tabs are intended to be soldered, IME.

author: IanB
author_rank: Super Contributor
timestamp: August 17, 2014, 01:17:11 am
content: You should definitely plan on soldering the mounting tabs for mechanical support. You never want to put undue mechanical stresses on electrical solder joints.Make isolated pads to solder the tabs to, and make the holes just slightly larger than the tabs (or use standard size holes in common with the rest of the board if this size is sufficient).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 17, 2014, 02:36:36 am
content: It's usually a matter of "why even bother plating it", for example when a plastic alignment peg or not-electrically-connected screw goes into the hole. In these cases, it doesn't usually hurt to have it plated anyway, and may be cheaper (when they go to plate the board, *everything* gets plated, whether you want it or not -- NPTH holes are drilled after plating, so it requires an extra step in the board production process).An example where it does matter is high voltage isolation. The metal inside the hole will short-circuit some of the creepage distance around it. If you have an isolation barrier, with a mounting hole smack dab in the middle, you definitely want that to be NPTH (and, obviously, to use plastic hardware in the hole as well).For components like the switch pictured, unless the datasheet specifically indicates otherwise, assume all metal is intended to be soldered into the board. Often, metal tails are kinked to improve retention strength in a poorly fitting (round) hole. Straight tails (like those pictured) might fit into slots, but those usually cost more (another step!), and you can't usually get a slot juuust tight enough to be useful, so you want it soldered anyway. And a round hole big enough to fit (when slots aren't an option) obviously needs enough solder to fill in the huge gap.Tim

author: zapta
author_rank: Super Contributor
timestamp: August 17, 2014, 04:13:41 am
content: A standard through hole pad with the required drill hole should do, and yes you want to solder it. If you want the switch to be on the edge of the PCB you may need to be creative or pick another switch (e.g. longer actuator).

author: Spike
author_rank: Regular Contributor
timestamp: August 18, 2014, 06:14:50 am
content: Thanks, everyone for you assistance. I've modified my footprint to make these plated holes and I'll plan to solder this at assembly.Thanks again.

author: KJDS
author_rank: Super Contributor
timestamp: August 18, 2014, 04:32:38 pm
content: If you want to wave solder the board then non-PTH mounting holes are preferred, otherwise you're into peelable resist or cleaning the hole out after it's been through the wave solder machine, though that obviously isn't the case here.