limited clearance on compact design using spring pins - Page 1
Forum section: electronics / projects

author: OwenH
author_rank: Contributor
timestamp: September 25, 2022, 02:37:02 pm
content: Hi,I'm working on a design using that uses high current spring pins and a flexible PCB routing power through a compact machined mechanical structure. I have 4 surface mount spring pins from Smith Interconnect (Part 100891) each rated at 15 amps providing power and 4 smaller Millmax pins for data. 2 spring pins are used for Bat + and 2 for Bat -. The voltage range of the device is 20 - 33v with a peak current of 20 amps although the typical operating current will be under 10 amps. The spring pins will be installed into a custom connector body machined from PEEK. A flex PCB is used because the connector is duplicated at both ends but needs to have the ability to flex a small amount (+- 0.2mm max) to accommodate any dimensional variation of the machined aluminium body.The issue I have is that there is little space between the aluminium body of the device and the lower diameter of the power spring pins and flex pcb once assembled. All the mechanical parts will be precision machined and should fit together nicely but would only leave about 0.5mm from the outside edge at the bottom of the spring pin to the aluminium body of the device.Because this is hard to explain I have a cross-section from the CAD software so you can see the clearance issues.I was wondering if I could just use Kapton tape and tape over the edge of the PCB and the connector? Maybe I could use potting compound? Kapton would be my preference for ease of use. Or is this just to little clearance and I should rethink this?As a note the aluminium body of the device is anodised and would provide some insulation but I do not want to depend on it.Thanks for any help!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 25, 2022, 03:39:31 pm
content: What isolation voltage, or should we assume the enclosure is ground with respect to those?Yes, Kapton(R) (polyimide) tape is commonly used for insulation, also polyester if you don't need the temperature rating.So that's a machined part, but the inside dimension varies by 0.2mm? That seems surprising...Such a tolerance could still be solved by a rigid PCB, with clever cuts -- though carrying all those currents may be a bit troublesome, or require more layers than otherwise. 4, even 6 layer might be comparable cost to a rigid flex layup? Not sure, haven't priced the latter before.The other way would be just the small rigid pieces and tacked-on wires for ~unlimited flexibility. Maybe just too tedious (14AWG probably won't fit in the clearance available, necessitating e.g. 4 x 20AWG per connection, or say, flat ribbon/strap in spaghetti tube?), or too much tolerance (all those solder joints and wiggly cables?) and the single part solution is preferable, despite the added cost. *shrug*Tim

author: OwenH
author_rank: Contributor
timestamp: September 25, 2022, 04:34:15 pm
content: The enclosure would be at ground.I was planning to use a Flex pub with a rigid stiffener at the ends where the spring pins are. It looks like I could get 2oz copper and run a trace width of up to 8mm. I could do 2 layers of 2oz with a trace width of 8mm and have minimal temp rise. Not sure how safe it is to use so much solid coper on a flex PCB but it would only need to flex once and only by a very small amount.The machined part will have a paper tolerance of more like +- .1mm but will likely be less than +- .05mm in reality. I just don't like to design in a way that it only functions if the tolerance of the machined part is very high.I would be curious what your thought are as to making a rigid PCB work in this application.Owen

author: ajb
author_rank: Super Contributor
timestamp: September 25, 2022, 04:45:06 pm
content: The fundamental question is: how much clearance do you need to meet your electrical requirements? 33V isn't crazy, so if you can guarantee 0.5mm minimum via the assembly, then as long as you don't have some other requirements (nasty transients? 120V logic levels on your signal lines?) then it seems like you should be fine?The CAD screenshots make it look like the PEEK body is a close fit around the pins (perhaps you're planning to fit them pins into the PEEK before soldering?) and contacts the nearest face of the aluminum body. Given that, it looks like the geometry of the PEEK body will enforce that clearance dimension pretty well. You'd want to make sure you tolerance the hole size and position correctlyrelative to that faceof the PEEK part to maintain the required clearance. (This is a good example of a part requirement that implies careful consideration of which dimensions are specified and with what tolerances when the design is issued for manufacturing. There are good reasons why whole books are written onGD&T!) You'd also need to apply some criteria to the electronic assembly, to ensure that the solder fillet around the base of the pin doesn't intrude into the clearance area, but that's a pretty simple requirement for any decent assembler.Actually, looking at the screenshots again, it doesn't look like you've allowed for any fillet at all around those pins. It looks like the edge of the pin base is right up to the edge of the PCB, and that seems like a bigger problem. What's your plan there?Anyway, if you wanted extra protection in that small clearance area, then either a strip of tape on the edge or an epoxy underfill would probably be fine. The epoxy probably wants to be something a bit flexible, and would need to be low viscosity to get under the PEEK, might be worth looking at BGA underfill materials as those will have similar properties. You might want to add a hole or two to the PEEK as injection/vent points for the underfill material if you go that route.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 25, 2022, 06:41:22 pm
content: Speaking of soldering, PEEK is high temp stuff, so it could double as a soldering jig. (Maybe that's why it was chosen..!?)You could possibly have a corner or flange of the PEEK piece extend down around the edge of the PCB to provide insulation without need of tape, unless that's a bother to mold or machine as the case may be.Tim

author: OwenH
author_rank: Contributor
timestamp: September 25, 2022, 08:36:59 pm
content: Yes, the footprint of the spring pin looks like it will be an issue. Smith Interconnect doesn't seem to provide a footprint for this part and I only created a CAD mockup of the PCB so far forgetting to add space for a solder fillet. The spring pin has a surface mount diameter of 3.85mm. Any recommendation with how much larger the footprint would need to be for a proper solder fillet? Maybe 4.4mm diameter? not sure how close to the edge of a flex PCB I can put my pins?The intent is for the pins to be pressed into the PEEK block then reflow it as an assembly onto the PCB.It looks like I would be line on line with the enclosure if make the PCB much larger to accommodate a larger footprint. I may have to spend some time trying to rework the enclosure a bit but I don't have much room to play with it.Owen