How much attention to pay to DSUB PCB connector strain and what to do about it? - Page 1
Forum section: electronics / projects

author: daqq
author_rank: Super Contributor
timestamp: July 18, 2016, 08:50:20 am
content: Hi guys,I am considering the following design:6 DSUB connector of various pin numbers and gender, all of equal height profile ( all from these series:http://www.farnell.com/datasheets/1802105.pdfwith a screw option of RM5) will be soldered into a PCB. The PCB will then be mounted in a sandwich type construction between two massive milled metal blocks - a classic RF type shielded box. See the attachment for details.My question is how much attention should be paid to the possibility of the PCB strain that could be created when both the 4-40 screw and the M3 screws are tightened? Assuming that the connectors have some height tolerance, say, a 0.1mm difference in height, this would create stress in the PCB - either the PCB would have to bend "forward" to accommodate for a lower connector, or "backward" to accommodate a higher connector.My questions are :Would the stress created in this way be a problem?If so, how to mitigate this?This kind of sandwich construction is attractive to us in this application both due to the relatively simple mechanics, the full shielding, the fact that it can be fitted specifically to our PCB and the mechanical sturdiness - basically, the bulk of the stress from the connectors inserted into the device would end up in the milled block rather than the PCB.Thanks,David

author: Kilrah
author_rank: Unknown
timestamp: July 18, 2016, 09:34:06 am
content: Mount the unsoldered connectors to the front plate, put PCB in place and install nuts instead of the bottom plate to hold it, then solder the connectors with everything in its final position?

author: TomS_
author_rank: Frequent Contributor
timestamp: July 18, 2016, 11:42:25 am
content: Kilrah's suggestion would ensure that the stress is minimised as the positioning will be uniquely tailored to each unit, catering for all applicable discrepancies that might come up during manufacturing of all parts (angles etc). But if you have to build multiple of these units then you wont just be able to build them in bulk and slap them in a case, at least when it comes to the connectors...Check out episode #438 to see what improper stress relief could cause:https://www.youtube.com/watch?v=HKNVIgsbYrA#t=27m35sIf absolute reliability is required then you might need to have some heat/cool cycle and vibration testing done to see what youre in for. Environmental temperature changes will cause parts to expand and contract which will create some flex or strain anyway. And then theres the question of what temperature the unit is assembled in and what will it operate in? Worrying about 0.1mm at 25oC may become more or less at 40oC, for example.Im no mechanical engineer, butin general0.1mm is a tiny amount. Its nearly flush. Convert to 0.0039" or a whisker under 4 mils. If tolerances and other clearances at this scale are going to be a concern, perhaps a small board for each individual connector with ribbon cable back to the main board would be better?

author: daqq
author_rank: Super Contributor
timestamp: July 19, 2016, 05:00:11 am
content: Hi guys,Thanks for the replies.QuoteIdeally yeah. The problem is that this is quite a space constrained application - the full height of the device must not exceed 25mm and board space is premium. It would probably be possible to do such a thing, but with 6 DSUB connectors that would mean carrying a lot of signals to the main board, some of which are differential pairs. I could probably make a flat flex PCB just for the connectors, but that would be quite expensive.QuoteI am planning on such a testing cycle, within my capabilities.QuoteThat might help! Thanks. It'll be a low volume production, so this solution would be acceptable.I've also wondered whether "socketing" the connectors would work - use PCB socket pins (something along these lines:https://www.mill-max.com/new_products/detail/2) and have the connectors mounted on the milled block. That would remove the mechanical problem, but assembling the whole thing might get tricky - aligning 76 pins without seeing them into very small holes isn't fun at all.

author: SeanB
author_rank: Super Contributor
timestamp: July 20, 2016, 09:44:05 am
content: You can simply make a small PCB with the identical drilled holes for the D connectors, and use it as an alignment jig for the sockets, so the pins are all aligned to the board and are all there, then simply place the main board over it with the sockets installed. The lip on the sockets will ensure alignment will occur, as the pins will be supported by the added board.Just means you have an added thickness of board without copper but with soldermask, silkscreen and the right non plated holes in it. Might need some extra routing on it if there are any components, but will be either an assembly jig if it is a thick ( 3.2mm thick 4 layer board) or a single use 0.9mm somewhat flexible board used as a spacer and as additional insulation to the back so any loose nuts do not drop between pins.