SMA Edge Connector 1.72mm to 1.2mm board - Page 1
Forum section: electronics / projects

author: jarez95
author_rank: Newbie
timestamp: February 12, 2020, 03:39:21 pm
content: Hi,I've just received my RF PCB's and noticed that the SMA edge connector doesn't quite match the thickness of the PCB (second part of the attached image), (luckily this seems to be the only issue!) It is about 0.5mm too short and consequently gives the SMA edge conn some play (before soldered). The ground pad which this connects to on either side has VIA's are through hole plated to let the solder pass through and give it mechanical strength. I do not expect the SMA edge connector to be under incredible mechanical strain, but I really do not wish the pads to be lifted up or torn in anyway throughout the product lifecycle.I am just wondering what would be the best solution to have it mechanically stable and to not aesthetically look like a huge mess?I was initially thinking two small 0.5mm pieces of Aluminium (perhaps some cut up washers) and placing them on the underside of the PCB between the edge mounted SMA.or just simply flooding the underside with solder as I know this makes a very solid connection and could be enough to support the connectorUnfortunately I do not have spare PCBs to test this theory on and I will have to instruct the assembly house what to do in this situation. I have sound experience with PCB design/debugging but I have not encountered such a situation which would require a little past knowledge of using an SMA edge connection which doesn't quite fit. Obviously, this is something that I would need to get right on the first time as making manual edits to these PCBs (after they have come back from the assembly house) is a little out of the question.The SMA Connector I have is:‎J502-ND‎‎142-0701-801‎Cinch Connectivity Solutions JohnsonCONN SMA JACK STR 50OHM EDGE MNTI cannot replace or buy new connectors as I already have a massive stock of these.Thank you for your time reading this.Best Regards, Jarez

author: ogden
author_rank: Super Contributor
timestamp: February 12, 2020, 03:55:12 pm
content: Quote from: jarez95 on February 12, 2020, 03:39:21 pmIf you can't replace connectors, then replace PCB.QuoteYou mean copper, right?

author: jarez95
author_rank: Newbie
timestamp: February 12, 2020, 04:15:13 pm
content: In an ideal world I would consider it, but given that these already took a great deal of time to get made I don't think I can. I will definitely consider increasing the core size for future spins.Yes, could also be copper.I should mention that these RF boards are part of a larger system, that very likely once setup and reliably working will not be touched again (i.e SMA cables screwed/unscrewed) etc

author: ogden
author_rank: Super Contributor
timestamp: February 12, 2020, 04:28:15 pm
content: Quote from: jarez95 on February 12, 2020, 04:15:13 pmI am saying: definitely not Aluminium! Copper or it's alloy only. You want your "hack" to be properly soldered, don't you?

author: jarez95
author_rank: Newbie
timestamp: February 12, 2020, 04:39:07 pm
content: Copper it is then.https://www.amazon.co.uk/pure-copper-sheet-TOOGOO-Copper/dp/B01MA0S9R6I guess something like this will do, cut into thin slices smaller than the pad so the solder can flood around it. I guess like this it might not even be visible.

author: pigrew
author_rank: Frequent Contributor
timestamp: February 12, 2020, 04:39:16 pm
content: I've not done any simulations, but my advice would be to flood the bottom side with solder. A copper shim could work (if soldered onto both the board and connector), and would give a little bit more mechanical strength. Don't use aluminium since you won't be able to solder to it (among other issues) The shim won't magically "fix" the impedance mismatch issues, unless the board layout is already designed for it.Comparing the mechanical drawings of the connectors for various board thicknesses, the pin diameter stays the same, it's only the height of the bottom ground "pins" that varies. The width of the RF PAD on the PCB does vary between board thicknesses, and is different than the width of a 50 ohm track. Note that the connector is designed for two-layer PCB (not four layer). The suggested footprint assumes some particular dielectric (FR-4 with 1 oz/ft^2 copper), so perhaps they are not optimal for you either. Designing the PCB RF track width to match 50 ohm impedance is a much more important than a perfect coax to PCB transition.Since you're using SMA, I'm assuming it's lower frequency (less than a few GHz), I doubt that the connector is so critical from an electric standpoint. If you care about precision, you'd be avoiding SMA (and use 3.5mm instead). Assuming your board isn't large (significant fractions of a wavelength), the accuracy of your transmission line impedance wouldn't have a large effect on performance.

author: ogden
author_rank: Super Contributor
timestamp: February 12, 2020, 04:45:05 pm
content: Quote from: jarez95 on February 12, 2020, 04:39:07 pmDon't overdo it. Try to do your "hack" for at least 10 connectors and see how it goes. Consult with assembly house. Most likely you need properly sized single piece of copper on PCB side opposite to central pin.

author: jarez95
author_rank: Newbie
timestamp: February 12, 2020, 04:51:32 pm
content: My apologies maybe I didn't make something clear.It is a four layer PCB and the ground plane for the RF is on the second layer therefore Impedance matching is not an issue, (it would be if it was two layer). (Designs are rated up to 6 GHz) and the Rogers dielectric has been selected to account for this (don't worry)There is also no issue with the pad dimensions on the top layer or the bottom. The only problem is that the PCB is not thick enough, I simply overlooked this when I migrated the stackup from a 6 layer design to a 4 layer.