Can you surface-mount components without pins? - Page 1
Forum section: electronics / general

author: victorsuciu
author_rank: Newbie
timestamp: January 28, 2023, 08:16:52 pm
content: Hi everyone, I'm new to electronics and looking for some soldering advice.ContextI'm designing an EMG sensor and I'm trying to use gold-plated electrodes. The board that touches your skin is 3mm x 30mm and has three header pins at one end. At 3mm wide, I could not find surface-mounted headers narrow enoughso I had to use through holes. However, that means the header pins would also stick through into your skin, creating unwanted electrical contact.The solutions I see are1. cover the header pins with an insulator, or2. find gold-plated electrodes long enough to extend way past the header pins.#1 on its own is not ideal because, if you put something taller than the electrodes next to them, it might prevent them from making solid contact on the skin.This leaves #2. There are gold-plated contacts that can be through-hole mounted, but their "pin" is usually 1.5mm minimum which feels too big for this 3mm wide board. Regardless, none have a flange end tall enough to surpass the header pins by even 1mm.QuestionThe best fix I can think of is to solder a long, gold-plated electrode directly to a copper pad on the pcb without using a through hole, so that the solder is sandwiched between the copper and the electrode. There wouldn't be a pin to put solder on top like a surface-mount component, you'd only be able to put solder underneath. Is that possible? If so, how would you do that?Here is a diagram of my idea.Full-size ImageThank you all for your time.

author: ataradov
author_rank: Super Contributor
timestamp: January 28, 2023, 08:50:37 pm
content: For hand assembly it is absolutely possible to do this. For automated assembly it might be tricky to tune the process, but still be possible. The standoff you are showing here is designed exactly for this. And spring loaded pogo-pin version of this is commonly available.You do not want to have the hole under it though. Just keep it a plain pad and use via nearby if you need to transition to a lower level.The issue I see here is that even 7 mm lift would not prevent your connector from touching the skin. So, you would still need to apply some insulation. Given the size constraints, epoxy may be a good option.

author: langwadt
author_rank: Super Contributor
timestamp: January 28, 2023, 08:56:36 pm
content: and trim the connector pins before soldering so they don't stick through the board

author: NorthGuy
author_rank: Super Contributor
timestamp: January 29, 2023, 02:55:09 pm
content: May be you can use standoffs to conduct signals instead of having a separate header.

author: thm_w
author_rank: Super Contributor
timestamp: January 30, 2023, 10:30:22 pm
content: "#1 on its own is not ideal because, if you put something taller than the electrodes next to them, it might prevent them from making solid contact on the skin."Then make it so the electrodes are slightly taller, no? I would not want to have bare PCB sitting on my skin, I'd want it in some sort of enclosure.Maybe look at how other devices on the market do it.https://www.environmentalengineering.org.uk/news/vertical-spring-contact-pins-for-smt-pads-3703/

author: tooki
author_rank: Super Contributor
timestamp: February 09, 2023, 04:28:14 pm
content: How about instead of a janky female header (which I assume you’d then connect to with male DuPont pins), solder surface-mount machined sockets onto the top of the PCB? For example, Mill-Max 5200. You can fit square DuPont pins into that, or you could use some solder cup machined round pins (like Mill-Max 0275, 0518, or 1107) soldered to nice soft wire, insulated with heat shrink. You could also do the reverse, soldering a surface-mount pin (like Mill-Max 1440) to the top of the PCB, and soldering a matching socket onto the wire (e.g. Mill-Max 0740, 5084, etc). Which gender you put on which part depends first on risk of shorting (don’t use unprotected pins on outputs!) and second on usage: sockets wear out and usually cost more.

author: tooki
author_rank: Super Contributor
timestamp: February 09, 2023, 04:41:37 pm
content: The other thing that comes to mind is this: if you want to have proper gold contacts on the skin side of the PCB (not just gold plating on the PCB pads), why not use “target pads” (normally used as the mating surface for spring-loaded pins)? You can get these that mount through the PCB, with either male pins or solder cups sticking out. For example, Mill-Max 1942, with mating 1024 sockets on the wires.