Need suggestions for PCB interconnect for automotive application - Page 1
Forum section: electronics / projects

author: rbm
author_rank: Regular Contributor
timestamp: July 23, 2014, 02:00:42 am
content: I am designing a circuit that will drive a 7-segment LED display. The display cannot be soldered to the PCB (1) because of limited board size and (2) flexibility to locate the display remotely from the PCB within the enclosure. The application is automotive and so the environment can be harsh. I'm using a through-hole 2 side PCB. I have to hand assemble the boards and I don't have access to sophisticated assembly / soldering equipment, just basic soldering iron and hand tools. I'm assembling about 10 units.I need suggestions for interconnects that will reliably mate the 7-segment display to the PCB. I'm thinking of a 10-position shrouded header 0.1" pitch for the PCB, and soldering individual 30GA wires to the LED display then encapsulating the back in conformal coating to secure the wires. I may have challenge selecting a female socket to fit the header that would be easy to solder (IDC is probably out of the question).I am concerned that the shrouded header may not be the best selection for this application and I would entertain other ideas from more experienced people.

author: David Hess
author_rank: Super Contributor
timestamp: July 23, 2014, 11:06:48 am
content: For a flexible attachment, I would use ribbon cable and insulation displacement connectors.

author: AndyC_772
author_rank: Super Contributor
timestamp: July 23, 2014, 11:12:38 am
content: Conformal coating may offer some protection from corrosion, but it doesn't offer any mechanical support.I'd probably use a crimp connector myself. Something like a 2.5mm JST connector with individual wire crimps should be reasonably robust.Be careful with soldering wires direct to the board; it's possible to get a stress concentration point where the stranded wire meets the solder blob, and under vibration this is where it will fracture. Using solderless crimps avoids this issue.

author: rob77
author_rank: Super Contributor
timestamp: July 23, 2014, 11:29:16 am
content: i'm using flat cable with these sockets soldered to the PCB.http://www.gme.cz/img/cache/800x600/826/007/samorezna-patice-xinya-kk18025c-obrazek-1.jpgso in your case i would make a small PCB for the 7segment and connect the 2 boards with flat cable with those sockets above soldered to the PCB on both sides.EDIT:the manufacturer is XINYA and part number is KK18025C for 18 pin version.

author: Precipice
author_rank: Frequent Contributor
timestamp: July 23, 2014, 11:56:55 am
content: Just chiming in with another vote for crimping. Whether that's ribbon or individual pins, I've seen both in roughly equal proportion.If you're doing 10-off I'm guessing it's not OEM, so you just want 'reliable', rather than 'approvable', and decent crimped connections will do.(One thing I do see a fair amount of in auto stuff is sparse ribbon - skinny wires on a 0.1" pitch, with webs between them. I don't know if the tooling is cheaper for those, or if they've got some actual charm, to compensate for being so wde.)

author: rob77
author_rank: Super Contributor
timestamp: July 23, 2014, 12:17:32 pm
content: when talking about crimping... 10 wires - 2 ends and 10 devices => 200 crimped connections , that's a lot of effort (unless you were trained in a Chinese factory).shrouded headers + flat cable or flat cable with the sockets soldered to PCB is much less effort and provides reliable connection.btw... if shrouded headers then definitely the ones with securing levers.

author: rbm
author_rank: Regular Contributor
timestamp: July 23, 2014, 01:17:30 pm
content: Thanks, These are great suggestions. I'm researching the JST connectors right now. I can find amanufactured 8-position JST-SH female connector to wire cable from Sparkfunwhich would be ideal for the display but they don't sell a corresponding PCB socket. The corresponding through hole PCB mount sockets are available from Alibaba but delivery is too long. I'll have to check other electronics suppliers.I forgot to mention that the cable assembly cannot be permanently soldered to both the display and PCB because the cabling has to be threaded through grommeted holes between different enclosures. Soldering the wires to the display pins and encapsulating them in epoxy is the best solution at the display end because the display is fitted to an instrument pod in a recess that just accommodates the display's outer dimensions. Consequently, a SIP connector like the JST-SH is ideal because it can be oriented in ways that ease passing it through the holes.Ribbon cable IDC connectors are definitely convenient and easy to manufacture. I will continue to investigate that solution also.

author: David Hess
author_rank: Super Contributor
timestamp: July 23, 2014, 01:49:45 pm
content: Quote from: rob77 on July 23, 2014, 11:29:16 amThat is what I had in mind but I am not sure what they are specifically called. Personally I find them annoying because the connections are permanent but without a plug and socket, they are better in a high vibration environment.

author: Precipice
author_rank: Frequent Contributor
timestamp: July 23, 2014, 02:07:33 pm
content: I've seen them called transition headers, and a search for that shows suitable things. No diea how general that is, though.

author: rbm
author_rank: Regular Contributor
timestamp: July 23, 2014, 10:27:45 pm
content: I finally decided on pre-made JST PH cable assemblies, one 4-wire the other 5-wire. Both are made by Iteadstudio but I was able to find a local brick-n-mortar retailer who sells and supports these products. The total wire count is 9, just right for a 7-segment display with decimal point and common connection. Using two cables ensures easy passage through any enclosure holes and different sized connectors means fool-proof connection to the PCB. The headers are low profile right-angled and 2.54mm pitch for easy soldering..This will work well. Cheers for the input!!