Building funcional test bench for PCB boards - Page 1
Forum section: electronics / general

author: cio74
author_rank: Regular Contributor
timestamp: December 28, 2017, 10:27:28 pm
content: Hello,I am looking to build a bench to do functional testing on some boards. The boards have a significant number of terminal block connectors. Various input sequences are generated using a DAQ board located in a PC and the response is read at board's outputs using the same DAQ board. The number of cables running to a test board is about 38 without power and Ethernet. One DAQ board can handle two such test boards.In order to save space I am thinking to use a tall 19" rack and sliding shelves. Each shelf should hold two test boards. The issue is about bending those cables every time I pull the shelf to setup a board then push it back to start testing. Considering I might test a significant volume of boards, I need a good solution to protect the cables from breaking and have them last for a high number of pull/push sequences.There is a DIN rail holding the 100pin cable from DAQ to a specialized 100pin terminal block. On the same rail I have the additional 2 Ethernet and 2 power supply terminal blocks. From this DIN rail I route 76 of AWG 24 cables to the test boards plus 2 Ethernet and another 2 AWG 20 cables for power supply.The issue is pulling the shelf. It will slide out for about 40cm. All cables should slide for the same distance. When pushed back, these cables should nicely go back with a minimum strain/stress. The idea is to function for a high number of test board replacement without breaking.I have the option to either have the 100 pin terminal block fixed on DIN rail or to have it mobile, attached to the shelf. If the latter then the tension will no longer be on the 76 separated wires but on the thick cable used to connect the DAQ board to the terminal block. Power routing for test boards and Ethernet can again be either fixed on the DIN rail or mobile with the shelf.I'm not sure what would be the better idea or perhaps there are other/better solutions?Thanks.

author: tszaboo
author_rank: Super Contributor
timestamp: December 28, 2017, 10:37:50 pm
content: Depends, how long the functional test takes. If you are testing the boards one by one, then I would rather look for an off-the-shelf pogo pin test solution. I worked with OTS and custom testing rig before. The custom one was calculated as 20KEUR to assemble a new one, and the first one was even more expensive, due to the non-recurring engineering costs. For that amount, you can just buy one, and save a lot of headache.

author: floobydust
author_rank: Super Contributor
timestamp: December 28, 2017, 10:42:19 pm
content: I wouldn't use a sliding shelf, unless you want to be troubleshooting broken wires and making new test cables for the rest of your life.A test fixture should sit rigid on a bench because the board-under-test is pushed onto the bed of nails, unless you are plugging/unplugging all these terminal blocks and connectors every time? That's a repetitive-stress injury for anyone doing it all day.If your setup time to install/remove a board is several minutes, then limited payoff having multiple test stations.

author: cio74
author_rank: Regular Contributor
timestamp: December 28, 2017, 11:09:49 pm
content: Quote from: NANDBlog on December 28, 2017, 10:37:50 pmI have a suite of tests, the burn in test last for days. However, the basic functional test completes in 24 hours.Quote from: floobydust on December 28, 2017, 10:42:19 pmThat is exactly the biggest issue I am thinking of, broken wires. It will completely mess up the entire testing procedure and will take long time to troubleshoot it.I would use terminal blocks instead of bed of nails to fully replicate the functional conditions. Injury can occur while pulling the connectors if tried in the confined space inside the rack, this is the reason I want the board to be fully pulled out when setting it up.It takes a couple of minutes to setup a board, what I want is to build a good solution that I can later replicate in a large number. A 42U 19" rack on wheels could be a very good test bench considering it's easy to add a rackable PC, a network switch for Ethernet, rackable power supply on DIN rails and even attach a touchscreen LCD to it.

author: tszaboo
author_rank: Super Contributor
timestamp: December 28, 2017, 11:29:00 pm
content: Then it is definitely a custom job.DIN41612 comes to my mind. You could break the connection, when pulling out the shelf. These have few hundred mating cycles, so it would last for years. Ethernet can pass though those connectors. Maybe have a small, throw away, replaceable PCB with the connectors always disconnecting?

author: Kean
author_rank: Unknown
timestamp: December 29, 2017, 03:13:24 am
content: Can you just use a bunch of Cat5 patch cables for the flexing interconnects? Ones with moulded cable strain reliefs.I suspect if you're only sliding the shelf open/closed every few days then the cables will last quite a while, but using a common patch cable makes it quick and easy to swap out. You might need a few, possibly all colour coded differently. You may also have to double up on conductors for the higher current ones, and check for crosstalk issues in the UTP. You could also just run a couple of separate high strand count silicone wires for the power supply.You can also get data cable rated for continuous motion (i.e. for robots) from several of the large cable manufacturers, like Lapp or igus. But it is *quite* expensive, especially for the number of conductors you're talking about. And they aren't all created equal:http://www.machinedesign.com/cables-connectors-enclosures/when-flexible-cable-doesn-t-flex-longThere are readily available cable management solutions for rack mount servers on sliding rails. Use velcro instead of cable ties.