'Free of charge'-board to board connection idea. Will it work? - Page 1
Forum section: electronics / projects

author: king.oslo
author_rank: Frequent Contributor
timestamp: October 30, 2012, 05:39:55 pm
content: Hello everyone,I need to connect two boards. One board is mounted flat in the enclosure, the other is at 90 degree angle. I thought: how about adding side plating/castellated edge to the 90 degree angled board, compressed against tinned contact pads on the bottom board.Could we mount the 90 degree board with slight pressure on the lower board so that bends slightly, using the rigidity of the FR4 material to keep a good connection. The solder on the contact pad would ensure that the contact point for the two boards bulge out sligtly.Do you think this would work well over time? Will the FR4 material keep its rigidity to ensure that the contacts keep firmly compressed?Thank you for your time.Kind regards,Marius

author: sacherjj
author_rank: Frequent Contributor
timestamp: October 30, 2012, 05:59:57 pm
content: I would worry that the bending of the board will subject the soldered components to stresses that would potentially break them. But that all depends on the size and pad layout of the components on the board.How many connections? What power requirements?

author: ejeffrey
author_rank: Super Contributor
timestamp: October 30, 2012, 06:04:55 pm
content: No, I don't think it would work well for a couple of reasons.First, the boards are relatively rigid. You have very little per-pin springiness, so you won't be able to reliably get all of the contacts working simultaneously. Small unevenness or motion or incorrect alignment will cause some of the pins to lose contact, or make intermittent contact.Second, the contact wouldn't be gas tight. In order to get good electrical connection you really need some sliding or scraping action to break through oil, dust, residue, and oxide and make a good metal to metal contact. If you look at normal connectors you will see that one side (usually the pin socket) is split and made of a springy material like beryllium copper. When you insert the pin, the spring scrapes along, displacing any contaminants. ZIF connectors have to be carefully designed so that the locking actuation performs the scraping action. Even press contacts like pogo pins, relay contacts, and spring battery connectors will slide some as they are compressed in order to improve the contact.Finally, in the end it wouldn't be free of charge: you would have to considerably over-design the enclosure mechanics to keep the mechanical registration in place. A normal card edge connector holds the edge of the board mechanically in place. They may use sliding rails or extra screws to make the assembly more rigid or easier to align, but they are not primarily responsible for keeping the contacts in the right place.

author: robrenz
author_rank: Super Contributor
timestamp: October 30, 2012, 06:09:15 pm
content: I assume you want more than one contact. If that is the case you you will not be able to ensure some level of equality of contact pressure on all the contacts in a row. That is why most contact systems use some form of spring to allow variation or "compliance" Even if the edge contract bumps were perfectly aligned the mating board is able to bow unless supported from behind, without which the center contacts would start to lose connection. The custom stuff to support the board and apply pressure would cost way more than existing technology. The idea itself is not bad and this is only one opinion.

author: sacherjj
author_rank: Frequent Contributor
timestamp: October 30, 2012, 06:19:24 pm
content: robrenz, that was my same thought when I asked number of contacts. Over two and you can't promise good connectivity.The only way I see the working reliably is in a situation where you never want to unmount the boards and are willing to solder them once joined. Otherwise, I think it is going to have many reliability problems.

author: king.oslo
author_rank: Frequent Contributor
timestamp: October 30, 2012, 06:25:43 pm
content: Power requirements: 1W. 5 contacts.I understand why it won't work. Thanks.I can't use card edge connector. I need some sort of spring contacts connector then.

author: king.oslo
author_rank: Frequent Contributor
timestamp: October 30, 2012, 06:32:43 pm
content: How about cutting slots in the flat mounted board,running all the way between each pad to the edge of the board. Then each finger would press onto the angled pcb as seperate springs.What do you think?M

author: robrenz
author_rank: Super Contributor
timestamp: October 30, 2012, 06:44:57 pm
content: That in principle will work but you will have to make sure the force required for reliable contact does not exceed the flexural strength of the circuit board material in the size beam that you end up with. You still need a system to hold the boards in proper relation to each other.

author: sacherjj
author_rank: Frequent Contributor
timestamp: October 30, 2012, 06:46:28 pm
content: Are these all power lines or do any of them contain signals? The spring will be an effective inductor, and will alter non-DC signals.

author: king.oslo
author_rank: Frequent Contributor
timestamp: October 30, 2012, 06:50:31 pm
content: Will the FR4 material loose it's rigidity over time? The contacts need to keep good contact for the life of the device.One of the lines has a 360kHz logic signal. The others are DC.M

author: sacherjj
author_rank: Frequent Contributor
timestamp: October 30, 2012, 06:57:57 pm
content: If you route slots in the horizontal board and tabs on vertical board, then you can make a place for the force of the spring to push against. This would eliminate the rigidity issues. It would just require a little more difficulty to get the boards together and make sure the springs are on the pad. If the pad extends to the edge and you make the tab widen after a little to control how far into the bottom board it can go, that would probably work. Or just go to a flat edge.You would need to see if the spring inductance is going to give you logic signal any issues. You are talking fairly low speed, which works in your favor. I doubt it would be a problem.Edit: The more I think about this, you would want spring tabs, rather than springs. Then these push in easily as you are dropping the board into place.

author: free_electron
author_rank: Super Contributor
timestamp: October 30, 2012, 07:06:27 pm
content: uless you solder them this will not work reliably. thermal expansion will rub off the gold flash over time exposing the nickel barrier underneath. nickel corrodes like the pest ...

author: robrenz
author_rank: Super Contributor
timestamp: October 30, 2012, 07:06:52 pm
content: Quote from: king.oslo on October 30, 2012, 06:50:31 pmAs long as it does not get hot it will not creep to any significant degree. You could put an elastomeric pad between the bottom of the board and whatever it is mounted to, just under the contact fingers so the majority of the contact force is from the elastomer,

author: robrenz
author_rank: Super Contributor
timestamp: October 30, 2012, 07:10:25 pm
content: Quote from: free_electron on October 30, 2012, 07:06:27 pmI was assuming in all my posts that he would have this area plated as thick as multimeter switch areas. (I know, added expense, no longer free)

author: sacherjj
author_rank: Frequent Contributor
timestamp: October 30, 2012, 07:22:24 pm
content: Did I miss the reason why something like this would be used other than cost?http://www.digikey.com/product-detail/en/7-5530843-6/7-5530843-6-ND/2259133Because I think all of the alternatives will most likely add more than $2 to the boards. Use multiple pads for power planes if more current is needed.

author: king.oslo
author_rank: Frequent Contributor
timestamp: October 30, 2012, 07:28:56 pm
content: There is a problem preventing the use of a card edge connector: The 90 degree board is actually going to be at about 80 degrees

author: robrenz
author_rank: Super Contributor
timestamp: October 30, 2012, 07:33:20 pm
content: Solder the edge connector in at 80 degrees and pot the 10 degree gap with epoxy

author: king.oslo
author_rank: Frequent Contributor
timestamp: October 30, 2012, 07:52:47 pm
content: It would be better to keep all smd construction. This would be my only through hole part.M

author: sacherjj
author_rank: Frequent Contributor
timestamp: October 30, 2012, 08:03:26 pm
content: Quote from: king.oslo on October 30, 2012, 07:52:47 pmAll of the other alternatives would be hard to do with SMD. Not sure how you place SMD springs and such. I don't know if there is a way to make this not be a physical step in manuf.

author: king.oslo
author_rank: Frequent Contributor
timestamp: October 30, 2012, 08:41:11 pm
content: I can get lots of smd battery terminals. However the remark which was made about thermal expansion left me feeling like I was clueless about how to proceed because all components will have a thermal expansion, so I feel like any way to proceed is pointless because it will fail due to thermal expansionM