Interlocking PCBs - A good idea? - Page 1
Forum section: electronics / general

author: m3g4by73
author_rank: Contributor
timestamp: July 26, 2016, 07:50:04 am
content: I want to mount an 8x2 alphanumeric LCD display at a right angle to the edge of my PCB. The trouble is the signal lines are usually at the top of the LCD a long way from my main PCB and I dont want to use flex cables because I'm looking for a mechanical solution that affixes the display to the PCB. I could mount the display upside down and solder it on with header pins to the board edge, but designing custom upside down characters could be troublesome.So, keeping the LCD display upright, I could have some custom bent PCB headers made up to handle the signals and hold it in place, but I'm thinking it might be cheaper to design a right-angled interlocking PCB instead that slots into a groove and you just solder the contacts. I've attached a photo to demonstrate what I mean.I'd like to know if this is an acceptable practise and whether there are any tricky engineering challenges I should plan for.E.g.1. Do PCB fab houses have troubles soldering the connections?2. Do they have troubles soldering the boards at close enough to 90 degrees?3. Using 1.6mm FR4, what size and tolerance would you specify for the slot width?

author: Niklas
author_rank: Frequent Contributor
timestamp: July 26, 2016, 08:12:06 am
content: What kind of environment is this board being subjected to? Vibrations or possible drop on the floor? You also have pushbuttons, so you will probably also have to think about mechanical fixation to the front panel as solder joints should not be subjected to mechanical stress. There is also the angle offset from 90 degrees that can also put a straining force on the joints. FR4 can be quite flexible and change between batches if not clearly specified.Q1 and Q2 are probably best to check directly with a manufacturer as it will be dependent on the production volume and location of production.Q3 - The tolerances on the PCB thickness is usually +/-10% or 0.16 mm and the positioning of routing/milling is somewhere between 0.05 and 0.1 mm. There can also be a demand for 0.1-0.2 mm clearance between the routed slot and the copper pads.

author: 2N3055
author_rank: Super Contributor
timestamp: July 26, 2016, 08:24:09 am
content: I had long term problems with solder joints breaking, in PCB that was sitting in a box, no external mechanical forces..It had 20-40°C thermal cycles daily (a box sitting in a sun).. This thermal cycling alone was reason for solder shearing of...Lead free solder was more brittle (how much, it depends on composition), leaded eutectic was much more reliable..After all those problems, I use connectors for this kind of interconnections..Also easier to repair ..And why custom bent connectors... zillions of right angle connector to chose from...And like Niklas said, you have buttons on the front PCB, so you have to mechanically attach it to the case... Don't give that job to a connector..Take care..

author: tszaboo
author_rank: Super Contributor
timestamp: July 26, 2016, 08:48:45 am
content: How do you guarantee that it will be 90 degrees?

author: m3g4by73
author_rank: Contributor
timestamp: July 26, 2016, 09:18:30 am
content: Thanks for the replies.The application is a small 8x2 Alphanumeric LCD that will sit behind a membrane inside an enclosure. No buttons, that pic I included was just to show the kind of PCB joint I was thinking of creating.I've attached a pic of the display and a drawing showing how it would mount if I used right angled header pins.As you can see in the pic, the signal lines are all up the top, so using right angle connectors they will have to have a very long vertical length (about 30mm) before bending and going to the display. I've searched Digikey but couldn't find anything like this.

author: tszaboo
author_rank: Super Contributor
timestamp: July 26, 2016, 09:25:17 am
content: Quote from: m3g4by73 on July 26, 2016, 09:18:30 amYour LCD probably has some commands to flip the screen, so you can mount it upside down. Check the datasheet.

author: m3g4by73
author_rank: Contributor
timestamp: July 26, 2016, 09:41:01 am
content: You had my hopes up for a minute! It uses a SPLC780C which is HD44780 compatible. Looking at their datasheet (http://www.buydisplay.com/download/ic/SPLC780.pdf) there is no mention of flipping or orientation.It *can* be done in software but there are a lot of caveats so a hardware solution would be preferable.

author: vzoole
author_rank: Regular Contributor
timestamp: July 26, 2016, 10:14:11 am
content: Maybe you should use IDC connectors with ribbon cable.

author: m3g4by73
author_rank: Contributor
timestamp: July 26, 2016, 10:22:28 am
content: They solve the electrical connection but offer no mechanical support.I found an alternate 12x2 display that has the electrical connections at the bottom so I will see if I can fit this display into my enclosure instead. It costs about 80 cents more than the 8x2 which is probably less than I'd pay for a custom interlocking PCB or long right angled headers. Now to order some samples and the wait begins again... Fingers crossed.

author: jolshefsky
author_rank: Regular Contributor
timestamp: July 26, 2016, 11:55:24 am
content: For what it's worth, solder joints should not be for both electrical and mechanical connections. The reason boils down to different needs for electrical and mechanical. Like your slotted soldered-into-the-board solution would be okay for electrical or mechanical.But if it's mechanical, the design is for some of the solder joints that are slightly out-of-alignment to fail leaving the remainder to do an adequate job, but a few solder joints failing is no good for electrical. If it's electrical, the integrity of every joint must be maintained, so the LCD needs to be mounted some other way—perhaps a slide-in support perpendicular to the main board, or a right-angle screw-in, solder, or crimp support.

author: KM4FER
author_rank: Regular Contributor
timestamp: July 26, 2016, 12:23:59 pm
content: Does the board have to be mounted below the display?Can you maybe flip the board over and mount it at the top where the connections are?

author: tszaboo
author_rank: Super Contributor
timestamp: July 26, 2016, 02:36:55 pm
content: Quote from: m3g4by73 on July 26, 2016, 09:41:01 amSorry, some does it...Good luck with the other display.

author: Ian.M
author_rank: Super Contributor
timestamp: July 26, 2016, 03:15:58 pm
content: I cant imagine any way the daughterboard and LCD could be soldered automatically without a very expensive wire feed automatic soldering machine. A typical character display wont stand up to the temperature profiles for wave or reflow soldering, so even if you come up with an acceptable way of soldering the daughterboard to the main PCB you've still got the problem of fitting the LCD. At that point, as its going to need manual assembly and soldering, you might as well simply mount it to a bracket that clips into place and use 0.1" pitch pre-cut and pre-tinned ribbon cable to connect it to the main board.

author: 2N3055
author_rank: Super Contributor
timestamp: July 26, 2016, 06:03:01 pm
content: Quote from: m3g4by73 on July 26, 2016, 10:22:28 amThe board you showed in a photo has no mechanical support, unless there is a structural element, a brace, behind the board, that triangulates to top of the front board to the bottom board..If those soldered joints are structurally only thing holding boards together, it'll break off first time someone presses a button...I would provide a mechanical structural element, of any sorts to hold the boards together, and then connect boards with connectors, i agree with Vzoole, i use a lot of cheap IDC connectors, directly at angle, or with a cable...

author: nctnico
author_rank: Super Contributor
timestamp: July 26, 2016, 07:34:42 pm
content: Quote from: vzoole on July 26, 2016, 10:14:11 amI second this suggestion and you can buy readily made IDC cables from Ebay for peanuts.I hate soldering 2 boards together; that is asking for big trouble because solder is mechanically very weak.

author: m3g4by73
author_rank: Contributor
timestamp: July 26, 2016, 11:25:09 pm
content: Quote from: KM4FER on July 26, 2016, 12:23:59 pmUnfortunately that's not an option for this product.Quote from: Ian.M on July 26, 2016, 03:15:58 pmGood points about manufacturing. Using a bracket to hold it in place and a flex cable for electrical connections sounds ideal. But do you know where to source such a bracket? I've never seen this sort of thing at Digikey and I know the costs of CNCing custom brackets out of Delrin will be prohibitively expensive for this product.

author: Ian.M
author_rank: Super Contributor
timestamp: July 26, 2016, 11:52:15 pm
content: That very much depends on the production volume.I'd go for a pair of identical triangular brackets, at rightangles to both the display and the PCB, one at each end of the display. They could be made from tin plated steel with lugs to go through the LCD mounting holes then folded over, and through slots in the PCB twisted to lock them in place.Depending on the volume, laser cut or punched with a custom punch and die.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 27, 2016, 12:28:12 am
content: Quote from: m3g4by73As others have said, it can be done, but needs care especially around mechanical stress managements (notvia the solder joints)Quote from: m3g4by73 on July 26, 2016, 11:25:09 pmYou could make brackets out of PCB, much along the lines of the slots you planned ?If youcanfit a standard 0.1" straight or 90' header, they will usually be better than copper-edge, but sometimes you just do not have the room...We have done slotted solder connections, but we took extra care to support the board elsewhere.In one product, we used PCB from the same panel as slotted support backets.

author: m3g4by73
author_rank: Contributor
timestamp: July 27, 2016, 01:29:02 am
content: Quote from: Ian.M on July 26, 2016, 11:52:15 pmNice suggestion, I never considered steel. Production volume is usually 100 to 250 units per batch. I had some delrin quoted (500pcs) at about USD$2.50 per piece so we're looking at double the cost of the display just to hold it in place. I will look into tin and see what it will cost to have it laser cut. Great idea having lugs that can just fold out instead of screws. Thanks!

author: Ian.M
author_rank: Super Contributor
timestamp: July 27, 2016, 01:34:26 am
content: Stiffen it up by soldering the tinn plate lugs through the PCB if its wobblier than you like!If you were still thinking of Delrin brackets, you'd need to design them for manufacturability, minimising waste to get the cost down. e.g. 2" square Delron bar, with a repeated pattern of holes drilled in each face for self tapping screws to fasten to the PCB and LCD, then rip-cut longitudinally on the diagonal to separate into two triangular bars with holes on their 90 deg faces, then cross cut into fairly thin individual brackets.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 27, 2016, 03:02:04 am
content: I've seen it done on old stuff like 80s era alarm clocks, radios etc.Probably two things:1. You can avoid this in the first place by e.g. buying a right-angle display. (But if you can't find one, already available, you'll have a hard time doing this for less than 100,000/yr quantity...)2. You can put more smarts on that little board, and wire it with a simple connector (like 3 wires for SPI, plus supply and grounds), then secure it anywhere in the box.3. Or if things are still best with rigid mounting, use right angle header strips to mate them. (Can even use female socket strip to make it a connector rather than a soldered connection, if at some expense.)As for assembly, I would think it would be okay, but:a. You should probably have some sort of bracket to fix the angle during assembly. It might be a jig, used only during soldering (and thus cost some NRE and tooling charge), or a bracket that's screwed or clipped in (you can find such items standard e.g. from Keyelco at the usual distributors), then left in, or removed after.b. You may have trouble forming a good solder joint. Ideally, such things would be mass-produced (where, for an assembler, "mass" might be more than a hundred or so?), which means wave soldering. You need to think carefully about the design of that joint.There's probably two ways to do it:You could do castellated holes. That would leave a relatively large gap between board edge and mating face. The 'barrel' (inside cylinder) of the cut-through hole faces the inserted board, and therefore leaves a lot of gap to fill. That's bad for getting solder to wick up. (You may need several tries to dial in the right radius and centerline, and the board fab may have issues with copper tearing off the milled edge -- that's always a danger with castellated holes, and why they add cost in tooling and yield.)You could do an unplated slot. Copper has to run right up to the edge, and that forms a butt-joint with the inserted board. The copper edges have to mate very close (which is why the copper has to be right up to the routed edge), otherwise when the solder wave flows over it, it'll just wick off the small fillet that you need.I'm pretty sure a big globby joint, like pictured, can only be done with hand soldering (or any of the really esoteric methods like laser plus solder wire/paste..). Fine for small quantities, but a huge deal in large quantity.As for strength, you definitely don't have strength if the solder fillet is tiny (which you can expect from wave). Even with brackets, vibration might be enough to fatigue it away rather quickly! Conversely, even with generous hand-soldered fillets, you have the problem of delamination, because there's no plating wrapping around either PCB, it's all along one edge or another. You get the strength of the laminate under the fillet, but that, too, will fatigue and fail (under harsher conditions).Oh, and obviously, you can't solder the top side, not with automated wave soldering. Putting solder fillets up there (as well as the bottom) would be excellent for strength, but awful for (automated) production. On the other hand, if you can make a plated slot work out, you'll be even better, because you get copper backed by laminate, plus whatever solder wicks along the whole joint!Tim

author: 2N3055
author_rank: Super Contributor
timestamp: July 27, 2016, 08:33:33 am
content: Quote from: m3g4by73 on July 26, 2016, 11:25:09 pmTake look a this...http://www.digikey.com/product-search/en/hardware-fasteners-accessories/mounting-brackets/2097394Google is your friendI also know there a loads of plastic elements like that... And inexpensive too..Take care!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 29, 2016, 12:07:03 am
content: FYI, noticed just today that the IPC has an opinion on the subject. Cheers:https://www.ipc.org/4.0_Knowledge/4.1_Standards/IPC-A-610E-redline-April-2010.pdfSection 7.3.5 or thereabouts.The rest is good reading too (if dry), for how a properly soldered board should be made, and should look, and all that.Tim