Stiffening options for long unsupported PCBs? - Page 1
Forum section: electronics / general

author: Dave8266
author_rank: Contributor
timestamp: October 06, 2020, 09:19:08 pm
content: I'm working on a project that involves some fairly long and narrow PCBs, which due to mechanical constraints, will only be able to be mounted/supported by the ends.The PCBs have more flex than I would like, so I've been investigating ways to stiffen them. Has anyone been down this route, or have any suggestions?I've found several references to dedicated "PCB Stiffeners" (that are also sold as bus bars), but they appear to be uncommon / non-stock / custom-order anywhere I look.I've also found products which are channel-shaped that slip over the PCB edges, but they seem to be designed for supporting boards during assembly/PNP, not intended for permanent attachment.I've thought too about running lines of cheap female 0.1"/2.54mm header strips. 64 pins (6.4"/162mm) is the longest common single-row piece before the prices shoot up. It would be straightforward and even machine assembly compatible, but I'd either end up withhundredsof extra through-holes in the PCBs, or a high parts cost if surface-mount versions were used.Another option, likely the cheapest, would be to leave some large bare copper pads near the board edges, and solder some long but narrow (0.2"/5mm?) matching PCB strips on edge (an inverted T-shape against the main board). Conceptually, it would be like making an I-beam out of FR4, just a different shape. It would be similar to the technique described in this Hackaday article from 2015, but just spot-soldering pads approximately every inch/25mm to limit the heat required:https://hackaday.com/2015/06/03/how-to-build-beautiful-enclosures-from-fr4-aka-pcbs/I'd epoxy the board to a backing structure if I could, but the layout necessitates components and connectors on both sides.Any thoughts or experiences in this situation?

author: wraper
author_rank: Unknown
timestamp: October 06, 2020, 09:36:49 pm
content: Add two L profiles along the board. Preferably in a way that they are screwed to enclosure at the ends. Or place narrow U profiles on it's edges so that they surround PCB edges.

author: eliocor
author_rank: Unknown
timestamp: October 06, 2020, 10:10:43 pm
content: I do not know quantities, length span, PCB dimensions/shape/weight, maximum allowed flex, ...so it is rather difficult to help you.Have you ever think about extruded aluminium profiles?if the profile is very simple the custom extruder can be rather cheap, but as I wrote, I have not the faintest idea of your real requirements!

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: October 06, 2020, 10:23:06 pm
content: Couple of things:Is there a reason why you can't mount the PCBA to something solid?If yes for above, can you use a thicker PCP? For example 3.2mm vs 1.6mm

author: Dave8266
author_rank: Contributor
timestamp: October 07, 2020, 12:22:06 am
content: Thanks for the replies.I can't be completely specific, unfortunately. It's approximately 36cm x 2cm, and is being added to a small slot in a piece of machinery to modify an existing process. The bottom of the board has sensors, the top side logic, and I have maybe 4-5mm of lengthwise board edge space available for strengthening. The flex I'm concerned about is long-term flexing or resonating fatigue in an environment with low/moderate vibrations. Because of the physical constraints, something which attaches to the board (rather than mounting the whole board in a fixture) would be preferred.These boards are not safety critical, nor inaccessible or irreplaceable, and this first round is an experiment. We just want to give them the best chances we can for survival on the first go.

author: eliocor
author_rank: Unknown
timestamp: October 07, 2020, 01:00:33 am
content: I don't know the English name, but maybe you can (if there are no vibrations) use the following as a stiffener:https://www.amazon.it/Fellowes-D103BL-Rilegafogli-Diametro-Confezione/dp/B00FHST088

author: PeteH
author_rank: Unknown
timestamp: October 07, 2020, 01:20:11 am
content: Use a second PCB along the length of it? PCBs are inexpensive...Either parallel or right angle with "periodic" fasteners/soldering.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 07, 2020, 02:57:33 am
content: If the back is flat, just double-stick-tape an aluminum or fiberglass angle to the back?Tim

author: IconicPCB
author_rank: Super Contributor
timestamp: October 07, 2020, 03:26:12 am
content: I had to make a PCB assembly approximately 1.9m long and 40 mm wide.A replacement for a fluoro tube supported by the ends and conventional tomb stone end pieces.I ended up with a T profile made up of conventional FR4. Adjacent lengths ofapprox 490mm were stitched together with a joiner piece whihch was riveted in place by four eyelets ( hollow rivets as used on shoes to hread shoe laces ) and only then sweat soldered.The vertical of the "T" was made up of five sections and similarly riveted/sweatsoldered and then applied to the top bar of the "T" in specific slots so that no joint in either the vertical or top section overlapped. The two pieces had solder tabs on the vertical and solderpads and slots in the horisontal tyo effect the solder joints.It proved reasonably robust.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 07, 2020, 11:46:13 pm
content: How thick can you make the board? It's not difficult to get 50% thicker than the usual 63 mils, about 90 mils thick, and you can probably ask for the stackup as thick as they can reliably make it. You will however have to to adjust drill sizes and check to make sure that extra via length isn't a problem. Through hole connectors often come with varying tail lengths.You didn't mention the number of layers.Your panel and assembly rails may also need revision. Check with the assembly house first.The only time I worked on some very long and narrow boards was for a light curtain.Thick FR-4 can be very structurally sound if thick enough...