SMT PCBs subjected to torsion and high G forces - Page 1
Forum section: electronics / projects

author: profdc9
author_rank: Frequent Contributor
timestamp: May 17, 2020, 06:20:37 pm
content: I am working on a project where a LQFP-100 part is subjected to signficiant G-forces and the PCB itself is twisted. The PCB is 1.6 mm thick 2-layer FR4. I am finding that the LQFP-100 pins are becoming detached from their pads and the solder joints are cracking. Has anyone else had this problem and found a good solution? I am thinking of using epoxy to glue several layers of a backing to the PCB to stiffen it. Or perhaps covering the entire chip with epoxy might help, but then if the joints fail they will no longer be accessible. Is lead-free solder more or less susceptible to cracking than leaded solder?

author: Domagoj T
author_rank: Frequent Contributor
timestamp: May 17, 2020, 06:37:51 pm
content: Instead of trying to prevent solder cracking when PCB is bent, why not try to prevent it from bending in the first place by modifying the mounting orientation or adding bracing?

author: jmelson
author_rank: Super Contributor
timestamp: May 17, 2020, 06:55:14 pm
content: Quote from: Domagoj T on May 17, 2020, 06:37:51 pmYes, this is right. it may be needed to attach the PCB to a stiff structural member that is loosely coupled to some outer structure, to act as a stiffener to the board, to prevent flexing. Flexing a board with high pin-count ICs is just never going to work. Many systems that have to work in harsh vibration environments use some combination of flexible mounts and stiffeners to protect the PC boards from flexing. They also work with vibration tables to assure that the board's natural resonant frequency does not coincide with some other structural resonance. (This latter testing would be hard to do at home.) The aerospace guys fight this kind of thing all the time.Jon

author: jmelson
author_rank: Super Contributor
timestamp: May 17, 2020, 06:59:32 pm
content: Quote from: profdc9 on May 17, 2020, 06:20:37 pmPure tin is definitely more fracture-prone than Tin-Lead. Some newer SAC305 solders are supposed to be close to Tin-Lead in resisting fracture, but still not as good. Low-temp solders with Bismuth in them seem to be extremely fracture-prone, taking almost no effort to knock components off.Jon

author: Gyro
author_rank: Super Contributor
timestamp: May 17, 2020, 07:16:45 pm
content: Note that if the relatively flexible leads of the LQFP are coming loose, then you are probably suffering hidden cracking damage to small rigid components, particularly MLCCs, too.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 17, 2020, 10:50:32 pm
content: My understanding is lead-free solder is stronger (SAC305, don't use pure tin), but does tend to fail in fracture while SnPb solder tends to fail more gradually.Yeah, this is a complete and utter failure of mechanical engineering -- PCB materials are strong and stiff enough for themselves, but they aren't meant for much load bearing, and a PCB with componentsdefinitelyisn't good for high strain! Absolutely, put a metal bracket under the poor thing. Maybe even isolate it from the bracket's strain by using a few mounting points in the middle, or a secondary bracket or something like that (with a vibration dampener on the end(s), to deal with the effective sprung mass this creates).Assembles for robust applications are often potted. This can range from a jelly consistency mainly for environmental protection (it will will still absorb quite some acceleration, as effectively the board is buoyant in the potting, distributing forces more evenly than hard points might), all the way to glass-hard materials that turn the circuit into a rigid brick. (Usually a hybrid approach is used in that case, where the circuit is conformal coated with something goopy to provide strain relief, then potted in harder or rigid material.)If for some reason, strain must be applied to the PCB, you can still do some isolation to help it. For example, rout a three-sided box around a given subcircuit, so that only strain from the one connected side can deform it. Various flextures can be made by routing, which will take up quite a lot of PCB area of course, and greatly limit electrical routing too, but can be used to make limited spring-mass isolation and filtering structures.What's neat is, isolation could be as simple as one or two routs, but I think there are very few instances where 1. that will be enough isolation to do the job, and 2. where the forces or mounting locations or component locations actually benefit from such a step. (A simple example might be, a screw mount that happens to be subject to a bending moment; slotting the PCB beside it allows local torsion, without bowing adjacent components (on the other side of the rout).)And obviously, you are left with less structural PCB, so the permissible load goes down dramatically (or the strain goes up, or the resonant frequency goes too low, or..).So, this is rarely done in practice I think. Consider this not so much as useful practice, but as interesting supporting information.One place it is kind of common to see, is the precision (voltage or time) reference in some test equipment. Dave's made some videos showing examples. The reference circuit might be isolated on a two-axis flexture. That is, a pair of facing 'C' shaped routs, which nearly cut out a rectangle from the main PCB. The rectangle is thus able to twist against the PCB plane, and isolated from one axis of stretch. Two sets of these cuts, nested and rotated 90 degrees, makes a bit of a two-gimbal linkage, allowing both axes of stretching and bowing, and twist (shear), to be isolated from the inner section. The thermal conduction path is likewise greatly lengthened, so that the section can be ovenized for thermal as well as mechanical stability. The resulting mass-spring system may be prone to resonance, but this may be partially addressed with dampening material which also serves as thermal insulation.Tim

author: profdc9
author_rank: Frequent Contributor
timestamp: May 18, 2020, 02:22:10 am
content: Ok thanks for your feedback. I am working on a spinning 3-D display. The board is being twirled by an induction motor at 1800 rpm. There are two brushes that attach to the bottom sides of the board that brush two stator connections. It detects a full revolution by detecting when the polarity changes.The problem is that the brushes torsion torques the board and caused the processor to become loose. Taking your feedback into account, I have some ideas:1. Add a cutout in the board where the brushes attach so that the brushes only torque the board in the direction mostly perpendicular to the axis of rotation, reducing the torsion.2. Clear out the back half of the board so that it is mostly flat and all the through-hole components are at the edge. Then a large flat metal backing can be epoxied onto the back side to make it rigid. This backing can relieve most of the stress on the board I think. Any stress from the brushes is mostly taken up by the back plate, and the PCB is going to be twirled by being pushed by the back plate.Here's the layout of the PCB as I currently have it. The back plate is going to be the entire board almost out to the edge of the axis, except it will not cover the outer 1/2 inch or so on either side. On the left side are the programming contacts, and on the right side is the antenna for the NRF24 radio which can not be over the metal plate for obvious reasons. The long slots will be cut by a rotary tool to make a complete cut out in the board.Does this seem like a good plan?

author: dmendesf
author_rank: Frequent Contributor
timestamp: May 18, 2020, 03:49:41 am
content: I would add more cutouts (full line from board end to board end) and make PCB 5mm longer to have more distance between TQFP and cutouts. Or you can run your display in vacuum to reduce torsion

author: moffy
author_rank: Super Contributor
timestamp: May 18, 2020, 05:29:47 am
content: You could also try the aluminium backed pcb material:https://www.pcbway.com/pcb_prototype/General_introduction_of_Aluminum_PCB.htmlIt would also help with any heat issues.

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: May 18, 2020, 06:05:25 am
content: Silly question, why not try a thicker PCB?I am quite surprised that a QFP is managing to wiggle itself off as the leads should somewhat help with the flex. I guess the other question is, does this IC come in a package with a thermal pad? If so give that a try as well

author: DaveW
author_rank: Frequent Contributor
timestamp: May 19, 2020, 05:42:11 pm
content: In a high G/torque situation, you will effectively have the PCB acting like a beam. Assuming I'm understanding correctly and the rotation is vertical as shown in the attached image, then the point of maximum inflection/bend will occur in the middle of the PCB. Right under your QFP, which will lever it straight off the board. You may well find just by moving it you eliminate the issue; in a previous job delivering stuff up to 1500g, one of the roles I wrote was not to have anything on the quarter or half lines of the board, and these combined with 2.4mm boards, supported circa every 30mm made everything survive.

author: Mecanix
author_rank: Frequent Contributor
timestamp: May 19, 2020, 06:20:46 pm
content: That sounds like a flex-pcb and redesign

author: profdc9
author_rank: Frequent Contributor
timestamp: May 20, 2020, 02:26:45 am
content: Ok try again...I don't see how I can add cutouts all the way across the board. That will completely sever the brushes from the rest of the board. My goal was to allow the brush to bend without torquing the PCB as much. I am thinking of cutting out a 1/4 inch 6061 aluminum plate and epoxying it to the back of the board to add stiffness as well.So I moved the brushes to the opposite site of the board and flipped the display over so that now the farthest side from the brushes is the LQFP-100 package. All of the components near where the torsion is applied are small components, that is LEDs, and those are lined up with the axis of rotation. Take a look:Perhaps this will take the strain off of the chip?

author: enjoy.cowboy
author_rank: Contributor
timestamp: May 25, 2020, 06:21:59 pm
content: Have you considered a solution by stacking smaller boards? I had high G and vibration issues when designing boards for a homemade rocket. Having them being circular and with screw-on board spacers to stack them with board to board interconnects more or less solved any bending