How to make a perfboard PCB rugged without destroying repairablility - Page 1
Forum section: electronics / projects

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: April 12, 2017, 08:09:42 am
content: Hey guys,I'm working on a project for an Airsoft/Paintball game prop. The devices is a suitcase in which there is a timer to register the teams controlling it (every team has a button they have to press to "claim" control of the device. Device will count the time each team controls it, team with most seconds wins).I'm actually "repairing" the device. It was designed by a prop-maker with very basic knowledge of electronics. While his design worked fine, it was very fragile, and on the first use it broke because it was dropped. I'm starting from scratch because it was easier than try and fix what was there.I would like to make this box quite rugged - it has to be able to take a beating. It will get thrown around and dropped, get bashed into trees while someone is trying to get it to a certain point. It's a metal case and has to survive being left in the scorching sun for hours in the summer. All my parts are severly overspecced. I'm using only Vishay and Panasonic 135C rated caps. Transistors are built for far larger currents than I'm using them for (I don't think I'm ever switching more than a few hundred mA, yet I'm using transistors rated up to many tens of amps).I will be screwing down the TO220 and such down onto the PCB. I would also like to glue down the voltage regulator module, as well as the electrolytics.Is there any way I can do this without ruining repair-ability? Are there glues that can be easily removed? (I know that goes against the entire purpose of a glue in the first place). Or are there any other techniques I can use to make the PCB able to handle more intense abuse without worrying about my caps and Vreg being torn off?

author: cowana
author_rank: Frequent Contributor
timestamp: April 12, 2017, 08:23:46 am
content: Your biggest enemy here is mechanical shock - with a bit of derating, achieving a working temperature of 'left out in the sun' is fairly easy to do (say 60 degrees max).You should look to keep the components as light as possible (can you use surface mount parts?), and for any big ones (TO220s) bolt them to the PCB (or glue them for things like electrolytic capacitors). A slightly compliant silicone such as non-acidic RTV will allow repairs to be carried out, as well as not causing issues itself due to unequal thermal expansion.You want to mechanically dampen the PCB from the chassis using rubber mountings (or Velcro as a cheap alternative), as well as making sure the PCB won't flex too much (which could crack traces). Often this takes the form of mounting the PCB to a metal/rigid plate, which is then mounted to the chassis with vibration isolating mounts.Try to avoid soldering wires (crimps are superior and less resistant to fatigue failure)

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: April 12, 2017, 08:30:25 am
content: Quote from: cowana on April 12, 2017, 08:23:46 amI would like to do it on a perfboard. Design of a PCB would take a lot of extra time, and manufacturing would take too long (if I send it out to china) or be too expensive (if I do it locally, I think it will be like 100 euros if I get them from Eurocircuits, which would be more than all the other parts combined). I think trying to use surface mount parts anyways will just make my life harder.I was thinking of finding some foam to attach to the bottom of the case, and then somehow fix the board to that, but I don't know if it will be any good.Quote from: cowana on April 12, 2017, 08:23:46 amDoes this include wires on the prototype board? Should I just ditch the prototype board and go for some other solution? As stated above, I'd rather not, but if it's the only way to get a reliable device...I'll have a look for non-acidic RTV silicone.

author: tautech
author_rank: Super Contributor
timestamp: April 12, 2017, 08:39:20 am
content: Nylon cable tie larger componentry to the PCB. Not fond of it but hot melt glue can be useful also.

author: cowana
author_rank: Frequent Contributor
timestamp: April 12, 2017, 08:41:10 am
content: Quote from: TheUnnamedNewbie on April 12, 2017, 08:30:25 amFair enough - perfboard works well enough for something like this! PCBs from China are very low cost - but if you have time as a constraint, then that sounds like it's ruled out.Quote from: TheUnnamedNewbie on April 12, 2017, 08:30:25 amIf there's nothing producing a lot of heat on the board, I'd be tempted to wrap the board in foam, put it inside a plastic project box, and fix that box securely inside your case. That way the box can't rattle around everywhere (as it's constrained in the project box), it's protected from shock (by the foam), and it's very repairable (as the foam is just wrapped round and easy to remove). Due to the fact perfboard is quite brittle and full of holes, I'd be wary of screwing the board to the case (even via rubber mountings) - the last thing you want is the entire of the board snapping off if it is dropped.Quote from: TheUnnamedNewbie on April 12, 2017, 08:30:25 amThat's only the case for wires which might have flex on them (such as the ones connecting the board to the outside world). Soldered wires tend to concentrate all the force at the junction between the solder and the wire - this can be mitigated by decent strain relief (drill holes in the board and thread the wires through that before soldering them) - but a crimped connector is a much more elegant solution.Quote from: TheUnnamedNewbie on April 12, 2017, 08:30:25 amThe important thing is it is designed for electronics - typical bathroom/silicone sealant includes acetic acid, which will cause corrosion over time.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: April 12, 2017, 08:58:00 am
content: Regarding the time-constraint: The guy I'm building this for is organizing a big game on the 22nd of april... I was originally planning to get a PCB from china, but that was before he told me that he wants it by then!Quote from: cowana on April 12, 2017, 08:41:10 amI only just found this out. I'm also worried now since I've been using regular bathroom silicone as a sealant for other purposes for a while now...There aren't much heatproducing parts on the board - I made sure of that. This is also why I went for a 7805-footprint switching module (ofcourse, the battery life was another major factor).Speaking of batteries - This will be powered with big RC/Airsoft batteries, since it's something that guy using will have a lot of with him already (as he provides airsoft guns for beginners that don't have their own). They are not ideal, but I felt like it would be the better choice for ease of use reasons.Because they are quite "low" capacity, most of them under 1.2Ah, I was going to put 2 or 3 connectors in parallel.However, they can provide a /lot/ of current, some of them rated to sustain over 50A or more. Although the batteries should be fine and the connectors are such that you can't plug it in the wrong way, I would like to fuse the connectors in some way, so a single reversed battery (the entire circuit has a reverse-battery protection circuit to avoid incorrect connection of the board with the battery compartment causeing damage) will quickly trip this fuse. I was thinking of just using 1W 100mOh resistors in series with the connector (wirewound resistors, as I have read they fail open). Will this work? Or should I instead bring out every connector to the main PCB and have proper fuses there? (I'm unsure how shock-resistant these fuses are tho...)

author: cowana
author_rank: Frequent Contributor
timestamp: April 12, 2017, 09:12:39 am
content: Quote from: TheUnnamedNewbie on April 12, 2017, 08:58:00 amThe only issue with this approach is if batteries with differing states of charge are mixed - for example paralleling up a fresh (4.2v/cell) pack and an empty (3v/cell) pack would result in the empty pack being charged very rapidly, which could cause it to overheat.A simple solution would be to add a series diode with each pack, ensuring your circuit pulls the majority of it's current from whichever battery has the highest charge level - this means the packs are (sort of) isolated from each other, and can be mixed and hot-swapped during a game.Are you planning to use the batteries main connector, or the balance connector? I'd be tempted to use the latter - smaller wires mean easier to work with, easier to add strain relief to, and less current if things go wrong. You could put the diodes in series with the wiring loom, or on the main board.Quote from: TheUnnamedNewbie on April 12, 2017, 08:58:00 amBetter to go with diodes between the packs to protect against one battery being backwards (or one unused connector being shorted), then a single fuse/over current protection on the board to protect against shorts.

author: Ian.M
author_rank: Super Contributor
timestamp: April 12, 2017, 09:31:14 am
content: To prevent reverse polarity or shorted pack disasters, stick a Schottky diode with a suitable voltage rating inline to each battery connector - it needs to be adequately rated, and smallish bore adhesive lined heatshrink used over each lead to wire joint, and a larger bore over the whole diode + both joints.You aren't doing yourself any favours at all by overrating transistors etc. as heavily overrated = bigger and heavier, which drastically increases the risk of board damage.As has been mentioned earlier, if the board is foam wrapped, it should survive, but that means you need to move any power devices (high dissipation) off-board to a heatsink. SMALL perfboards (1" square) siliconed to the heatsink should survive OK if you need free-wheeling diodes, base resistors or snubbers directly at the power device pins.Build a mechanical mockup by sticking the heavy parts you think you need in their approximate final locations to a perfboard. soldering their leads but don't bother with building an actual circuit. Put it in a stout box with thinner padding than the real thing and drop test it. Examine the board with a strong backlight to detect cracks after each series of impacts before increasing the height for the next series.For your temperature issues, it would be a big help if the casing can be polished metal, gloss white is next best and matte black is absolute worst. A matte black object in direct sunlight can reach as much as 50 deg C above ambient, so you'd have to design your electronics to work up to 95 deg C. If it cant be shiny bright, perhaps keeping it shaded could be part of the gameplay - if the hotter it is the shorter the time interval before the button has to be pressed again to count as retaining control!

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: April 12, 2017, 09:59:53 am
content: Quote from: cowana on April 12, 2017, 09:12:39 amI'm going to be using the main connector. They still use a lot of NiMH batteries as well, and those don't have the balance connector. The main connectors are also a lot bigger and clunkier, so it seemed like a better idea to use those for ruggedness reasons.Quote from: Ian.M on April 12, 2017, 09:31:14 amAcording to my back-of-the-envelope calculations I'll not really need any heatsinks. Everything in terms of regulation (a LED driver and the 2 voltage regulators needed) is switching. All MOSFET's will be switching a few hundred mA at most and the transistors I'm using should have an Rdson of under 50mOhm at my gate-source voltages. Even using worst-case calculations (taking Rdson 100mOh, assuming everything just happens to be drawing the maximum power it can, and then add 25% more) I get a maximum temperature increase of 30C, so even if it's in a 90degC box (it's a metal/alu shiny case so I doubt it will ever get there) it should still work in theory.Now I just have to try and get the parts before the weekend, and then I can get started on putting it together!