Illuminated tact switch SMT soldering problem - Page 1
Forum section: electronics / general

author: jrowberg
author_rank: Contributor
timestamp: January 14, 2021, 04:13:00 am
content: Hey all,I'm working on a board that has about 90 illuminated tact buttons on it. It's about 200 x 250 mm, 4 layers (1.6mm thick) with traces on the top/bottom and solid GND/5V planes for the two inner layers. The problem is that a bunch of the pins on the buttons are not making good electrical or mechanical contact after the reflow step, and I can't figure out why. There are 100+ resistors and another 50+ 1206 LEDs, all of which work fine. I've included a close-up photo showing an example of where the problem occurs.For the current test builds, I'm using Kester 63/37 leaded paste and reflowing in a modified toaster oven powered by the Controleo3. I have reflowed many small boards and large panels with this same setup, and to my knowledge it works fine. This is the first board I've had this problem with, and it's only with the tact switches.My first thought was that the pins on the switches were bent upward too much to make contact with the paste during reflow. I haven't ruled this out 100%, but it's more of a problem I'd expect to see maybe 5-10% of the time, at worst. I'm seeing more like an 80% failure rate on these things. I also did pulled a dozen of the tact switches from the reel and placed them on a blank bit of FR4, and all of the pins in every case seemed to sit flush against the board as expected. Certainly nothing so high that it wouldn't make it into 0.3mm of paste.My second thought was that a very large board with two solid inner layers needed more time to come up to temperature during the reflow, so maybe the paste wasn't getting quite hot enough. I tried another partial assembly with a longer soak @ 160 C, slower ramp up to 225 C, and longer hold at peak to be sure. I also watched the paste visually through the small uncovered portion of front glass. The paste quite obviously flowed well below peak and stayed molten the whole time until cooling. The outcome was the same--most of the tact switches didn't light up properly.My third thought was that maybe my paste was too old, since I'd run the earlier tests using a large-ish jar of paste that I've been dipping into for a while. But when I tried again with a small, unopened jar of the same 63/37 paste noted to expire six months from now, the result was yet again exactly the same--over 50% failure rate.You can see in the attached photo that the solder just isn't flowing onto the pin. It's almost like there's some chemical that is preventing it, or it's the wrong metal or something. (I don't currently have specs on what kind of metal it is.)Have any of you encountered a problem like this before? Can anyone recommend a possible solution, or further tests that I might run to be able to identify the cause?Thanks!

author: SVFeingold
author_rank: Frequent Contributor
timestamp: January 14, 2021, 04:31:21 am
content: It may well be the wrong metal. Where'd the switches come from? Is it possible they're from a mystery OEM or counterfeit? I ask because I had a similar situation at my last company...we ordered some custom gold plated SMT clips and none of them would solder with anything less than acid flux. Sent them off to EDX and lo-and-behold...0.0% Au detected on the surface. This was a "trusted" supplier too. It could be that the plating is not actually what it should be.It's also possible that the switches aren't heating up enough. I've got a similar setup (custom oven with a Controleo 3) which has worked perfectly for years. But occasionally on high thermal mass boards and components I have to crank it way harder than you would think for things to reflow reliably. But these switches aren't so massive so that'd be a surprise...Have you tried soldering them with an iron? That work any better?

author: 48X24X48X
author_rank: Frequent Contributor
timestamp: January 14, 2021, 05:49:38 am
content: Where is the thermocouple tip located? Hanging in the air?

author: jrowberg
author_rank: Contributor
timestamp: January 14, 2021, 05:34:48 pm
content: Thanks for your replies. I've attached two more photos showing the oven outside and inside; I tried to follow the Controleo3 guide as well as possible, and I think it turned out well. You can see the thermocouple hanging in the middle, coming from the right wall. The tray is the aluminum one provided with the complete Controleo3 kit, not the original one from the toaster.As noted before, I think the oven is getting hot enough; the paste I'm using visibly flows around the 190 C point (as reported by the Controleo3 display), and then the temperature is brought up to 225 C peak. So whether the thermocouple is reading higher or lower than it ought to, it can't be too far off. And you're right, the switches don't appear to have a lot of thermal mass, and the pins themselves are quite small and thin. Further, the pads on the board are not large, and although some of them go to GND, they're connected through a thin trace into a via rather than a large pour on the top layer.The switches are from a Chinese manufacturer, sourced by the guy for whom I'm assembling the boards. He reached out to them about the material they use...apparently it's some kind of phosphor copper and they're looking into a better quality phosphor bronze...? I'm not used to having the option to modify the build of components I'm using, but apparently it's a possibility in this case.I can achieve mechanical + electrical reliability if I solder with an iron, but of course that doesn't scale when you want to build a batch of 20 boards that have 80+ buttons with 6 pins each.But yes, it's possible to get them to work. However, even in that case, I don't end up with a clean solder joint. Even iron-applied direct heat doesn't cause the solder to flow nicely onto the pins.

author: SVFeingold
author_rank: Frequent Contributor
timestamp: January 14, 2021, 09:56:22 pm
content: Based on your description the problem is most likely just terrible metallurgy on the mystery switches. It's one of those things that most people don't have the tools to verify and so regardless of what you request, the crappier suppliers will just melt down some old bicycles and call it whatever metal you want. I assume you're using a decent quality solder as well?Is it out of the question to try ordering some new switches from Digikey/Mouser?

author: jrowberg
author_rank: Contributor
timestamp: January 14, 2021, 10:31:01 pm
content: Huh. Interesting. We'll see with this supplier.We're planning to test some switches from Digi-Key that have the same footprint, but I haven't found any that match the footprint, plunger height, *and* color/temperature of the internal LED. Even the ones that don't match are considerably more expensive. I guess there's a reason for that.

author: floobydust
author_rank: Super Contributor
timestamp: January 15, 2021, 05:50:28 am
content: My theory is the LED's leads are cold. Based on the pics, I have issues with your reflow oven. Where is the circulation (convection) fan?There are three ways heat is transferred. With the quartz heating elements and no fan, it's only radiation.So the thermocouple does not see an accurate air temp (it's basically lit up by the heating element near it), and the switches LED leads cannot get heat. They're shiny and reflective to IR so convection heating is the only means, which will take forever here without forced air currents. The PCB is on a hot plate, great for SMT but the top-side larger parts will stay cold.

author: Psi
author_rank: Super Contributor
timestamp: January 15, 2021, 06:17:07 am
content: Some possibilities.1)Is that switch supposed to be mounted upside down through a hole in pcb?I've seen people get confused with SMT LEDs that are intended to be reverse mount.I don't think this is your issue but it did occur to me as a possibility.2)This one is more likely.Have a look at the underside of the smt switch, the part that touches the pcb. Does it have little plastic bumps?If it has bumps those are supposed to go into alignment holes in the PCB. If your PCB doesn't have those holes the switch won't sit flat and you would get this issue.3)Or it could just be bad parts. Someone selling factory rejects or similar.

author: Kean
author_rank: Unknown
timestamp: January 15, 2021, 08:03:26 am
content: If the pins are low quality metal or tarnished due to poor storage conditions, the flux in your paste may just not be able to clean the surface for proper wetting.What happens if you apply a small amount of the same paste to the pins of a factory fresh switch and reflow that with hot air, or in the oven but placed upside down on a dummy PCB (legs in the air).Does it flow nicely all over the pin?You could try rubbing the switch legs on some fine sandpaper just prior to placement, and see if the solder flows better after that. It may be quicker than hand soldering, but still time consuming. At least it may help confirm the issue.Or you could try a solder paste with RMA or RA flux (mildly activated or activated) for more aggressive flux action. That may lead to cleaning issues as you likely can't immerse those switches in a cleaning bath.

author: 48X24X48X
author_rank: Frequent Contributor
timestamp: January 15, 2021, 08:17:42 am
content: As someone that makes reflow controller for a living, I would suggest trying to put the tip on the thermocouple touching the PCB. If you have a through hole to do that, it will be even better. I also own those China desktop oven which have the thermocouple hanging in the air setup, the difference between the PCB temperature and the oven can be around 20-25 Degree Celsius.Secondly, always use fresh solder paste for reflow and don't skimp on that. I use a new jar every 1.5-2 months. The point your soldering iron can solder it manually gives a good hint.

author: thinkfat
author_rank: Unknown
timestamp: January 15, 2021, 12:20:16 pm
content: If the paste doesn't wet the pins even with an iron, it's old.

author: SVFeingold
author_rank: Frequent Contributor
timestamp: January 15, 2021, 09:39:39 pm
content: To Flooby's point: when you say you watch the paste melt, is that the actual paste on the pins in question? Or just paste elsewhere on the board? I kind of assumed the former, but if the latter than yeah...you can't assume it'll reflow everywhere just cause it did in one place.I don't have to mess with profiles too often in my oven (similar to yours but w/ a fan and more insulation). I've got one for GC10 and one for leaded and they work just fine. Even so, I will always take a bare PCB (always get extras!) and apply paste and then reflow it to make sure it all reflows properly. It's also much easier to visually inspect. If there's that one section of the board that's only molten for a few seconds before the cool-down starts, that's a problem! Obviously adding components to the board changes the thermal equation but it's a good place to start.My best guess based on your difficulty with the soldering iron is that either your paste is bad or the pins are bad. If you used a good spool of solder instead of paste for that test and it STILL didn't reflow, I would blame the pins IMO.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 17, 2021, 07:34:21 pm
content: Judging by the way the resistor is clumsily soldered next to it - it is not aligned with the surface tension, I can assume an insufficient temperature at the peak of melting. To all the hypotheses listed earlier, I can add one more: the button is high enough, it can create a shadow for direct IR rays. Because of this, some of the pins and the poor resistor are not heated. Your funny oven doesn't seem to be very capable of sending rays reflected from the walls to the board.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 17, 2021, 07:43:08 pm
content: Another hypothesis: the button may want to find work at Musk, contains some substances or moisture inside and tries to take off when heated.Or it deforms when heated and one pin is lifted.

author: Matt-Brown
author_rank: Regular Contributor
timestamp: January 19, 2021, 05:51:58 pm
content: You could change the stencil1) 3MM that is thick 1.5MM should easily be thick enough.2) Extend the aps on the stencil out onto the soldermask a little (3% a side) for the switch leads and do inverted homeplate. You may just be raising the switch up so high on the solder it is floating as the solder melts and not getting s chance to go around and along the leads

author: tooki
author_rank: Super Contributor
timestamp: January 23, 2021, 10:39:11 pm
content: Quote from: Psi on January 15, 2021, 06:17:07 amOf course not. If it were, all of the force of a button press would become tension against the pads on the reverse side. It wouldn't last more than a few presses before the pads rip off.

author: jrowberg
author_rank: Contributor
timestamp: February 22, 2021, 10:46:46 pm
content: Hi all,I'm finally back, and the problem is solved. TL;DR answer:Quote from: SVFeingoldBingo! I'm going with 95% likelihood of sub-par metal in the switch contacts, 5% likelihood of ENIG plating on the PCB making a difference, though I never had a similar issue with non-ENIG plating and component adhesion from this vendor before, and I've used them for years.To address a few individual comments--thanks for all the input, by the way--see below.Quote from: S. PetrukhinI admit the resistor looked shabby, but that's because I added them in after reflow using hot tweezers, because I forgot to place them during my quick test assembly. The oven is in fact quite capable of getting heat to the edges of the board. I will attempt to do a more scientific test with a thermocouple tip Kapton-taped right to the PCB surface, but it's clear from watching through the front window of the oven that the paste reflows thoroughly all over the board well before the peak temperature is reached.Quote from: floobydustThere's no circulation fan in this build, but it's a small oven with great pains taken to add reflectivity and seal off all leak points. As noted above, the paste reflows easily every time, and this oven has has no troubles building panels of boards as big as this before.Further, I took apart one of the buttons to see the LED portion of the button clearly, and it's an incredibly small thermal mass. Even if heating were an issue in some cases, this shouldn't be one of those cases.Quote from: PsiInteresting guess, but nope. Definitely not upside-down. Coincidentally, I actually did end up with a hundred LEDs (but not buttons) intended for this product which were mistakenly the bottom-mount variant, and I had to flip them by hand to make them work. Go figure.Quote from: PsiGood question for sure, but these buttons have no alignment bumps on the bottom.Quote from: KeanManual paste application and hot-air reflow on the original button appeared to work, but it's hard to say for sure. The additional heat could have impacted things, and that technique wouldn't be so good for the plastic housing. I wondered whether I needed some special kind of paste, but that question was rendered moot by the success of new buttons.Quote from: 48X24X48XI'm definitely going to do this. If I had to guess, I'd bet the PCB actually gets hotter than the thermocouple in the air based on the nominal temperature the paste is supposed to reflow at. We'll see though, after I get a chance to perform the test.Quote from: 48X24X48XI'm bad about this, but even with unopened, unexpired paste, the original buttons wouldn't attach. With the new buttons, even my old paste had a 100% success rate.Quote from: Matt-BrownI misremembered the units (oops). I was thinking of 3mils, but actually the stencil I used is 4mils. Good tip on the extra aperture dimensions though.