Criticisms on my reflow soldering process. I can't tell where I went wrong. - Page 1
Forum section: electronics / projects

author: roymoran
author_rank: Contributor
timestamp: February 15, 2023, 04:14:48 pm
content: I'm working through assembling a project but failed on the initial try. This is my first attempt at reflow soldering so I'd appreciate some advice. I'm using a hot plate with a hot air station. Here's my process -1. Started by applying solder paste (Chip Quik TS391SNL a non-leaded solder paste with melting temp at ~217-220C) to the board using a stencil. The result of applying the solder paste, IMG_3373. I wasn't happy with this result because I assumed some pads had excess solder so I redid the solder paste application. I didn't take a picture of the second application. The result was a little better but there still seemed to be some footprints with solder paste spread across their pads including the 144 LQFP. 40 pin FFC, and some ICs. Truthfully I don't know how much solder paste is too much on certain footprints. How can I tell when I've applied too much? I kind of assumed that some spreading across pads is okay since it would be wicked up by the pad once it melted. But am I wrong here?2. I hand placed all the parts on the board. Including the resistors, capacitors, inductors, diodes, sd card connector, usb-c connector, stm32 microcontroller, ffc connector, and other ICs. I pressed down on each part lightly to make sure the pins/leads were touching the pads. Some of the solder paste got spread around the pad but I figured this was okay.3. I set the board on the hot plate which at this point was at 150C, IMG_3390. The board must have come to temperature quickly because it was only a few seconds before it started releasing smoke. Was this the solder paste "working"? I let it sit at this temperature for the rest of the soldering process. I quickly started applying hot air to the board at 255C-280C. This is where it got difficult. I waved the hot air around the area I wanted to solder. When it wasn't working I cranked the hot air up from the 250C range to 260C range. The solder paste still struggled to melt so I began focusing the hot air onto certain pads on the ics/passives. This worked great for most parts on the board. But for the life of me I couldn't get the solder paste on certain pads to melt including the microcontroller/ffc connector. So I continued by increasing the temperature until I reached 280C. Little to no solder paste on the microcontroller pads melted. Same for the FFC. After spending several minutes across the STM32/FFC I threw in the towel and walked away. IMG_3391.This morning I inspected the board. I noticed that the pads where the solder paste did not melt were consistently ground pads which are connected to the ground plane through vias on the bottom side of the board. The entire bottom is intended to be the ground plane. Screenshot included with image of the pcb layout, ground connections are highlighted. My best reasoning right now is that the solder paste on these pads are just not coming to the melting point temperature. May the heat be dissipating too quickly on these pads? Should I be cranking up the heat on the hot plate or even the hot air gun. Looking at the difference in IMG_3390 and IMG_3391 I clearly applied a lot of heat to the FFC connector with the brownish tint. Or could this be something the needs to be fixed in design? Advice here is greatly appreciated, I'm looking to figure out my next steps to resolve this.

author: coromonadalix
author_rank: Super Contributor
timestamp: February 15, 2023, 05:14:18 pm
content: ill talk for myself, i use 350 degree temps and paste solder with a small dab of flux in itover time the solder paste dry a bit and i have to "wet it"seeing the photos : i could criticize a bit the cpu pads "too much"and in general, the solder paste on many parts is too big, the solder paste while melting will give some pulling action and try to center part(s)I dont know your stencil quality / precision, or it was not correctly put or the holes are too bigor it moved a lotCheck all your small SOTxxx ic's, way too much pasteA good stencil application would have the same size of the pads for the solder paste application on themand you have grain / balls dimension in the paste ...

author: Weston
author_rank: Regular Contributor
timestamp: February 15, 2023, 07:21:30 pm
content: It looks like your stencil lifted up / shifted during paste application. How are you fixturing the stencil?If the stencil hangs off the edges of the PCB and is unsupported it will flex downwards and lift the stencil over the PCB up, allowing paste to go between the PCB and stencil, increasing your total paste applied and smudging the shape.I typically surround the PCB I am stenciling with extra PCBs of the same thickness to make sure the stencil can not flex and make sure the PCB does not shift. Also, you need to be careful to make sure to lift the stencil directly up once you have applied solder paste, any lateral motion will smudge the paste. A little bit of smudging is ok though, a bigger issue is the stencil lifting and causing more paste to be applied.In terms of your soldering process it sounds like you are not getting the board hot enough. Reflow profiles for lead free solder peak at ~250C, but that is with monitoring and more exact temperature control. The hot air gun is going to heat your PCB up much less than that. You have a strong visual indicator on when the solder is reflowing, so its a bit difficult to over-heat the board (also, aside from connectors, its a lot harder to thermally damage things than you expect). Try running the hot air gun at ~375C or so.Preheating the board would allow less heat to be applied from the hot air gun. This board does not look like it has a lot of thermal mass so its probably overkill though. One thing to worry about with preheating is baking out all the flux in the solder paste. When the flux gets hot is activates and can start to evaporate. If you leave the board on the hot plate for a while (5+ minutes) it may bake out all the flux which would make getting the solder to melt and reflow correctly more difficult.As far as the connector goes, it's a bit of a lost cause. With lead free solder its almost impossible to not have plastic connectors get a bit of thermal damage without a perfect reflow cycle. I would recommend hand soldering it or removing the ZIF retaining clip before soldering and replacing it after, as its often the most heat sensitive part.

author: tooki
author_rank: Super Contributor
timestamp: February 15, 2023, 08:33:21 pm
content: I agree with Weston. Preheating at 150 degrees isn’t enough; if the preheated itself is at 150, then the board you place atop it will be less.Take a look at some solder reflow curves. You see a few distinct phases. First you quickly heat to about 150-200, then you “soak” there (let the heat penetrate), then quickly increase to reflow temperature and “dwell” there for a short time for the solder to melt, then cool down.The key thing is that the soak needs to be effective enough that you don’t need to add much heat to push you to above reflow.Applied to hand reflowing, it means you really want the board heated thoroughly so that you can come in with relatively gentle hot air. If you don’t soak enough, then you end up needing excessively high heat, and it still might not work, since the cool rest of the board is acting as a big heatsink. This may not be intuitive, but it’s gentler on the board to heat thewholeboard to, say, 190 (and then only need to use hot air to add another 30 degrees) than to heat the whole board to 150 and then need to add another 70 degrees with hot air. Doing the latter requires the air to be a LOT hotter, and that means risk of damage, as you saw.Remember also that a hot air gun isn’t a laser; the temperature of the jet of hot air plummets with distance. So if it’s set to 250, the hot air actually hitting your board isn’t going to be 250 unless the hot air tool is in practically direct contact. So it’s absolutely normal to use a set temperature of 350-400 degrees C, because with the low airflow speeds you need (to not blow away your components), the air actually hitting the board will be less. And air is a terrible heat conductor, so it isn’t very effective at getting that heat into the piece.Also, consider this: when you cook food in the oven, we need temperatures far in excess of our target internal temperatures. You bake a chicken at 180-220C, but the internal temperature of perfectly cooked chicken is 60-65C. Now, with sous vide cooking, where you cook with liquid at or only slightly above your target temperature, it takes food far longer to cook through, even with water’s much better thermal conductivity.

author: roymoran
author_rank: Contributor
timestamp: February 16, 2023, 02:36:54 pm
content: Quote from: coromonadalix on February 15, 2023, 05:14:18 pmThanks for the advice. I got the stencils through oshstencils so I imagine the quality is reasonable. It's a stainless steel 4mil thick stencil. I do need to practice applying the paste - I'm not sure if I have the technique down yet. The stencil did lift once or twice so I'll keep this in mind when I give this another shot in the next day or 2. I did research the grain/ball dimension in the paste before buying and I landed on T4 since I was reading that this is a good choice for finer pitch surface mount components.

author: roymoran
author_rank: Contributor
timestamp: February 16, 2023, 04:07:33 pm
content: Quote from: Weston on February 15, 2023, 07:21:30 pmRight now I am using a small acrylic jig set to keep the pcb in place and then applying some tape to secure both the stencil and jig set, IMG_3374. I do think the stencil is lifting slightly so I need to adjust my technique when applying the paste. Right now I am applying paste section by section so I am making multiple passes across an area which I'm sure doesn't help when there is a small lift. What I think I'll do is practice on the finer pitch components - apply the paste, lift the stencil, see how it looks, wipe off the paste, then practice again with a different component. It might be a bit wasteful but I think it's worth the practice to get my technique down.I did look at the reflow profile for the solder paste I am using and noted that it peaks at about 250C so I figured anything above that by 10's of degrees would work. I was nervous about going beyond 280C but I will crank up the heat as suggested.I was working with the board on the hot plate for well beyond 5-10 minutes so I wouldn't be surprised if the flux burned off making it harder for the solder paste to reflow. I imagine that I can add some liquid flux or flux paste to reactivate the solder paste. Thank you for the guidance and tips.

author: jpanhalt
author_rank: Super Contributor
timestamp: February 16, 2023, 04:49:32 pm
content: Your setup for applying paste is almost identical to mine, including using a SS stencil from OshStencil.Presumably minor differences:1) I used Kapton tape instead of masking tape.2) I taped both ends of the stencil.3) I used a metal spatula instead of plastic4) My jig is on aluminum MIC-6 tooling plate and has a CNC milled surface. I don't think the difference in flatness is significant.My finest devices were a TQFP chip (0.65mm) and Molex FFC socket (0.5mm). It was also the first actual PCB I did, but I did a few practice runs with waste. None of the practice runs included those fine-pitched devices. Pictures attached.I had much less paste on my pads than you show and overall, I was quite happy with the result. Sorry for the poor pictures. My setup for that is now a little better.PS: I later cleaned the board with my ZEP cleaner and git rid of the tiny bit of white junk between the pins.

author: tooki
author_rank: Super Contributor
timestamp: February 16, 2023, 07:38:55 pm
content: Quote from: roymoran on February 16, 2023, 04:07:33 pmOne reason people use PCBs to make their jigs is that you know the jig will be exactly the same height.With that said, I suspect your paste spread problem is caused by something else. It looks like the entire jig is set up on top of a silicone mat. That is going to provide flexibility that allows everything to shift or flex a little. That’s the exact opposite of what you want. If you look at commercial stenciling equipment (even manual ones), they’re all about stiffness, so that nothing wiggles: heavy steel plates, rigid frames, etc.Applying section-by-section is not recommended. If the stencil lifts even the tiniest bit, then the next pass, solder balls raise the stencil just a bit, allowing more paste to spread underneath. Your squeegee is wide enough to print your board in one pass, so you should. It’s not to say that I’ve never touched up an incomplete pass with a second pass, but I give the print extra scrutiny, since excess paste is problematic. (I recommend getting a metal putty knife — their sharp edge works better — or at least stiffening the plastic one so only the last 10mm or so can flex.) A related tip: NEVER, EVER lift the stencil and put it back down again. It doesn’t matter how precise your jig is, it will smear the paste a bit going down and, worse, get paste on the bottom, causing the same spread problem as above.If your prints are not coming out clean, clean the bottom of the stencil. It’s very obvious when paste is spreading on the bottom.On the topic of stencils, I have had good results by further reducing the paste apertures in my layout program. (IIRC, 20% reduction worked well.) One could accomplish a similar effect by ordering thinner stencils, with the caveat that then they’ll be even more fragile.

author: Jeroen3
author_rank: Super Contributor
timestamp: February 16, 2023, 08:04:00 pm
content: Such diy jig is very tricky for stencils when you need this fine pitch. Some alignment holes are neede.dI got much much better results with the ec-registration stencils and theec-stencil-fix, and also buying the 100um instead of the default 130um helped a lot.After stencil is done right, reflow is more tolerant. But from what you initially described you're dwelling in preheat too long or too hot activating the flux before the solder can melt.Then you can heat it whatever you want, it won't ever solder right because the solder will not flow.What you can try is royally coat the parts in flux paste and reflow it again. It will smoke -a lot- but it should solder.

author: jpanhalt
author_rank: Super Contributor
timestamp: February 16, 2023, 08:38:07 pm
content: I knew I had better pictures. Took me awhile to find. Attached are pictures of:1) DIY stencil printer w/o stencil Note: stencil can be raised vertically on the shoulder screws.2) Stencil in place w/o tape (no pix w/tape)3) Spatulas used. Either HD or Lowes, cheap, 0.020" thickness4) TQFP before final cleaning5) TQFP after final cleaning w/ ZEPI wanted a spatula that was stiff enough to give a little flex, but not so soft that it would dig into the apertures. They were pretty much the cheapest I could find and had slight burrs on the edges from manufacturer. I used a cloth wheel w/ red rouge to remove burrs. I also put a very small radius on each corner. As I recall, I made two passes with the smaller one to cover the entire stencil.Nothing you show seems grossly different from what I did. The extra tape and spatula were may be the greatest differences. I also used Kester EP256 T3 paste w/ 63/37 leaded solder.Practice with a stiffer spatula, and have both PCB and stencil secure. Try lifting the stencil vertically; although, many of the tutorials (e.g., Adafruit) just lift at the hinge. I don't know whether that makes any difference.

author: rstofer
author_rank: Super Contributor
timestamp: February 16, 2023, 09:07:56 pm
content: I have never reflowed a board complicated enough to need a stencil so...I put the PCB on the hotplate with the temperature control OFF and the plate at room temperature.I turn the temperature to high and wait until all the solder has reflowed. I can watch for problem spots like vias to ground planes. BTW, my board manufacturer has a via for power and ground plane that has spokes to keep the mass of the board from affecting the soldering at the via. It's called a 'thermal pad to filled plane' I believe.Once the solder has fully flowed, I lift the board off the hotplate and set it on a hot pad to cool. Then I turn off the hotplate.It has always worked well. I also have a converted toaster oven that also works well and at least makes an attempt at a reasonable profile.

author: Bud
author_rank: Super Contributor
timestamp: February 16, 2023, 09:30:52 pm
content: It is better to have reflow done in a closed box such as a toaster oven, this much improves hot air mass being applied uniformly across the board. Also you do not place the board on a preheated plate, place it on a room temperature plate and then turn the heater on. This will preheat the board evenly. As the other poster said, look at the typical reflow profile, they are often included at the bottom of datasheets. If you have a thermocouple (these days they come with multimeters), use it to monitor board temperature. You do not have to be spot on on temperature and duration of each phase, just use your judgement and common sense to follow recommended reflow profile curve in general.

author: roymoran
author_rank: Contributor
timestamp: February 17, 2023, 02:15:48 pm
content: Quote from: tooki on February 15, 2023, 08:33:21 pmThanks for thoughts around the preheating/hot air gun. I'm going to do a second run tomorrow, and will tweak my process with the comments. I'll pay closer attention to the reflow profile.

author: roymoran
author_rank: Contributor
timestamp: February 17, 2023, 02:27:54 pm
content: Quote from: tooki on February 16, 2023, 07:38:55 pmThanks tooki for these tips. I've also adjusted my setup to remove the silicon mat from underneath. Instead I'm securing the jig/stencil/pcb directly onto the table.

author: roymoran
author_rank: Contributor
timestamp: February 17, 2023, 02:32:03 pm
content: Quote from: jpanhalt on February 16, 2023, 04:49:32 pmYeah I think I just need to work on how much paste is deposited on the pads. I'd be happy with these kind of results. When you inspect the board do you use a microscope to take these pictures? I like that you're able to see that much detail.

author: roymoran
author_rank: Contributor
timestamp: February 17, 2023, 02:41:17 pm
content: Quote from: rstofer on February 16, 2023, 09:07:56 pmThat was my mistake on not following the reflow profile better. I made a mental note of the reflow profile before starting the whole process but still made the mistake. I'll fix this on my next try.

author: roymoran
author_rank: Contributor
timestamp: February 17, 2023, 02:45:03 pm
content: Quote from: Bud on February 16, 2023, 09:30:52 pmThanks Bud, I have a thermocouple I'm using. I'm keeping a closer eye on the reflow profile moving forward.

author: jpanhalt
author_rank: Super Contributor
timestamp: February 17, 2023, 03:23:07 pm
content: Quote from: roymoran on February 17, 2023, 02:32:03 pmYes, those most recent pictures were made with a typical binocular dissecting microscope ("dumpster salvage"), a Nikon CoolPix 990 (circa 1998) camera, and a homemade adapter. It's a nice little camera, but its memory card is huge. Sensor is only 3.3 megapixel. Earlier photos were with an inexpensive "digital microscope" like one sees advertised everywhere. I didn't like the distortion and resolution of the latter. My current plan is to find an "affordable" macro lens for my new Z30 camera. Just waiting for the next Nikon sale.I am not stuck on Nikon, but I do like the capabilities of a real camera+macro lens v. $70 Amazon digital versions.On the subject of keeping the stencil and PCB from moving relative to each other, I wonder what would happen if you sprayed the bottom surface of the stencil with a light coat of repositionable adhesive (e.g., Elmer's, Krylon, many others), let it dry but still be tacky, and then applied to the PCB. The adhesive might also help prevent paste from getting under the stencil.John

author: jmelson
author_rank: Super Contributor
timestamp: February 17, 2023, 04:22:06 pm
content: I do production on a limited scale, but have made over 2000 boards this way. I have a stencil printing rig that holds the boards by the edge, with a step the thickness of the board. I tape the stencil on one edge ONLY to the adjustable frame. So, the frame is parallel to the top of the board. I use .003" (0.075mm)stencils, and reduce the size of the stencil apertures to reduce the amount of solder. I reduce the apertures much more agressively for fine-pitch components, it really doesn't matter so much for larger passives. I use a GE toaster oven with a thermocouple ramp and soak controller. This is like the common thermocouple controllers seen on eBay, etc. except that it can take multiple setpoints with ramp time between. I poke a micro thermocouple into a plated through hole in the board to measure actual board temperature. I then set up the controller to ramp over several minutes to 180 C, then ramp as fast as the oven can do to 236C for lead-free solder and 220C for leaded solder, and then cool fast. This works amazingly well, and if my stencil alignment is good and the parts are accurately placed, I get excellent soldering results. I use GC10 solder, which is amazing stuff!Jon