#54 Solder Paste Technique without Stencil

[Music] welcome back today I wanted to do a quick video about how i transfer solder paste from in here to over there for example without using one of these syringes now this is a syringe which has some flux in it but you can buy these and you can also buy the individual needles but that rarely works because sometimes you get way too much out of it and sometimes you get nothing out of it despite how hard you press on this side so about a month ago somebody asked me on one of my videos how I transfer my solder paste to the board without using a stencil now this can be quite difficult but what I found is if you mix this this is some it's from Aliexpress it's called pee-pee warning bepd pro paste it's a let free paste I think this is a medium temperature maybe even low temperature solder paste it doesn't really matter I believe this is Type three that's also type 5 which has some smaller solder balls but when you try and for example take one of these cocktail sticks and try to stick it on the board it ends up all over the place except on the pads and usually you're just moving solder paste around without actually having it sticking to the board so today I'm going to show you my method and it involves using a little bit of white paper can be another color just plain paper and this tape now I don't know what this is called but it's this Brown packing tape and I used this because it has some plastic called him on it and in a minute you'll see why so what I do is I take a little bit of tape and stick it to the paper and I get one of these cocktail sticks whatever they're called and just grab a little bit of solder paste that's actually a lot God it's okay and then I mix this with some flux let's get in a bit closer and I'm gonna add a decent amount of flux because I want this paste to be more viscous and that really helps when transferring it to the PCB and now I'm just gonna mix this [Music] which doesn't always happen really quickly but you can take your time and add flux if needed and now you can see why I'm using the sticky tape instead of just paper because the paper would shock out all the moisture of the solder paste and I actually need it to be quite moist and you can already see it doesn't look as dry as it did before and what I'm actually looking for is you see when I lift the cocktail stick it forms these kind of strings and that's what I want because I'm actually going to use gravity to transfer the solder paste to the board and this is quite big if you look at this in comparison to the board that's quite big so what I'm actually going to use is a lead of a resistor it's just a resistor I used for something else I saved the lead and this is going to be my transfer device let's move over to the microscope where I can show you what I'm doing in detail so as you can see it's quite viscous at the moment and this is about the consistency I wanted in see those little strings form that's what I want so here's the board it's a qfn 20 with a point five millimeter pin spacing which is actually the distance between the center of one pin and the center of another pin and I'm going to sold it to this a don't know if you can see this on camera hmm don't know it's a ft 2 3 1 X Q and the Q meaning qfn and it's a usb-to-serial chip by FTDI now I'm going to grab the resistor lead so I'm just touching the pads very lightly using gravity to flow the solder onto the pads now because of surface tension this doesn't have to be really precise but it saves time reworking the chip if you take your time now that's a bit much all right it's a bit much and some places but because this is prototyping I expect I have to do some reworking so the main thing is I want the chip to be in its correct place and that is going to happen because of the surface tension now for this chip it's not needed but some chips do need a connection on the pad so I need to put a bit of salt paste on there as well I'm gonna make some noise and I've set my hot-air gun to 400 degrees which is quite a lot but the airflow is way down I'm just gonna heat up the board I kind of want to keep my hands out of the way as well because I not want to burn myself and I have to get in view it's all quite difficult to do but let's see how it goes did you see the chip move it's actually moving into position now maybe do the other side as well so there are some sort of riches to fix but I knew that was gonna happen clip first let me check the alignment put the microscope in autofocus and as you can see this is pretty well aligned so a few things to fix back to manual focus and what I usually do is put my soloing on the first pin and then just drag along sometimes making it worse but eventually fixing it so these look okay maybe this one and a bridge over there just going to do the same thing put it on the first pin of the row or on this side and just drag across there we go oh that's still a bridge you can see that earlier clean it clean my soul my mind that's better you're the last side and there we go that's inspected one more time these look good this also look good also good although that right one it's not quite but I think it's okay and last side great now one thing I have to check is whether the pad underneath the chip is actually soldered to the board and I could measure continuity between this pad and this pad but that wouldn't tell me anything because these are already connected on the PCB now I know this FTDI chip has two other grounds one of them is on pin 3 and the other one is on pin 13 so if the bottom pad is correctly solid I'm gonna have continuity between this pad and pin 3 and pin 13 so I'm gonna do that over here that's okay and now between the ground plane and pin 13 also continuity and they'll also be continuity between these two pins now then another check you can do is check for shorts between two pins and I usually do that like this put one probe in one pin the other probe and pin next to it and I move the probe over to this pad that will have continuity of course this won't this won't and if everything's correct neither pin will be connected to the pin next to it there we go no shorts between pins and a nicely solid qfn 20 chip without using a stencil thanks for watching see you next time bye bye

Channel: Dustin Watts

Views: 48,052

Rating: 4.6789474 out of 5

Keywords: Solder paste, Syringe, stencil, PCB

Id: xPFujTJbUkI

Length: 13min 52sec (832 seconds)

Published: Sun Mar 29 2020