Hotplate Reflow Technique
See also Toaster Oven Reflow Technique.
Soldering SMT parts is simple and easy. Its actually quite forgiving, and is very accessible, even for a beginner! You don't even need any fancy tools. Its quite possible to get by with things you already have in your house. There are some tools that make it simpler, but they are not required. The only thing that you absolutely need is a syringe of solder paste, some decent tweezers, and a hotplate. A magnifiying glass can be very helpful, but if you have good eyes, you can easily get by without one.
For more info on what tools you need to do SMT soldering, check out our SMT Toolkit Guide.
The basic process for soldering a board that has SMT parts is this:
- Apply Solder Paste to all the SMT pads.
- Place all SMT components in the appropriate place.
- Heat up the entire board to melt the solder and solder all parts simulaneously.
- After the board has cooled, fix any solder bridges.
- Solder in the remaining through-hole parts, if any.
Applying Solder Paste
Solder Stencil<div class="thumb tright">
There are a few different techniques for applying solder paste. If you are doing runs of 5+ boards, you will probably want to make a solder paste stencil. This is simply a stencil made from Mylar or Kapton that you lay over the board, wipe solder paste over, and then remove. It makes the solder paste application process quick and painless. Making these stencils can be somewhat pricey or difficult if you do not have access to a lasercutter.
More info on using a solder stencil forthcoming once we have some experience with it.
Solder Paste Syringe<div class="thumb tright">
The simple way for small quantities of boards, such as making a kit or a prototype is to use a syringe of solder paste. This allows you to apply the solder directly to the pads where you want it, and its cheap too. This is the method that we use for constructing our prototypes as well and for making small amounts of kits. The hardest part is applying the paste to pads for chips like SOIC and TQFP. However, it's easy to fix if you put too much solder paste on the pads.
Also, if you do manage to completely mess up and apply too much solder to a pad, you can easily wipe it off with a towel or swab of some sort. Applying the solder paste is completely reversible and if you want to try it a few times just to practice, then go for it.
The basic technique for applying the solder with a syringe is to press down on the syringe plunger and dab the solder onto the pad. You will want to practice this a few times, but you'll quickly get the hang of it.
A large number of SMD parts have fairly big pads, especially the SMD part that we use for RepRap. Resistors, capacitors, LEDs, 7805s, etc. have large pads. Simply apply solder to those pads and you're good. The key is to put a bead of solder paste, but not to flood the pad. If you put too much solder paste down, you'll end up with solder balls. These are not fatal, but could theoretically cause you problems, so its best to put down the right amount in the first place.
The trickier pads to apply paste to are the chips. SMD chips come in a variety of sizes, and we try to use only the large ones to make it easy on all of us. The big ones are things such as SOIC, which we use wherever possible. There are smaller chips which come in a package called TQFP. This package is pretty small, but its the only SMT package that many chips come in, such at atmega chips.
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For SOIC chips, you can generally just apply the paste directly with the syringe. If you're good with the solder paste, you can lay a thin line of the paste on the pad, and do that for every pad. This is the recommended way, but it can be tricky. If you want to cheat, simply lay a bead of solder paste along the entire line of pads. Don't put too much, otherwise you'll definitely get solder bridges. You really don't need too much solder paste to make a good solder connection.
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For TQFP chips and other fine-pitch components, its almost impossible to lay down individual beads, so you have to switch to a different strategy. Lay down a bead of solder paste along all of the pads, and then use your finger to smear it so that it is just a thin layer. You will want to clean up the excess outside the pads with a Qtip or something as well. Most likely you'll get a few solder bridges no matter what, so just assume that it will happen and move on.
Place the Components<div class="thumb tright">
Placing the components is pretty easy, but it can be intimidating at first. Don't worry... those little parts won't bite you. Just try and relax and pretend what you're doing is normal, because after doing it a couple times, it will be!! Keep in mind that if you mess up on the placement of a part, its easy to try again. Nothing you've done yet is permanent, so just stay chilled out and it will go well for you. Frustration is the enemy, so keep a smile on your face... you're part of the future now.
You'll want to place most parts with some tweezers. If you spend money on any part of your setup, get a good pair of tweezers. Seriously, it will make a huge difference. A nice pair has thin, pointy tip. I like the ones with curved tips, because they make it easier to position the parts.
The most important, and trickiest part about placing SMT parts is getting them out of their package. No joke. Most of the components come pre-packaged in strips. There is a thin plastic film that keeps them in their package. Carefully, and with your tweezers you'll want to peel back the plastic filament and then carefully tap them out onto your work area. Be very careful! If you are not gentle, then you may send parts flying across the room. They're very small, so its easy to use too much force and send them flying. Just take it easy, and peel the plastic off slowly.
Once they're out on the work area, just follow the build instructions and place them in the appropriate places. The solder paste will help them stay where you put them, which is handy. Alignment is not super critical, especially on simple parts like resistors. When the solder melts, the surface tension will pull it into the proper alignment automatically. Your job is to get it close enough so that the solder can finish the job.
For more complicated parts like chips and such, you need to get it as accurate as possible, because its a heavier part with smaller tolerances. When placing a chip, the easiest way is to do an initial placement that is mostly good, and then to nudge it into the exact position you want it. This way, you'll be able to take your time and get it right.
If your hands are shaky, there are a few things you can do to make it better.
- Use your other hand to steady it. It really works well. To use some RepRap terminology: Use your main hand as the XY positioning, and use your second hand as the Z positioning. It gives you better accuracy.
- Get some magnification. An interesting fact about your brain is that the more feedback it gets (say, from a magnified image) the better it can do fine motor control. Using magnification will help you place components easier.
- Stop drinking so much caffeine! ;)
- Take a deep breath, calm down, etc. RepRap is a fun project, so take your time and enjoy it.
Solder the Board<div class="thumb tright">
Once you've applied solder paste and then placed all the components, its time to actually solder this beast! This process is pretty magical. Your solder paste will go from dull gray to shiny silver in front of your eyes. It's really fun, and a cool process to watch.
The basic idea is that you put the circuit board on something (like a hot plate or a skillet) and then heat it up until the solder melts. Then you let it cool off and its done! I use a hot-plate because I don't have much space and they are cheap enough to use. You will definitely want to use something that will NOT later be used to [cook food]. Soldering + Food = Death / Sickness. Avoid it at all costs. Mark your hot plate as soldering only so nobody else accidentally uses it for food.
Now, most hot plates get REALLY hot. You don't want it to get that hot. You want it to get just hot enough to melt the solder, but not much more than that to avoid damaging chips and melting the epoxy that bonds the circuit boards. This is around 180-190C depending on your solder. Its a good idea to measure your hot plate to determine what setting generates this heat. You can even use the thermistor or thermocouple sensing circuit that RepRap has developed to make it happen. On my hot plate, this setting is 'Warm', so most likely yours will be a low setting too. Its best to start with temperatures that are low and gradually work your way up, because high temperatures CAN and WILL destroy things. Burning circuit boards smell REALLY bad.
Once you know the right temperature, put your board onto a cold/off hot plate. Turn the dial up to the right temperature, and wait. The solder paste will dry out and eventually it will 'pop' and turn into liquid solder. You will want to keep a close eye on this process. Sometimes resistors will stick together, so you need to be ready with tweezers to push them apart, if possible. You may also want to rotate the PCB during heating to get an even cook. It is normal for the solder to let off a bit of smoke as the flux burns out, but keep an eye out if it starts to give off lots of smoke that really smells bad. That means it is burning and you'll want to kill the heat right away.
After all the things have soldered, kill the heat. I usually then slide the pcb off the hot plate onto a paper plate or other flat surface to cool down. You may want to leave it on the hot plate. Its all personal preference. Be careful, because it does take it 5-10 minutes to completely cool down. Don't suddenly move / drop / hit the pcb when its cooling down as the solder may still be molten and you may knock components off. That is bad, as you'll have to do stuff over again.
Fix Solder Bridges<div class="thumb tright">
A solder bridge is when solder connects two pins that are next to each other that are not supposed to be connected. They are a fact of life when dealing with tiny components and soldering them by hand is solder bridges. They're easy to fix, so don't sweat it if your board ends up with ten of them.
The first step is identification. Magnification can definitely help with this, but its not crucial. I regularly spot solder bridges on TQFP parts with just my normal human eyeballs. The easiest way is to hold the board up to eye level so that you're looking across the 'plane' of the board at the leads. This allows you to really see between them, with the black of the chip as the background. Having good, strong light really makes this much easier. Looking from the top and the sides is also good to. Basically just look over every single component, focusing on the chips, and especially focusing on the TQFP parts like atmega chips. Another good technique to spot solder bridges is to use a digital camera with a nice macro mode to take photos of the leads. Sometimes you can spot bridges that would otherwise have been hard.
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Once you've identified the solder bridges, you'll want to remove them. This is also really easy. You'll need a soldering iron with a decent tip, and some desoldering braid / desoldering wick. Trim the tip of the wick so you have clean, virgin copper. Place the tip of the solder wick on the solder bridge and then gently press your soldering iron on it. Wait a few seconds and you'll see the solder start to move up the braid. Now remove your soldering iron and wick, and inspect where the bridge used to be. Hopefully it is now gone! If it's not, simply rinse and repeat. Make sure you always start from the tip of the wick (don't start in the middle!!) and always trim off the used portion of the wick.
The vast majority of the time, you'll remove the bridge, but there will still be the right amount of solder for a solid connection. In the rare case that you take up too much solder, simply get some more paste, or some more normal solder wire and add a tiny amount of solder to the component lead. Of course, if you add too much, then you just use the solder wick to fix it again. Don't do it too much, as it will get messy on your board.
Solder in Remaining Parts
Now that the SMT soldering is done, simply solder the rest of the components in however the instructions tell you to. Since you're now a soldering pro, this should be a piece of cake for you. If you need some help with that, check out our soldering guide for some pointers.