I used to fear surface mount components... those damn teeny tiny sexy components that were too good for my weak soldering foo. Through hole components worked just fine for my low speed microcontroller circuits until one of my projects started to take off and we were looking at assembling a bunch of them. The temptation of soldering a bunch of boards all at once finally seduced me to start the journey towards SMD . This blog post will help you start cooking small batches of PCBs and making your own low cost solder paste stencil.
Someday, I'll write a more advanced post covering the large improvements that I've made to my process since I started, but it may be out of reach of those getting started due to cost and complexity.
Assembling a through hole board you may follow a simplified process like this:
- grab component(s), insert into PCB, and cut leads to length
- somehow fix components to PCB (tape/bend leads/jig) so they don't fall out when you flip the board over to solder
- flip board over
- clean/tin soldering iron and solder joints
- repeat above until finished for this board
- repeat above for next board
For a board with lots of components, there is a lot of wasted time (especially if you work for NASA!!!). With SMD technology, you apply the solder paste first, then all of the components, and finally, solder a bunch of boards all at once which can save lots of time.
One of the other large benefits of switching to SMD that I didn't anticipate, was the fact that it makes PCB layout much easier; you have many more options for running traces on double sided boards because the board isn't peppered with through holes for component leads. This allows for even more compact PCBs.
Sparkfun has a lot of SMD soldering tutorials, and I decided to try and follow the skillet approach where you essentially cook your PCB on a skillet or hot plate. One important thing to note is that all the soldering Sparkfun tutorials I've read assume you are using toxic leaded solder and just not licking your hands :P Lead free solder melts at a higher temperature and makes the job quite a bit harder (discussed below). I only use lead free solder so keep that in mind while reading on.
If you are just getting started, consider using low temp lead free solder (more brittle/weak) or stick with the leaded stuff until you get the hang of things. It will make your life easier.
SMD soldering is much easier when you have a stencil to apply the solder paste on the board. You can use a syringe like this one to directly squeeze out some paste on individual pads, but you'll likely end up with way too much solder on each pad (the paste sticks like goo to the pad) and have numerous solder bridges on SOIC parts to fix. Finer pitch devices may end up being one big bridge you'll need to fix.
TIP! Buy a flux pen or dispenser. It will make fixing solder bridges ONE MILLION AND FIVE PERCENT EASIER! I wish I had learned that trick many years ago. I've since saved numerous friends from licking 240V in an attempt to end their rework woes.
If you don't have many components to solder, you might consider just hand soldering the devices as making a stencil can take some time. Ideally, we would all have access to a laser cutter to make stainless steel solder paste stencils, but the world is a cruel, unjust place. You can also order laser cut plastic stencils for about $80 shipped to Canada, but for small runs, I cut mine out of overhead transparency sheets.
Step 1: make the stencil
Print out your solder paste layer on some paper and tape it to the transparency sheet. You can then use a sharp exacto knife to cut out the large pads. I use a ruler to cut out a strip that runs along the pads for an IC instead of trying to cut out each individual pad which is impossible on a fine pitch chip like a SSOP. The key is to make your strip the correct width that it covers roughly the same area your solder paste pad footprint covers.
I tend to use mainly 1206 and 0805 sized SMD components because they are easier to work with. The larger pads also means that I can use a drill bit to cut out the pads. It isn't a perfect fit, but it is good enough for protyping with large 0805+ components.
Drilling out the pads saves a lot of time and precision cutting, but creates its own problems as shown below.
Each hole has a bit of plastic pushed through on the other side that needs to be cut off so that the stencil will lay flat.
I flip over the stencil and use a sharp exacto knife to very carefully trim off the excess plastic. Have to be patient here. It is easy to accidentally slice open a hole. I've tried different metal/sanding dremel bits to remove the excess plastic, but nothing worked very well. I imagine there is a fancy drill bit out there that would do a better job.
Step 2: apply the solder paste
The Sparkfun tutorials cover this part well, but I make a little jig so that I only need to line up the stencil once and then tape down one edge so the stencil is like a hinge. When I lay the stencil down over a PCB, I also gently pull the stencil taut and tape down the opposing edge to help keep it down.
Put out a good amount of solder paste, and wipe it across. Other tutorials do a good job explaining this part so I won't go into too much detail. It takes some practice to get it correct, but even mild smearing of the paste will be OK for lots of parts. Below are some results from early prototype boards that I stopped to take pictures of.
Notice how the solder paste tapers off on the top left diode in the above picture? In that case, I didn't have enough paste on my squeegee as I passed over the diode. However, there is still lots of solder on that pad so it will reflow fine. Once you have applied the solder (ideally in your first wipe), you will need to run your squeegee back over the stencil scraping off the excess so you don't leave too much on each pad.
Here's a birds eye shot showing my first ever attempt at making a stencil for a TSOP package. A few mistakes here. (1) The biggest mistake is that I cut my stencil strip a bit too wide so too much paste gets applied. My memory is hazy, but I believe this particular paste application resulted in a solder bridge in the middle of the bottom row and at the right of the top row. I've since learned to use a small tool or wire to carefully remove excess solder paste when I apply too much now; it is easier to remove excess paste than to add extra. (2) I didn't hold the squeegee properly to apply even pressure so the bottom right pad of the TSOP didn't get enough solder, but it still soldered fine. (3) the paste extends to the outsides of the pads. This isn't really a problem for reflow, but it makes lining up a placed component much more difficult as you can't see the pads. In this case, I had intentionally put the paste here because I had to solder a SSOP package to a TSOP footprint which just barely fits.
Step 3: place the components
Get a good set of ESD tweezers on ebay. You'll want a straight set for larger ICs and a curved set for smaller components. I experimented with various techniques for holding the component's tape down, but I'll have to cover that later. You can always use double sided tape to hold them down in a pinch. If you don't hold them down with some method, you'll find that the tiny components tend to bounce around when you are picking them out with tweezers.
Step 4: bake them boards!
Here's an example of my initial hot plate setup. I've since moved to convection ovens.
Might just change your mind on the skillet method.