You can do your own Surface Mount Technology based PCB assembly with just a handful of tools and some patience. At the heart of my SMT process is stopping to inspect the various steps all while trying to maintain a bit of cleanliness in the process.
Surface mount or Surface Mount Technology (SMT) is the modern way to assemble Printed Circuit Boards (PCB) and is what is commonly seen when opening a modern piece of tech. It’s much smaller than the older Through-Hole (TH) technology where the component leads were inserted into holes in PCB, and act we called “stuffing” since we had to stuff the components into the holes.
A few specialized tools make this a lot easier, but resourceful hackers will be able to pull together a solder paste stencil jig, vacuum tweezers, and a modified toaster oven with a controller that can follow the reflow profile of the solder paste. Where you shouldn’t skimp is on the quality, age, and storage of the solder paste itself.
Join me after the break for my video overview of the process I use in my workshop, along with details of every step of my SMT assembly process.
Component Size Matters
Surface mount package names can be a little bit confusing. In the video I talk about pats that have a pin pitch of 0.5 millimeters, but when it comes to resistors, we use numbers like 0603 and 0402 which measure 0.06×0.03 inches. It’s always to good idea to have examples you can reference to ensure you’re using the right size package.
Looking at my trusty Adafruit ruler with component sizes shown I will say that I commonly do the 0603 size components on a regular basis, though I have done 0402 for RF components where less size means less things like inductance which fight you as you go up in frequency. I don’t work with the smaller 0201 components as I am afraid of inhaling them they are so small.
Pretty much all of the SOIC parts you see can be done using the methods I show in the video and I commonly go down to 0.5 mm lead spacing, though in the video I solder some chips which have 0.4 mm lead spacing, the smallest I do in my home lab.
Caring for Solder Paste
Nowadays we “place” a component on PCB’s without the need of holes for the components leads, and instead of applying solder after the fact in its molten form, we use a solder paste consisting of flux and solder. The paste can be packaged in a variety of ways but I buy my paste in syringes and I keep them in a small refrigerator and only for a limited amount of time as solder paste has a shelf life depending on the kind and where you buy it.
Solder paste is denoted by the flux that it is based on, I use “no clean” which means pretty much what it says, the end result is operable without cleaning the PCB. If I am delivering the board to someone else I typically clean it anyways which I go over in the video.
The solder is an amalgam of little beads of solder held in suspension with flux and can be the touchiest thing about DIY soldering as the solder has a definite shelf life of weeks if not days depending on the manufacturer. Simply put if you don’t remember when you purchased your solder, throw it away. I buy a solder formulated for better shelf life named Zeph paste which keeps longer than a production mix, and I store it in a small dedicated refrigerator (think six-pack cooler) and stored upright in syringes with the needle pointed down.
Solder also comes in many types, starting with leaded vs non-leaded and then variations depending on what kind of flux it contains. I use the “no clean” version which doesn’t have to be cleaned, or can be cleaned with alcohol instead of the stronger flux cleaners.
Applying the Solder
There are several ways to get the solder paste on the PCB; it can be manually applied using a syringe and good old fashion squeezing to deposit the solder onto the connector pads of the PCB. I also have an air pressure operated dispenser that I don’t really use.
My go-to method is to use a paste mask stencil and squeegee the paste onto the PCB as seen in the video. There is a trick to squeegeeing the paste to not press too hard while forcing the paste through the paste mask screen. Paste mask stencils are available for a few dollars from the same sources I buy my PCB’s at, for larger runs I would look at companies like OSH Stencil.
Inspect Your Paste, then Place Components
I always inspect the PCB before placing the components on the board and I use a stereo X10 microscope for this step. What I am looking for is anything that would create a solder short later, I.E. if its shorted now it will most likely be shorted later.
The trick is to place components on the board, kind of snuggled into the solder paste really, without smearing the paste or bending the component leads. Tweezers work for smaller components like resistors and capacitors but often for Integrated Circuits (ICs) one must be careful to lay the package flat. I have a variety of ways to do this using suction including the little suction cup with a squeeze bulb for a fully manual method or when doing a lot I might be inspired to fire up my little suction powered parts picker-upper made out of an old aquarium pump.
At Last, the Oven
I have tried a variety of reflow ovens including the T-962 (complete with mods per the Hackaday articles on the subject) but always found it wanting, especially with tall, dark components as the radiation patter inside the oven is somewhat uneven.
In order to make a better small scale oven I started with a Black & Decker convection oven and bought the Controleo3 oven controller which comes with an auxiliary heating element to boost performance. After applying lots of reflective tape and high temp RTV the oven is sealed against as much heat loss as feasible. The oven controller takes an hour for it to learn the oven’s characteristics but once learned I got really good and repeatable results.
An important consideration is the Solder Profile which is a chart of heat vs. time. It is pretty much the opposite of just sticking the boards in a hot oven until the solder melts, which is pretty much guaranteed to just bake off the flux leaving little clumps of solder as opposed to the smooth reflow that we want to achieve. The Solder Profile takes care of various phases such as preheat and rapid cooling in addition to the actual reflow stage.
I installed an additional heating element on my oven. The Controleo3 calibrates itself to the oven and even the size of what the oven is loaded with. I re-train the oven if doing large PCB’s using a blank board.
True to my nature I slap the final soldered item under the microscope one last time and look carefully at each lead noting any solder bridges or voids in the process. I am also looking for the solder to have melted thoroughly and not look “grapey” or where the solder may have coalesced into tiny clumps instead of a properly whetted solder joint.
I like to give the boards a trip through my ultrasonic cleaner that I keep just for PCBs. While not usually necessary it gives the boards a nice professional look and makes inspection easier.
Hopefully one can see that doing DIY SMT assembly is certainly possible and can even be done for small scale production. In my case I can solder components down to 0.4mm lead pitch which is pretty small as long as I do ample inspection. Keep it clean, keep your solder paste fresh, and inspect at every step and you’ll get great results in your own workshop.
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