I've always wanted to be able to use surface mount components in my projects, but like most hobbyists, I found it to intimidating and expensive. How do you get those tiny little parts on the board? More and more components these days are only available in a surface mount form factor, and are usually cheaper than their through-hole counterparts. This little page hopes to demystify the process and open up a whole new set of possibilities for the DIY engineer.
The first real intimidating thing about doing a surface mount project, after you have the boards made, is the assembly. How the heck do you solder a resister about the size of a grain of rice? How do you solder those fine pitch leads on a 44TQFP micro-controller? A little research with google will illustrate how it's done professionally, and that it's probably too expensive to do it that way yourself. A low end hot air rework iron is around $300 and a IR solder reflow oven is around $10,000. Ouch.
These tools are all well and good if you need to assemble a few 1000 boards, but if you're only going to do 1 to 10 then the following will probably work just fine. I've built around 30 boards with these tools, and while some required a little touch up afterwards, they have all came out beautifully in the end.
The soldering paste comes in a little 35gm syringe with a cap on it. No tip or plunger is included, you have to order these separately. The solder paste has a short shelf life and doesn't like to stay warm for too long. The paste is tiny beads of solder suspended in flux and water, when cool it has the consistency of toothpaste. At room temperature it will become more fluid, and will eventually separate. Because of this you need to order it 2nd day air or faster, and then place it in your refrigerator when you're not using it. Don't put it in the freezer, bad things will happen.
Be sure to let the syringe return to room temperature before soldering. The solder doesn't flow smoothly if it's too cold. I broke the threads on a tube once trying to force the solder out.
| It's probably a good time to mention that your going to be dealing with extreme heat, poisonous substances, and dangerous vapors. Don't lick your fingers, don't put your hand on the hot burner, and don't use your significant other's toaster oven from the kitchen. You really don't want to eat the baked on residue from solder paste. |
I've been told the solder paste will keep for about 6 months refrigerated, I've had mine for a about a year now and it's still useable. A 35gm syringe will go a long, long way. If you have a group of people you work with I suggest getting one for the group at large. If you use up one syringe on a project it better be a motherboard!
The fluid dispenser tips will screw into the top of the syringe. I don't bother removing it once it's on. I just stand the syringe on end without the plunger when I need to set it down. You want to use the smallest tip you can find. I use 0.016" tips. Unfortunately they come in boxes of 50, which is probably enough to last a lifetime.
Let's call the paste application method I'm going to describe the Syringe Method. There is also the Stencil Method for high precision and the Hand Soldered Method, which is a royal pain. For the syringe method there is a certain class of parts you can easily work with, and a certain class of parts you should avoid unless you've got robotic hands and x-ray vision.
I tend to stick to 0.4mm pitch parts and larger. The limiting factor for how small a part you can work with is directly related to how little solder you can apply, (and to some degree how evenly). For finer pitch parts or boards with a high part count use the Stencil Method.
For resistors, capacitors, diodes, and other descrete components we measure the parts by their width and length. The sides are measured in 1/100ths of an inch. I use 1206 resistors in my projects, which means that the resistors are 0.12x0.06" in size (3x1.5mm).
to be continued...
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-- Mike Estee.