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SMT assembly techniques
Building small stuff from smaller stuff

When you're ready to build a circuit using SMT parts, there are 4 different ways (that I'm aware of) to get your surface mount components actually soldered to a PCB. Mind you, I haven't done this (yet) myself, so here I'm just passing along information from folks who actually have done this.

  1. Discrete pin soldering by hand
    You'll need a needle tip on your soldering iron, SMT solder (low melting point solder in a very fine diameter), a water-soluble solder flux pen, a steady hand, and some sort of magnifier (to see what you're doing on such small things).

    Start by laying down a thin layer of solder flux on the pads. Place the chip on the pads (watch the alignment!) and gently press it in place. The flux should have just enough "stick" to it to hold the chip. Carefully solder one pin on one corner, then verify the chip's alignment. Now solder one pin on the opposite corner, then verify alignment again.

    If all is well, carefully solder each pin in order, working your way around the device. Use the iron to heat the pad, then flow the barest amount of solder from the pad under the pin. Watch for solder bridges; if you get any, use a very fine solder wick to remove the bridge.

    One version of this is as follows: lay a piece of solder across the ends of the pins (so it touches both the pins and the pads) along one side of the chip. Touch the solder with a hot iron at the end of the solder. It will melt onto the pad and pin. Without moving the solder, do the next pin, and the next, till the side is done. Each time you touch your iron down, the end of the solder will be perfectly placed for the next joint.

  2. "Painting" pins
    Use a large tip (1/8th inch wide or more) soldering iron. Stick the chip onto the layout using the flux technique given above. Tack one lead in place at each of two opposite corners, to hold the chip in position. Coat a fair amount of solder onto the iron's tip then, in one smooth stroke, paint all the leads on one edge by wiping the iron's tip down the length of the chip. Repeat for all edges.

  3. Hot air stream -- reflow "pen"
    Here, you'll need water-based solder paste, an iron or hot-plate and frying pan, and some sort of gizmo that can produce a thin stream of very hot air (you can spend about $700 US on a commercial one, or see here for a way to build one for about $20). For SO components, put a dab of solder paste on each pad. For TSSOP components, put a thin line of solder paste across each row of pads. Put the chip on the pads and push it down. Preheat the board with components on an inverted (flat side up) clothes iron, or an old frying pan on a hot-plate. Carefully "spray" the pads with the hot air to melt the solder.

  4. Toaster oven
    Here, you need water-based solder paste and a toaster oven. Note that the oven needs to be "dedicated" to this use; small bits of lead will (at least potentially) move around while you're heating boards, so you won't want to use this oven with food after using it for circuit boards. For SO components, put a dab of solder paste on each pad. For TSSOP components, put a thin line of solder paste across each row of pads. Put the chip on the pads and push it down gently. Put your board into the oven, heat it up to melt the solder, then turn off the oven and open the door. Wait for the solder to cool and solidify before you take the board out of the oven.

    This is essentially an at-home version of commercial "reflow" soldering. The Seattle Robotics Society has details on this method (oven temperature profile, pictures, etc.) here.

For more information...

You can learn how to build your own SMT reflow air pencil here.

Another neat trick for getting surface mount ICs lined up correctly is here.

For another take on working with SMT parts, I found "Steve"'s description of making a solar engine with them.

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Page author: Eric Seale
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