Computing stuff tied to the physical world

Assembling the JeeSMD

In AVR, Hardware on Jul 8, 2010 at 00:01

The JeeSMD is a kit with tiny “Surface Mounted Device” (SMD) components. SMD was designed for automated assembly with Pick & Place machines, but with a bit of patience it’s fairly easy to assemble a board by hand – see this post for an overview of what you will need.

You won’t be able to do this without at least a fine-tipped (0.4..0.6mm) soldering iron plus the following tools:

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A magnifier lamp also helps, I know I couldn’t do this without one anymore!

This is a step-by-step guide on how to assemble the SMD kit, which consists of these parts:

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The tiny ones (don’t sneeze!) are hardest to tell apart:

Jee smd Closeup

(thanks to Steve E for the macro shot – I just added some labels)

There are 2 10 kΩ resistors in there, although you only need one. That lets you get started without having to worry too much – if you mess up completely, just remove it and start over with the other one.

For a fantastic resource with detailed videos about hand-soldering SMD, see this Curious Inventor page.

So let’s get started. First thing to do is to apply flux wherever you’re going to solder things. The flux is essential because the flux in your solder wire will have evaporated longe before you can move your tip from the wire to the part.

I’m right-handed, so that’s how I hold my soldering iron. That leaves only my left hand for the tweezers – and no hand at all for the soldering wire:

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Use your tweezers for all these parts, and don’t let go – once a 0603 part flies off or drops on the floor, your chances of finding it again are next to zero. Best is to work on a clean flat surface with everything around you removed.

The trick is to place the part and then push it down while you touch it with your iron with solder already on it. The moment a part is soldered down on at least one pin, it becomes a lot easier:

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The matte “stain” you see around these pads is the flux, which has dried up but is still active.

(Remainder continued after the break…)

Ok, apply solder and add the other three 0603-sized caps. Don’t forget to add solder to both sides, of course:

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It’s not hard, it just takes some patience.

Try to get better at it, and don’t touch the parts longer than necessary. These ones will take a lot of beating, but the next ones are less happy with too much heat.

Next is the voltage regulator, it’s in a 3-pin SOT-23 package. Again, once the first pin is soldered on, the rest is relatively easy:

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Now a tricky one: the 16 MHz resonator. This is a three-pad package. I tend to put it slightly off-center, and always start with the middle pad:

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Make sure it’s properly aligned, and make sure it’s still flush against the PCB. Otherwise, re-heat from the side until everything is right. Be patient – you can fix it with just one pad soldered, you can’t fix it later.

I use the middle pad, because it’s easy to check whether the connection is good: just try to lift the part with the tweezers. If well-attached, the board will lift as well.

Now carefully apply a little solder to the other two pads. The top is a metal can, you don’t want any of the solder joints to connect to that:

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Now comes the pièce de résistance – the ATmega328 as a 32-TQFP package. Start by soldering only 2 pins on opposite sides of the chip:

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Whoops – I used too much solder and created a bridge. Help!

No problem. Ignore it. This happens all the time. Just make sure the chip is well-aligned on the pads, and don’t proceed before it is. Fix the problem now, because there will be no way to do so later. Once you’re satisfied, do all the remaining pins:

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Yep, two more solder bridges. Again, no problem.

Once all the pins have been soldered, make sure each one of them really has a solder joint to the pad. If not: add flux, and touch the pin with a drop of solder again. All that matters here is that every single pin is connected to its pad.

Now we’re going to fix the solder bridges. That’s where the solder wick comes in. Solder wick is nothing but thin copper braid, which clings to solder like crazy once hot enough. So the idea is to put a fresh & unused bit of wick on top of the pins with too much solder, and then heat the whole sandwich with the iron. At one point, the wick wil start to soak itself with solder, removing it from the pins and pads below.

This process will make the pads look completely different, almost as if there is no longer any solder, even:

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Not to worry. If the ATmega pins are flat and flush against the pads, then there wail always remain a bit of solder between them (due to capillary forces) – exactly where it matters.

Every time you use solder wick, you have to cut off the bit soaked in solder, to have a new piece ready for the next repair. Note that copper is a very good conductor – also of heat – so the wick can get very hot, everywhere.

Ok, if you’ve got this far: you’re done! Congratulations. Your board will now look something like this:

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Tomorrow, the easy final part: adding headers, programming it, and trying it out.

(Reminder: the Jee Labs shop will be closed starting July 14th. All orders before today will be sent out before then – I hope! – the rest as well, but I might run out of stock on certain items. Please get in touch to check if you don’t want to risk having to wait until the end of August)

  1. After soldering a SMD IC, I usually apply some more flux and then slowly smear across each row of pads with the soldering iron to even out the amount of solder between the pads and to remove bridges. This results in joints that I think are as close to reflowed ones as you can get with manual soldering and avoids using solder wick unless there is way too much solder on the pads of one side. But even then, this technique often allows to push out the excess solder as a pearl at one side, so that still no wick is needed.

    BTW, solder wick is more than just copper. The wires are coated with flux to prevent corrosion and increase the capilar attraction of the braid.

  2. I have never soldered smd, but it occurred to me that you might be able to glue the tiny s*ckers to the board with CA glue (“secondenlijm”), at least temporarily. this will enable you to use your left hand for other jobs.

    the trick will be to keep the CA off the soldering pads. When modelling I use a toothpick to apply the CA and that works with accuracy.

    But, as said, I have no personal experience. try it on a piece of scrap first.

  3. I’ve just done one, and it’s really not too bad. I actually bought two kits as I was sure I would destroy one, but the first kit worked perfectly, so I now have a second one to play with.

    I didn’t use any specialist tools either. The soldering iron was an 18watt Antex I’ve had for years, I just bought a smaller 0.5mm tip for it. The flux was a pot I found in the garage, which I applied with a small paint brush. It was quite thick stuff so it was probably plumbers flux! It worked great. Held the components in place nicely, and I washed it off with flux remover afterwards so I doubt it matters.

    I didn’t even have any tweezers (although I wished I did have a few time). I just pushed the parts about with a small screwdriver until they were in the right place and then held them firmly in place with the tip of a screw driver whilst I soldered them. I didn’t do that on the resistor and it tombstoned (stood up on end), which was amusing, so I learnt to hold them down when doing the first contact.

    The one thing you really must have is desoldering wick!

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