Saturday, January 28, 2012

Hand soldering SMT packages to stripboard

Prototyping a new design can be tricky enough, but it can get even more so when the part you want to use is not available in a through-hole version. I needed a p-channel FET to run the high side switching of an LED matrix and they did not exist in a sensible package, so I made one.

Now this makes me proud - i fear no package!

Hand soldered SMD transistor package on stripboard

This is a Rohm P-channel MOSFET (RTQ030P02) in a TSMT6 package. The black package body is 2.9mm by 1.6mm.

The process to make this little guy is pretty straight forward, although kinda fiddly.

Fine tip soldering iron
0.5mm / 0.02in / 28AWG solder or similar.
1.5mm / 0.6in Desoldering braid
Solder sucker (optional)
Fine point tweezers (optional)
Sharp stanley/craft knife
Steel ruler or straight edge (optional, but you'll want one if you like your fingers)
Hands free magnifying glass, or natural short sightedness.
Jewelers loupe (optional)

4x PCB header pins
2 by 5 stripboard or Veroboard(tm)
The package to be fitted


Before we start, make sure that your stripboard and PCB pins are completely free from oxides and finger grease. The easiest way I've found is to use a regular pencil rubber, plunging the pins into it and spinning them around.

First up, take the knife and, using the ruler, cut two lines through the copper across both sides of the middle most lower hole. Remove the remaining copper between these two lines. You can clearly see where this track has been removed on one side of the hole, the other side is hidden under the package. We do not have to repeat this on the upper hole as pins 1, 2, 5 and 6 are all drain pins.

Once you've done this, cover the tracks with a very thin tinning layer. Remove any excess with the sucker or braid. This will make is infinitely easier later on.

Next, I chose to solder in the PCB pins, as I then used them to secure the whole piece in a corkboard for later on. It also means that you don't risk desoldering or nudging the FET package when you're putting them in later.

Place the FET across the track and the cut hole in the right orientation. If you haven't got the tweezers any very sharp pointy thing should be fine - freshly sharpened pencil, sewing needle, etc. With the tip of the iron, melt one corner pin into place to the tinning layer while holding the package still. Once you've checked that it's in the right place, proceed to solder on the other pins, starting with the one diagonally opposite the first. Remove any excess solder with the braid.

When I did this the first time, I cycled through soldering too much, removing too much a couple of times before I had it right. Be diligent about not letting the package get too hot - just let it sit for a while between attempts and you should be fine.

Once you're done, inspect all over for any solder bridges - especially under the package itself where they may not be as visible.

Finally, if you're as petty, OCD and pedantic as I can be, you can wash it with propan-2-ol to remove any flux reside - but I'm sure you've got far better things to be doing with your time than that.

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