Introduction: How to Neatly Solder (without Loads of Wires!) Decoupling Caps on SMT Microcontrollers.

This instructable is written to teach you how to use a neat and tidy method of prototyping with SMT microcontrollers (or other devices) on an adaptor board.

After struggling to make a neat job of effectively decoupling the power pins on my PIC18F I decided something needed to be done!

This instructable shows what I did.....

This is my first instructable (I thought it was time to give something back!) so go easy on the comments ;-)


The first picture (below) shows the completed board - looks neat (apart from the burnt flux!) doesn't on!

Step 1: Lets Begin...

The first step is to solder your device onto the adaptor board as can be seen in picture below.
I find that the best way to solder these fine (ish) pitch devices is to use plenty of good flux and some leaded solder (dont tell the environment though ;-) !).

Step 2: Copper Tape and Capacitor Lead Forming.

First you need to cut a ~20mm x ~20mm piece of copper tape and stick it to the centre of the underside of the adaptor board.
Next you need to poke the capacitor legs through the adaptor board with one capacitor leg placed through the positive supply (to be decoupled) hole and the other leg placed through the ground pin connection. This is fairly easy with the PIC18F since the power and ground connections are usually close together.
Next you need to solder small pieces of solid core wire onto the ground legs of each cap as shown in the images below. These pieces of wire then need to be formed such that they overlap the copper tape in the centre. This copper tape will become our makeshift ground plane.

Step 3: Soldering the Decoupling Capacitors.

Once you have formed the solid core wire over the copper tape you need to then carefully solder the wire to the tape. This is best done quickly and with minimal solder.
The image below shows the soldered connections.

Step 4: Insulating the Negative Plane.

The next step is to deal with the positive connections.
First you need to insulate the existing copper tape 'ground plane' with some tape. I used Sellotape but i am sure there is something more suitable! Maybe Kapton tape?
Make sure that the ground is well insulated and then move on.
Next you need to stick another similar sized piece of copper tape over the insulated ground plane.
The image below shows the ground plane insulated with Sellotape.

Step 5: Add the Positive Plane.

Once you are happy that the ground plane is well insulated you should stick the second piece of copper tape onto it as shown in the images below.
Make sure you get the copper tape well stuck down!

Step 6: More Capacitor Lead Forming!

Next you need to take some more single cored wire and solder one end to the positive side of the decoupling capacitor and the other should be formed such that it is over the copper tape. This copper tape is to become our positive 'power plane'.
See images below for an idea of how it should look!

Step 7: A Bit More Soldering.

Now you need to solder these positive connections.
Again, make sure you make a neat job of the soldering and don't use too much solder.
If you look close you can see the Sellotape just poking out from the edges of the copper tape.
See image below :-)

Step 8: Connecting the Supply Wires.

Next you need to take some red and black wires and solder the black one to the ground connection on the top side of the adaptor board and the red one to an adjacent positive connection.
See image below for details.

Step 9: Making Sure You Haven't Shorted the Whole Thing Out :-)

Next you need to use a DMM (Digital Multi Meter) to make sure you have not shorted the ground connections to the positive connections. If they are shorted you are in big trouble since this connection method (with tape etc) is not easily re-workable!
The DMM (set to Ohms measurement) should show open circuit between the red and black wires if all is connected correctly.
See image below showing what a working board should show on the DMM.

Once it works you have a nice, reusable board suitable for prototyping and showing off to people without heaps of wires etc.

The finished article 'should' provide decent decoupling of the supply pins and will show some of the benefits of using a ground and power plane although the capacitance between the two copper 'plates' is likely to only be a few nF (nano-Farads) at best.

If you liked this instructable and have an interest in electronics please pop over to PCBPolice and say hello :-)

Many thanks,