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Electronics Design and Construction

Surface Mount Devices

I have been using SMD components at my place of work for a few years now and have decided to begin using them at home for some of my projects. Smd resistors and capacitors and many other parts are available from RS Components or Farnell in smaller quantities with quantities of 50 0805 resistors costing not much different than the same quantity of similar valued leaded parts from your local electronics store. The use of surface mount parts reduces the number of holes required in home made Printed Circuit Boards which is great because "I hate drilling circuit boards!".

In my experiance SO packeged IC's can easially be fitted to home made PCB's but the finer pitch devices may cause some difficulty especially since home made PCB's don't include solder masks.

One thing to be aware about is that many SMD components don't have their full value on them, some have a very short code that must be looked up in a data book and others (especially capacitors) don't have anything at all to tell you what value they are. Because of this if you buy any SMD parts ensure you keep them in a labled container or better yet if they are supplied in tape write on the back of the tape their value.

Buy a few extras as these tiny components have a habbit of flicking out of your tweesers only to be lost forever!

Useful tools for SMD work

• fine pointed stainless tweezers
• 0.5mm solder (flux cored), you can get by with 0.7mm but the finer stuff is better.
• fine pointed temperatur controlled soldering iron
• liquid flux in a syringe or pen
• thin desoldering wick.
• good lighting
• small x6 or greater handheld magnifier
• magnifying lamp,desk mounted (useful but not necessary).

For prototyping designes I have found it is possible to slice the tracks on vero board lengthwise to allow mounting of a small outline ic (SOIC) on veroboard. fine wire such as that sold for wirewrapping circuit boards is then used to connect the pins and their half tracks out to other components. This method works ok provided you don't overheat the tracksthat you cut in half as they come away from the substrate much easier than a whole track. The higher quality prototype boards based on fiberglass substrates with tinned tracks are a little more robust in this respect.