Nuts and Volts - January 2009

■ A chip once cut for removal.

■ MOB controller for OSMC boards (example of surface mount and through-hole parts).

http://store.nutsvolts.com. In the June 2008 issue, there is an article on a controller you can add to a regular toaster oven.

The other method uses a regular soldering iron and is what we'll focus on here. There are two slightly different variations to this method which both provide excellent results. The first only applies if the PCB (printed circuit board) was made with a reflow solder finish. (This is where the board already has a thin layer of solder applied by the PCB manufacturer on the pads.) In this case, you just need to heat each pin and pad so that the solder from the pad will stick to the lead of the part. When using this method, you should have a nicely tinned tip (important for heat transfer) but not any excess solder. I usually only use this when installing very fine pitch parts like the TQFP devices. If the board doesn't have any extra solder, then you'll have to do more traditional soldering.

NOTE: During assembly, make sure you keep the soldering iron tip clean. Wipe it often on a wet sponge or cloth then apply a little solder to the tip. This will protect the tip and enable you to make good connections. When the solder tends to "ball" or does not stick to the tip, the tip needs to be cleaned and re-tinned.

■ Boards with pads to accept surface mount or through-hole parts.

Do not rush! After perfecting the techniques,

you'll naturally get faster along the way. A good surface mount solder connection forms the electrical connection between two parts, such as a component lead and a circuit board foil. With surface mount parts, it also provides the mechanical connection, as well. Make sure there is enough solder to leave a nice fillet between the component lead and the pad.

NOTE: Before installing any components, plan ahead and think about the final placement of each part and what the final board will look like. With that in mind, it should help with the assembly order so that each component will not block the next one installed.

The alignment of the parts is critical. Small components like the 1206 capacitors and resistors are more forgiving and are good to start out with. Others like a TQFP microcontroller are absolutely critical in regards to placement with their fine pitch leads. Before installing a surface mount component, apply a small amount of solder to one (and only one) of the pads on the PCB where the part is going to be installed. Then, while warming the solder on that pad use the tweezers to set the part in place. If the alignment isn't right, you can do adjustments while the solder is still molten. If it takes too long, let the solder cool to ensure the part is not damaged by excessive heat. Once cool, the joint can be warmed up again and the alignment can be adjusted until it is perfect. Before soldering any other joints on the part, use the magnifying glass to verify that the alignment is okay. At the moment, the exact orientation of the part in relation to the pads is what we're concerned with. Next, go ahead and solder an opposing lead on the part. As long as everything looks good, continue soldering each one.