Circle On Reader Service Card

Other than that, all you need to worry about is a reasonable amount of clear space, and places for tools and components. If you are using a space that has other purposes (i.e.* your kitchen tabic) it's easy enough to keep your tools and components in trays so they can be easily put aside when you arc not working.

Soldering 101

Entire magazine articles, even books, have been written about soldering. So how can I hope to teach you to solder with a few paragraphs and illustrations? Easy, Soldering is not difficult, and the basics are easily within your grasp if you have the right soldering iron, the right solder, and a little hii of practice.

Practice is important, so if you are new to soldering, please take the time to do some before we start on the project! You can practice on any old components and a bit of scrap circuit board material or skip ahead to unsoldering, remove a couple of components from a junk circuit board and resolder them. Kit suppliers w ill tell you that 90% of all problems in kit building are a result of poor soldering. How can that be, if soldering is not difficult?

Simple ... carelessness and ignorance. We'll fix the ignorance problem right now—carelessness is up to you,

Soldering is a process of amalgamating metals to provide a good electrical connection. Solder is a mixture (alloy) of two or more metals with a relatively low melting point, that will How onto the surface of other metals creating a low-resistance electrical connection. Ordinary solder is not very strong, and you should never rely on solder alone to hold components together physically. The mechanical connection should be secure before you apply solder, and the parts should not be able to move in relation to each other The flux is vaporized by the heat of the iron and the vapors will clean the surfaces of any oxidation (often invisible to the naked eye), allowing Lhe solder to flow freely onto the metal surfaces.

The purpose of the soldering iron is to transfer heat into the work to be soldered; [he solder should melt upon contact with the work. The iron must be at the correct temperature to do this, and some elementary principles of thermodynamics are involved here. Fortunately. 24 73 Amateur Radio Today * November 1997

we dpn't have to worry about the details too much—a 15-30 W iron will heai up to an appropriate temperature and won t get too hot under ordinary circumstances, Rut let's look at the basics anyhow, because they will help you to understand what is going on, and also influence your decision to buy a temperature-controlled soldering iron later!

The wattage of an iron is a measure of the power that is used to generate heal. Your soldering pencil is always running at that level of power consumption, and it is always generating heat. The tip has a specific mass which can absorb heat. As long as power is supplied, it will continue to gel hotter until it rcachcs equilibrium at its maximum temperature. Heat Mil be conducted away from it (into the surrounding air) as quickly as it is generated by the applied power. Heat will transfer out of the up more quickly when it is in contact with the work—the rate at which that occurs will depend on the size and shape of the tip, the amount of its surface that is in contact with the work, and the nature of the work (how quickly heat is conductcd away from the point of contact). When your soldering pencil is sitting idle it very definitely gets much hotter than required for soldering, but its heat drops almost instantaneously when you apply it to the work, and the applied power sustains the working temperature. When it's idle, though, at higher temperatures, its surface is much more susceptible to corrosion. So turn it off when you are not actually soldering (for more than five minutes or so). Otherwise, you can expect to replace or refinish the tip fairly frequently. Leaving it on overnight once will ruin the tip. Once the tip has been overheated and cannot be tinned (see below) you can file or grind it down and start over, but it is usually a lot easier just to replace it.

Ail else being equal, the wattage of an iron is a poor indicator of its performance because its main effect is in how quickly the iron will heat up to its maximum equilibrium temperature, or how fast it will create new heal for transfer into the work—not necessarily how hot that temperature will be! That's why the best irons, if somewhat more expensive, are temperature-controlled and not "variable output/' I finally worked that out for myself after going through perhaps a hundred soldering iron ¡ips.

From this point on, Pll be talking about soldering components onto a printed circuit board, but the principles apply to other soldering such as wire connections to controls.

Allow the iron to heat until solder flows freely on the tip, "tinning" it. This means there should be a thin, shiny coating of solder on the working surface of the tip—it should not "ball upn" and drop off. Apply a small amount of solder to the tip and then wipe it off quickly with a soft cloth or a damp sponge. You can probably do three or four joints in immediate succession without having to repeat this tinning process, but if you stop soldering to place components on the board you will need to repeat it.

Here are the steps in soldering a component into a circuit board.

1. Inspect the board and the component leads, and make sure they arc clean, Older components may be oxidized and require cleaning (use fine sandpaper, or scrape with the edge of your hobby knife). Most circuit boards do not require cleaning before use, but it can't hurt. Wash the board with soap and water, and use a mild abrasive (Scotch™ scouring pad, for example) or metal polish only if absolutely neccssary, The surface of the tracks should be shiny and tree of smudges and fingerprints. Some builders (and kit suppliers) wiii recommend cleaning a board before use and completing it in one session, but I have never found this to be necessary—that's why there's flux in the solder!

2, Mechanically install the component. Use your long-nose pliers to bend the component's leads so thai they will go straight into the holes in the board, If the spacing permits, hold the lead with lhe pliers and bend the end of the lead against the jaws of lhe pliers. Otherwise, watch what you arc doing and make sure you are not exerting excessive pulling force on the lead—you can easily ruin a diode or inductor by pulling on the lead. Check the value before you insert it in the board. If it is a polarized component such as an electrolytic capacitor double check the orientation. If the component isn't polarized (for example, a resistor or ceramic capacitor) then it doesn't matter which way it goes, but it's a good idea to mount it so that you will be able to read the value later. I usually put resistors in wjih the tolerance band to the right or bottom depending on how the resistor is mounted, and capacitors with the value facing me or to the right (unless they are very close to a larger component, in which case I turn them around), The aim is simply to make it easy to see and verify all of the component values after the board is complete. Before you Mulder it, recheck the value, the orientation, and that it is in the right holes! In most cases, the body of the component should be snug against the component side (opposite from the "track" or soldering side) of the boards. Obvious exceptions arc transistors and other components which might run hot. Ltxiking at the solder side of the board, bend the leads outw ard at about 45 degrees to hold the component in place.

3. Inspect the unsoldered connection. Make sure you know where solder is supposed to go. For example, if there is a pad for another component very close to where you are going to solder, you can memorize the pad layout and be sure that there is no unwanted snider bridge when you finish the connection. If you don't do this, ii s often hard to tell whether two points should he connected or not. Examine Fig, 3 lor an illustration of this.

4. Solder the connection. Tin and wipe the tip of the iron as described above. Apply the lip to one side of the pad. wedging the tip against the lead where it protrudes from the hole, as shown in Fig* I. Count to three and apply solder to the opposite side of the pad. and it should flow across the pad. around the lead, and slightly up the lead from the surface of (he board. Do mt apply the solder to the tip of the iron, as it w ill melt instantly and may flow onto the joint without bonding properly. Fig. 2 shows a good joint and a bad joint. The bad joint is often called a * cold" joint because it is most often caused by inadequate heating of the joint It doesn't just look ugly; If I can coin u new phrase here, it's "electrically ugly/ offering no electrical connection between the two surfaces, or a weak one which is bound to fail, or (worst of all) an intermittent fault.

5. Inspect the soldered connection. Use a magnifying device of some kind, ideally 5-1 Ox pow er, and make sure the


Fig. 1. Placing the soldering iron on the work.

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