What you are expected to learn

At the end of this module, you should be able to:

• Identify the purpose of different types of soldering irons and tips.

• Describe proper soldering iron tip care.

• Describe how to solder components on an etched printed circuit board.

• Identify the appearance of a properly soldered connection.

• Understand and observe all safety precautions associated with the fabricating, assembly, and soldering of a printed circuit board.

• Discuss the importance of electrostatic discharge (ESD).

Soldering Precautions

When soldering, always observe the following precautions:

1. Always assume the soldering iron is hot. Never touch the tip to see if it is hot.

2. Always place the soldering iron in its holder when idling.

3. Never shake excess solder off the tip; wipe it on a damp sponge or approved tip cleaner.

4. Never pass a soldering iron to another person; place it in the holder and let the other person take it from there.

5. Never solder on a circuit that has power applied to it.

6. Always use a grounded-tip soldering iron.

Solder and Soldering Iron

Soldering is the joining of two pieces of metal alloy having a melting point below 800° Fahrenheit. Solder includes a combination of tin and lead. Tin-lead solder ranges from pure tin to pure lead and all proportions in between. In electrical soldering, the alloy mix is usually 60% tin and 40% lead (60/40) or 63% tin and 37% lead (63/37) (Figure 39–7). For soldering surface mounted devices, a solder is available with 62% tin, 28% lead, and 2% silver (62/36/2).

Characteristics of alloys of tin and lead are plotted against temperature. An alloy of 63/37 has a eutectic point (a single melting point). All other combinations start melting at one temperature, pass through a plastic stage, and then become a liquid at a higher temperature. Any physical movement of the components being soldered while the solder is in the plastic stage will result in a fractured solder connection. Such a connection will appear dull and grainy, and is mechanically weaker and less reliable. Therefore, 63/37 or 60/40 solder is commonly used in electronics because it does not remain in the plastic stage very long.

The Ten Soldering Commandments

In truth, the Ten Soldering Commandments are more like guidelines than commandments. But if you heed them, things go well with your solder joints, your children’s solder joints and your children’s children’s solder joints. So let it be written; so let it be done:

I. Thou shalt wear eye protection whenever thou solderest, lest thy get molten solder in thine eye.

II. Thou shalt not touch the heated end of thy soldering iron, lest thy burn thyself.

III. Thou shalt not fashion molten solder into false globs.

IV. Thou shalt wash thy hands after thou solderest, to remove vile contamination from upon thy hands before thou eatest.

V. Thou shalt provide bright illumination upon thine objects which thou solderest, that thou might see clearly the way unto which the solder may be applied.

VI. Thou shalt not spill thy excess solder upon thy neighbour’s pad lest thy create unintended pathways through which current may flow.

VII. Thou shalt not leave thine hot soldering iron unattended.

VIII. Thou shalt not covet thy neighbour’s professional-grade temperature-controlled soldering station.

IX. Thou shalt not apply solder directly upon thine soldering iron, but shalt instead apply solder to the objects which thou solderest, that their heat may melteth thy solder.

X. Thou shalt always place thine hot soldering iron in a suitable holder.

• The soldering iron is the primary tool for installing, removing, and replacing components on a printed circuit board.

• Soldering guns should not be used on printed circuit boards.

• A 25–35 W soldering iron is recommended for printed circuit board work.

• The pencil soldering iron is the least expensive type of iron.

• A temperature-controlled soldering station accurately maintains the tip temperature when soldering.

• Iron-clad tips are the best selection for soldering on a printed circuit board.

• A bevel tip is best for general printed circuit board work.

• Tinning is the process of applying solder to the soldering iron tip.

• Idling refers to placing a soldering iron in a stand to remove excess heat.

• Use a systematic method for installing components on a printed circuit board, such as installing all resistors first, then capacitors, then ICs, and so on.

• Use leg warmers on LEDs and small transistors to avoid overheating.

• There is no standard color code for wiring discrete components to a printed circuit board.

• The basic goal of soldering is to electrically and mechanically join circuit components to the printed circuit board.

• In electrical soldering, an alloy mix of 60% tin and 40% lead (60/40) or 63% tin and 37% lead (63/37) is used.

• Soldering surface mounted devices requires a solder with 62% tin, 28% lead, and 2% silver (62/36/2).

• Flux helps the solder alloy flow around the connections by cleaning the component leads of oxide and film,

allowing the solder to adhere.

• A pencil soldering iron is the primary tool used in hand soldering.

• A properly soldered connection will be bright and shiny.

• Soldering on a printed circuit board requires careful attention. Too much heat with too much pressure can cause a pad to lift from the board substrate.

• During the soldering process a flux residue forms on the printed circuit board. Flux should be removed because

it is sticky and foreign particles can attach to it.

Basic Soldering Lessons

The most common form of soldering when creating electronic projects is soldering component leads to copper pads on the back of a printed circuit board (PCB), If you can carry out that task, you’ll have no trouble with other types of soldering, such as soldering two wires together or soldering a wire to a switch terminal.

The following steps outline the procedure for soldering a component lead to a PCB:

1. Pass the component leads through the correct holes. Check the circuit diagram carefully to be sure that you have installed the component in the correct location. If the component is polarised (such as a diode, electrolytic capacitor or integrated circuit) verify that the component is oriented correctly. You don’t want to solder it in backward.

2. Secure the component to the PCB. If the component is near the edge of the board, the easiest way to secure it is with a crocodile clip. You can also secure the component with a bit of tape.

3. Clamp the PCB in place with your vice. Turn the board so that the copper-plated side is up. If you’re using a magnifying glass, position the board under the glass.

4. Make sure that you have adequate light. If you have a desktop lamp, adjust it now so that it shines directly on the connection to be soldered.

5. Touch the tip of the soldering iron to the pad and the lead at the same time. You have to touch the tip of the soldering iron to the copper pad and the wire lead. The idea is to heat them both so that solder flows and adheres to both. The easiest way to achieve the correct contact is to use the tip of thesoldering iron to press the lead against the edge of the hole.

6. Let the lead and the pad heat up for a moment. They take only a few seconds to heat up sufficiently.

7. Apply the solder to the lead on the opposite side of the tip of the soldering iron, just above the copper pad. The solder begins to melt almost immediately. Don’t touch the solder directly to the soldering iron. If you do, the solder melts immediately, and you can end up with an unstable connection, often called a cold joint, where the solder doesn’t properly fuse itself to the copper pad or the wire lead.

8. Feed just enough solder to cover the pad when the solder begins to melt. As the solder melts, it flows down the lead and then spreads out onto the pad. You want just enough solder to cover the pad completely, but not enough to create a big glob on top of the pad. Be stingy when applying solder: too much solder is a more common problem than too little. Adding a little more solder later if you don’t get quite enough coverage is a lot easier than removing too much solder.

9. Remove the solder and soldering iron and let the solder cool. Be patient – the solder takes a few seconds to cool. Don’t move anything while the joint is cooling. If you inadvertently move the lead, you create an unstable cold joint that you have to resolder.

10. Trim the excess lead by snipping it with wire cutters right above the top of the solder joint. Use a small pair of wire cutters so that you can trim it close to the joint.

Desoldering Technique

• Desoldering requires special tools and materials to make the task easier.

• In desoldering the first step is to apply more solder.

• SMT components are much smaller and must be handled with tweezers with the