From BEAM Robotics Wiki
Soldering is a process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, the filler metal having a relatively low melting point. Soft soldering is characterized by the melting point of the filler metal, which is below 400 °C (752 °F). The filler metal used in the process is called solder.
Soldering is distinguished from brazing by use of a lower melting-temperature filler metal; it is distinguished from welding by the base metals not being melted during the joining process. In a soldering process, heat is applied to the parts to be joined, causing the solder to melt and be drawn into the joint by capillary action and will cling to the materials to be joined by surface tension. This is often refered to as wetting. After the metal cools, the resulting joints are not as strong as the base metal, but have adequate strength, electrical conductivity, and water-tightness for many uses.
Fixed type can be as simple as a Low-power Soldering iron that typically uses a 15 to 30 watt heating element. It therefore they tend to operate at a temperature that is within the safe range for soldering small to medium wires, and/or various electronic components to printed circuit boards. However the initial heat up can be a bit slow, as can be recovery after making several consecutive solder connections.
There are also Temperature-Controlled Soldering Irons or "Soldering Station" as they are often called. These are generally higher wattage irons, which allows for quicker initial heat-up and recovery (100F (~38C)) in approximately 11 second). They typically consists of a power supply and soldering iron with some form of thermostat (often inside the tip itself) which switches power on and off to the element, thus maintaining a relatively constant temperature at the tip.
Variable Soldering Irons can be as simple a unit that utilizes a variable power control (similar to a light dimmer) that together with the loss of heat from the iron to the environment roughly sets the temperature.
Better units regulate the actual temperature of the soldering tip; keeping it at the level selected by the user for a particular task.
 Kinds Of
Solder is a fusible metal alloy with a melting point or melting range of 90 to 450 °C (200 to 840 °F). The Solder used in electrical and electroncs work is a fusible metal alloy that typically has a melting point between 360-460 °F (180-235°C). It is available in a variety of diameters; with 0.031 (22 AWG) and 0.062 (16 AWG) being the sizes most commonly used in electronics.
Unfortunately Lead-free solderrequire higher soldering temperatures. Where-as Lead-based solder require 250 °C to 300 °C, lead-free soldering needs about 350 °C to 400 °C. Also, producing a good quality solder joint is more difficult with Lead-free solder. That is, require a lot more practice so if you are new to soldering it is recommended that use the Lead-Tin type.
Solders are also designated by the type of flux-core they have. Flux is an additive in solder that facilitates the soldering process. It removes and prevents oxidation and improves the wetting characteristics of the melted solder. The types of flux cores available include:
- Rosin which is the most commonly used by hobbyists. After soldering, it leaves a brown, sticky residue which is non-corrosive and non-conductive, but can be cleaned if desired with a solvent such as isopropanol (also called isopropyl alcohol or IPA).
- No-clean flux which leaves a clear residue after soldering that is non-corrosive and non-conductive. This flux is designed to be left on the solder joint and surrounding areas.
- Water-soluble flux are typically more active (i.e more aggressive) and leave a residue which must be cleaned with water. This residue is corrosive and may damage the board or components if not cleaned correctly after use.
- Solid core solder contains no flux and is usually used for copper water pipe.
 How to Solder Electronics
The importance of good solder connections cannot be over stated. A poor soldering practices can result in short circuits, open circuits or what are called "cold solder joints", which can increase the resistance through the circuit, or lead to intermittent current flow.
This section deals mainly with the soldering of through-hole components into printed circuit boards (PCBs). Through-hole components are those which have leads (meaning wires or tabs) that pass through a hole in the board and are soldered to the pad (an area with metal plating) around the hole. The hole may be plated through or not.
Soldering of other electrical items such as wires, lugs, have slightly different steps but the general principles are the same.
 A Few Cautions
- Soldering irons get very hot. Do not touch the tip with your skin. Also, always use a suitable stand or holder to keep the tip up and off of your work surface.
- Use Eye protection. This is not a joke. All it takes is one careless move and suddenly there is a blob of molten solder in your eye. Safety glasses or goggles are a small inconvenience that could save your sight.
- Solder fumes can cause irritation to the eyes and upper respiratory tract and even lead to asthma or make existing asthmatic conditions worse (See: "Solder Fume and You" for more information). It is a good practice to ware a particle mask and/or use a vent hood or small table fan to draw these fumes away from the workspace.
- Use a soldering iron stand. This will not only help protect the work surface, it will also reduce the chances of accidental burns caused by inadvertently touching the hot end of the soldering iron with your hand or arm.
- Solder contain hazardous materials. Especially lead-based solder. Be sure to wash your hands after soldering, and be aware that items containing solder may require special handling if you dispose of them.
 Things you will need
- Soldering pad or a a piece of 12 inch by 12 inch ceramic tile.
- Eye protection
- A good Soldering Iron.
- A Soldering Iron Stand
- A Damp Sponge
- Flux-cored Solder
- Items to be soldered and circuit board (if used)
 Soldering Steps
1) Plug in and/or turn on your soldering iron and allow it to heat up.
2) Tin the end of the soldering iron (as needed) by applying a small amount of solder to coat the tip, and wiping off the excess solder won the damp sponge
3) Select the component to be soldered.
4) Line up the component leads (bending them if required) with the appropriate holes in the PCB, and push them through the holes.
5) Clinch (bend) the components leads on the underside of the PCB so that the component will remain in place when the board is tuned over for soldering.
6) Carefully place the tip against both the component lead and pad of the PCB.
Note: Melting a small blob of solder on end of the soldering iron first will help improve the transfer of heat to your work.
When the lead and pad are hot enough, the solder will begin to melt. The molten solder will be drawn into the joint and will cling to the pad and component lead.
8) Once a sufficient amount has melted into the joint; pull the solder and Soldering iron away and allow the joint to cool.
Using to much solder not only wastes solder, it encourages solder bridges. A solder bridge is a short circuit that results when the solder makes an unwanted connection to another nearby circuit point.
9) Inspect the solder joint
The connection should be smooth and relatively shiny (see Figure 8).
If the joint looks dull and does not appear smooth (as in Figure 9) you have what is called a cold solder joint. This occurs when the solder does not meet and bond properly to the surface of the component lead and/or the pad on PC board. Possible causes are:
- The component lead and/or PC board pad did not get hot enough for the solder to adhere properly.
- The component lead moved before the solder compleatly cooled and solidified. Cinching the component lead usually prevents this, but it can still occasionally happen.
You should now have a good clean electrical and mechanical connection between the items being soldered.
 Hints & Tips
- Use only as much heat as you need to use. Components like diodes, transistors, etc. are quite susceptible to heat damage.
- Make sure that your soldering iron tip is in good shape. If it is pitted or misshapen then replace it. Using a bad tip will lead to bad solder joints.
- The tip of a soldering iron tends to get stuck (in the soldering iron) with time. Especially if the iron is frequently used. This is due to oxides that build up between the copper tip and the iron sleeve. Plated tips do not usually have this problem. If the copper tip is not removed from time to time, it can become permanently stuck in the soldering iron! The Soldering iron then becomes all but useless. It is a good idea therefore (after 20 - 50 or so hours of us) to remove the tip (when cold) and put it back in several times so that the oxide scales can come out. This will help keep the tip from locking in place, and your soldering iron will last for many years of use!
- There are small aluminum (heat sink) clips available that can be used to reduce the chances of ruining the component. Simply attach the clip to the component lead on the top (non soldered) side of the PC board. If you do not have a set of these heat sink clips, a small alligator clip will also work.
- Use something to hold the components or board in place while you are solder. (See: Helping Hands for more on this subject)
 External References
- "Instructables" How-to-solder PDF
- The following are a series of YouTube videos posted as part of Expert Village Channel:
- The following are a series of YouTube videos posted by CuriousInventor
- Solder Fume and You Useful safty information