How to
How to Solder Electronics Learning to solder through-hole components is an essential skill for any amateur hobbyist or electronic professional. You can learn what equipment and skills you'll need to get started soldering electronics properly.
Getting the Necessary Equipment
1 Use a soldering iron with the appropriate heat control. For soldering electrical components into printed circuit boards, the best soldering irons are
Electrostatic Discharge (ESD) safe, temperaturecontrolled, high-power irons. These will let you solder for hours, and are good for complex amateur radio projects. For simple kits, an inexpensive pencil iron will do just fine. 
Use a fixed power soldering iron, 25W for small jobs, and 100W for larger jobs with heavy cabling.
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If possible, variable temperature irons are available, which will make for the safest treatment of the boards. The tip temperature can be controlled to suit the size of the job.
2 Use solder wire of an appropriate alloy. The most common solder alloy used in electronics is 60% tin and 40% lead, sometimes notated as 60/40. This alloy is recommended if you are new to soldering,
though it is somewhat hazardous, requiring proper ventilation, breathing protection, or a soldering iron with a vacuum attachment. 
Various lead-free alloys are becoming popular recently. These require higher soldering temperatures and do not "wet" as well as Tin-Lead alloys. However they are safer and can be more effective. 96.5 % tin to 3.5% silver is the most successful and will produce a joint with less electrical resistance than any tin-lead alloy.
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Both lead and lead-free formulations are available online at places like solderdirect.com and in your local Radioshack or Home Depot store.
3 Try to get flux-cored wire if possible. Flux is an additive in solder that facilitates the soldering process by removing and preventing oxidation and by improving the wetting characteristics of the liquid
solder. There are different types of flux cores available for solder wire.
Rosin is 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 isopropyl alcohol. There are different grades of Rosin flux, the most commonly used is "RMA" (Rosin Mildly Activated).
No-clean flux leaves a clear residue after soldering, which is non-corrosive and non-conductive. This flux is designed to be left on the solder joint and surrounding areas.
Water-soluble flux usually has a higher activity that leaves a residue which must be cleaned with water. The residue is corrosive and may also damage the board or components if not cleaned correctly after use.
4 Get the necessary board and components. Mostly, electrical soldering deals with "through-hole" components, which are pressed into printed circuit boards (PCBs). Through-hole
components have leads (wires or tabs) that pass through a hole in the board and are soldered to the pad of metal plating around the hole. The hole may be "plated through" or not. 
Soldering other electrical items such as wires or lugs, have slightly different techniques, but the general principles of operating the solder and iron are the same.
5 Get a clamp to hold the components. Electrical components are usually quite small, and you'll need tongs, needle-nosed pliers, or tweezers to hold them in place while you operate the soldering iron and negotiate the solder. It can be a balancing act.
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Some kind of clamp or stand is usually best to hold the board in place while you solder the components.
Soldering the Components
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Prepare the components for soldering. Select the correct component by checking it's type and value carefully. With resistors, check their color code. Bend leads correctly, if necessary, being careful not to exceed the stress specs, and clinch leads to fit the board.
2 Be extremely careful and solder in an appropriate location. Always solder in a wellventilated area, using breathing and eye protection. Make sure to prop up the iron when it is on but not in use. Irons can start fires quite easily by burning into your workbench. 
Seven to twelve inches of space should be between the electronic components and your face, or it may jump into your eyes. Solder may splatter.
3 "Tin" the solder. Melt a small blob of solder on end of the soldering iron. This process is called tinning and it helps to improve heat flow from the iron to the lead and pad, keeping the board safe from the heat.
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Carefully place the tip (with the blob) onto the interface of the lead and pad. The tip or blob must touch both the lead and the pad.
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The tip of the soldering iron should not be touching the nonmetallic pad area of the PCB, or any of the fibreglass area surrounding. This area can be damaged by excessive heat.
4 Feed the solder onto the interface between the pad and lead. Flux from the solder wire is only active for about one second maximum after melting onto the joint as it is slowly burned off by heat. The lead and pad should be heated enough for the solder to melt not the connection point. The molten solder should "cling" to the pad and lead together by way of surface tension. This is commonly referred to as wetting. 
If the solder does not melt onto the area, the most likely cause is insufficient heat has been transferred to it, or the surface needs to be cleaned of grease or dirt.
5 Stop feeding the solder interface is filled. No more than a drop or two of solder should be necessary for each joint, though it will vary slightly for different components. The correct amount of solder is determined by:
On plated-PCBs, you should stop feeding when a solid concave fillet can be seen around the joint.
On non-plated PCBs, you want to stop feeding when the solder forms a flat fillet.
Too much solder will form a bulbous joint with a convex shape, while too little solder will form a concave joint.
Soldering Well
1 Move quickly. Unfortunately, it's quite easy to damage a component or the board with too much heat. For the most part, however, you can keep the
components and the board safe by moving swiftly. Keep a finger on the board nearby to ensure that it doesn't get too hot. 
Try to err on the side of irons that are slightly less powerful than you think you might need. Use a 30 watt iron and practice soldering quickly so as to avoid excessive heating.
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If working with a double-sided circuit board check both sides for good solder joints. A good joint will look shiny and cone shaped. if it looks frosty and dull then it is a cold joint.
2 Consider using heat sinks for sensitive components. Some components (diodes, transistors, etc.) are quite susceptible to heat damage and require a small aluminum heat-sink
clipped on to their leads on the opposite side of the PCB. Small aluminum heat sinks can be purchased through electronics supply houses. Hemostats (small) can also be used.
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Learn to recognize when there is enough solder present. After a proper application of solder, the solder will be shiny and not dull. Visible indications are the best way to know if your solder joint is good. The solder needs to melt with the surface of the electronic components, rather than the tip of the soldering iron. This way, when the solder cools, it forms an alloy with the surface of the metal. 
The solder joint should coat the surface of the component evenly, not too much such that it forms a glob, nor too little such that it does not completely coat the surface.
4 Keep the soldering iron clean. Burnt flux, rosin from the core of the solder, or plastic sheaths from wires may all burn onto the soldering iron. Such contaminants prevent the formation of a clean alloy
between the electronic components. This is undesirable, because it raises the electrical resistance and also reduces the mechanical strength of the solder joint. A clean tip is shiny all the way around, without burnt gunk on it. 
Clean the iron in between each component that you solder. Use a damp sponge or steel wool to clean it thoroughly. Tip thinner may also be used.
5 Make sure to let the solder cool completely before moving the components. This cooling should only take five to ten seconds. If the components are too hot to handle, use needle nose
pliers, or a tool called helping hands which consists of two alligator clips attached to a little articulated stand. If you watch carefully, the cooling solder will settle right before your eyes.
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Practice on junk components. It's important to practice on throwaway stuff before you move straight to trying to solder something important. Gather some old junk boars and burnt out components from a junked radio or other electronic device to practice on. 
Nobody is perfect, not even the professionals. Don't be ashamed to repeat a bit of soldering work. It will save you time in troubleshooting later.
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