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Soldering station

A soldering station

About[]

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. The filler metals are typically alloys that have liquidus temperatures below 350°C. It is distinguished from welding by the base metals not being melted during the joining process which may or may not include the addition of a filler metal. 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 to bond to the materials to be joined by wetting action. 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. There is evidence that it was employed up to 5000 years ago in Mesopotamia.

One of the most frequent applications of soldering is assembling electronic components to printed circuit boards (PCBs).


Printed Circuit Boards[]

For attachment of electronic components to a PCB, proper selection and use of flux helps prevent oxidation during soldering, which is essential for good wetting and heat transfer. The soldering iron tip must be clean and pre-tinned with solder to ensure rapid heat transfer. Components which dissipate large amounts of heat during operation are sometimes elevated above the PCB to avoid PCB overheating. After inserting a through-hole mounted component, the excess lead is cut off, leaving a length of about the radius of the pad. Plastic or metal mounting clips or holders may be used with large devices to aid heat dissipation and reduce joint stresses.

A heat sink may be used on the leads of heat sensitive components to reduce heat transfer to the component. This is especially applicable to germanium parts. (Note the heat sink will mean the use of more heat to complete the joint.) If all metal surfaces are not properly fluxed and brought above the melting temperature of the solder in use, the result will be an unreliable "cold solder joint".

220px-Cold solder joint2

An improperly soldered 'cold' joint

To simplify soldering, beginners are usually advised to apply the soldering iron and the solder separately to the joint, rather than the solder being applied direct to the iron. When sufficient solder is applied, the solder wire is removed. When the surfaces are adequately heated, the solder will flow around the joint. The iron is then removed from the joint.

Since non-eutectic solder alloys have a small plastic range, the joint must not be moved until the solder has cooled down through both the liquidus and solidus temperatures. Visually, a good solder joint will appear smooth and shiny, with the outline of the soldered wire clearly visible. A matte gray surface is a good indicator of a joint that was moved during soldering. Too little solder will result in a dry and unreliable joint; too much solder (the 'solder blob' very familiar to beginners) is not necessarily unsound, but tends to mean poor wetting. With some fluxes, flux residue remaining on the joint may need to be removed, using water, alcohol or other solvents compatible with the process. Excess solder and unconsumed flux and residue is sometimes wiped from the soldering iron tip between joints. The tip of the iron is kept wetted with solder ("tinned") when hot to minimize oxidation and corrosion of the tip itself.

220px-Gebrochene loetstellen

Broken solder joints on a Flyback transformer

Lead Free[]

More recently environmental legislation has specifically targeted the wide use of lead in the electronics industry. The RoHS directives in Europe require many new electronic circuit boards to be lead free by 1 July 2006, mostly in the consumer goods industry, but in some others as well. In Japan lead was phased out prior to legislation by manufacturers due to the additional expense in recycling products containing lead.[1]

It is a common misconception that lead free soldering requires higher soldering temperatures than lead/tin solder; the wetting temperature in lead/tin solder is higher than the melting point and is the controlling factor - Wave soldering can proceed at the same temperature as previous lead/tin soldering. Nevertheless many new technical challenges have arisen with this endeavor; to reduce the melting point of tin based solder alloys various new alloys have had to be researched, with additives of copper, silver, bismuth as typical minor additives to reduce melting point and control other properties, additionally tin is a more corrosive metal, and can eventually lead to the failure of solder baths etc.

Lead-free construction has also extended to components, pins, and connectors. Most of these pins used copper frames, and either lead, tin, gold or other finishes. Tin finishes are the most popular of lead-free finishes. Nevertheless, this brings up the issue of how to deal with tin whiskers. The current movement brings the electronics industry back to the problems solved in the 1960s by adding lead. JEDEC has created a classification system to help lead-free electronic manufacturers decide what provisions to take against whiskers, depending upon their application.

Tools[]

Hand-soldering tools include the electric soldering iron, which has a variety of tips available ranging from blunt to very fine to chisel heads for hot-cutting plastics, and the soldering gun, which typically provides more power, giving faster heat-up and allowing larger parts to be soldered. Hot-air guns and pencils allow rework of component packages which cannot easily be performed with electric irons and guns.

Wire brush, wire wool and emery cloth are commonly used to prepare plumbing joints for connection. Electronic joints rarely require mechanical cleaning, though copper traces with a dark layer of oxide passivation (due to aging), as on a new prototyping board that has been on the shelf for about a year or more, may need to be polished to a shine with steel wool before being soldered.

For PCB assembly and rework, alcohol and acetone (one or the other) are commonly used with cotton swabs or bristle brushes to remove flux residue. A heavy rag is usually used to remove flux from a plumbing joint before it cools and hardens. A fiberglass brush can also be used. Some fluxes for electronics are designed to be stable and inactive when cool and do not need to be cleaned off, though they still can be if desired, while other fluxes are acidic and must be removed after soldering to prevent corrosion of the circuits.

A heat sink, such as a crocodile clip, can be used to prevent damaging heat-sensitive components while soldering. The heat sink limits the temperature of the component body by absorbing and dissipating heat (reducing the thermal resistance between the component and the air), while the thermal resistance of the leads maintains the temperature difference between the part of the leads being soldered and the component body so that the leads become hot enough to melt the solder while the component body remains cooler.


References[]

  1. DKL Metals. accessdate 2011. FACT AND FICTION IN LEAD FREE SOLDERING


Links[]

See also[]

Video[]

thumb|300px|right|How and WHY to Solder Correctly

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