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CATEGORIES (articles) > Electrical, Systems, Wiring & Technologies > Circuits & Components > Soldering overview and techniques

Soldering overview and techniques

(De)soldering a contact from a wire.

Soldering is a method of joining metal parts using an alloy of low melting point (solder) below 450 C (800 F). Heat is applied to the metal parts, and the alloy metal is pressed against the joint, melts, and is drawn into the joint by capillary action and around 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. Soldering is an ancient technique that has been used practically as long as humans have been making articles out of metal.

Soldering can be done in a number of ways, including passing parts over a bulk container of melted solder, using an infrared lamp, or by using a point source such as an electric soldering iron, a brazing torch, or a hot-air soldering tool. Flux is usually used to assist in the joining process. Flux can be manufactured as part of the solder in single or multi-core solder, in which case it is contained inside a hollow tube or multiple tubes that are contained inside the strand of solder. Flux can also be applied separately from the solder, often in the form of a paste. In some fluxless soldering, a forming gas that is a reducing atmosphere rich in hydrogen can also serve much the same purpose as traditional flux, and provide the benefits of traditional flux in re-flow ovens through which electronic parts placed on a circuit card are transported for a specific period of time.

One application of soldering is making connections between electronic parts and printed circuit boards. Another is in plumbing. Joints in sheet-metal objects such as cans for food, roof flashing, and drain gutters were also traditionally soldered. Jewelry and small mechanical parts are often assembled by soldering. Soldering can also be used as a repair technique to patch a leak in a container or cooking vessel.

Soldering is distinct from welding in that the base materials to be joined are not melted, though the base metal is dissolved somewhat into the liquid solder - this dissolution process results in the soldered joint's mechanical and electrical strengths. A "cold solder joint" with poor properties will result if the base metal is not warm enough to melt the solder and cause this dissolution process to occur.

Brazing is similar to soldering but uses higher melting temperature alloys, based on copper, as the filler metal.

"Hard soldering", or "silver soldering" (performed with high-temperature solder containing up to 40% silver) is also a form of brazing, and involves solders with melting points above 450 C (800 F). Even though the term "silver soldering" is more often used than silver brazing, it is technically incorrect.

Since lead used in traditional solder alloys is toxic, much effort in industry has been directed to adapting soldering techniques to use lead-free alloys for assembly of electronic devices and for potable water supply piping.

Basic electronic soldering techniques

All solder pads and device terminals must be clean for good wetting and heat transfer. The soldering iron or gun must be clean, otherwise components may heat up excessively due to poor heat transfer. The devices must then be mounted on the circuit board properly. One technique is to elevate the components from the board surface (a few millimeters) to prevent heating of the circuit board during circuit operation. After device insertion, the excess leads can be cut leaving only a length equal to the radius of the pad. You may use plastic mounting clips or holders for large devices to reduce mounting stresses.

Heat sink the leads of sensitive devices to prevent heat damage. Apply soldering iron or gun to both terminal lead and copper pad to equally heat both. Apply solder to both lead and pad but never directly to the tip of soldering iron or gun. Direct contact will cause the molten solder to flow over the gun and not over the joint. The moment the solder melts and begins to flow, remove the solder supply immediately. Do not remove the iron yet. The remaining solder will then flow over the junction of the lead and pad, assuming both are free of dirt. Let the iron heat the junction until the solder flows and then remove the iron tip. This will ensure a good solid junction. Remove the iron from the junction and let the junction cool. Solder flux will remain and should be removed.

Be sure not to move the joint while it is cooling. Doing so will result in a fractured joint. Do not blow air onto the joint while it is cooling; Instead, let it cool naturally, which will occur fairly rapidly. A good solder joint is smooth and shiny. The lead outline should be clearly visible. Clean the soldering iron tip before you begin on a new joint. It is absolutely important that the iron tip be free of residual flux. Excess solder should be removed from the tip. This solder on the tip is known as keeping the tip tinned. It aids in heat transfer to the joint.

After finishing all of the joints, remove excess flux residue from the board using alcohol, acetone, or other organic solvents. Individual joints can be cleaned mechanically. The flux film fractures easily with a small pick and can be blown away with canned air. In solder formulations with water-soluble fluxes, sometimes pressurized carbon dioxide or distilled water are used to remove flux.

Traditional solder for electronic joints is a 60/40 Tin/Lead mixture with a rosin based flux that requires solvents to clean the boards of flux.

Environmental legislation in many countries, and the whole of the European Community area, have led to a change in formulation. Water soluble non-rosin based fluxes have been increasingly used since the 1980's so that soldered boards can be cleaned with water or water based cleaners. This eliminates hazardous solvents from the production environment, and effluent.

Desoldering and Resoldering

Due to the dissolution of the base metals into the solder, solder should never be reused; once the solder's capacity to dissolve base metal has been achieved, the solder will not properly bond with the base metal and a cold solder joint with a hard and brittle crystalline appearance will usually be the result. It is good practice to remove solder from a joint prior to resoldering - desoldering wicks or vacuum desoldering equipment can be used. Desoldering wicks contain plenty of flux that will lift the contamination from the copper trace and any device leads that are present. This will leave a bright, shiny, clean junction to be resoldered.

The lower melting point of solder means it can be melted away from the base metal, leaving it mostly intact though the outer layer will be "tinned" with solder. Flux will remain which can easily be removed by abrasive or chemical processes. This tinned layer will allow solder to flow into a new joint, resulting in a new joint, as well as making the new solder flow very quickly and easily.

Lead-free electronic soldering

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 1st July 2006, mostly in the consumer goods industry, but in some others as well.

Many new technical challenges have arisen, with this endeavour.

For instance, traditional lead free solders have a significantly higher melting point than lead based solders, which renders them unsuitable for use with heat sensitive electronic components and their plastic packaging. To overcome this problem solder alloys with a high silver content and no lead have been developed with a melting point slightly lower than traditional solders.

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. Somehow, 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 kind of provisions they want to take against whiskers, depending upon their application criticity.

Stained glass soldering

Historically soldering tips were copper, placed in braziers. One tip was used; when the heat had transferred from the tip to the solder (and depleted the heat reserve) it was placed back in the brazier of charcoal and the next tip was used.

Currently, electric soldering irons are used; they consist of coil or ceramic heating elements, which retain heat differently, and warm up the mass differently, internal or external rheostats, and different power ratings - which change how long a bead can be run.

Common solders for stained glass are mixtures of tin and lead, respectively:

  • 63/37: melts between 355-365F
  • 60/40: melts between 361-376F
  • 50/50: melts between 368-421F
  • lead-free solder (useful in jewelry, eating containers, and other environmental uses): melts around 490F

Pipe/Mechanical soldering

Sometimes it is necessary to use solders of different melting points in complex jobs, to avoid melting an existing joint while a new joint is made.

Copper pipes used for drinking water should be soldered with a lead-free solder, which often contains silver. Leaded solder is not allowed for most new construction, though it is easier to create a solid joint with that type of solder. The immediate risks of leaded solder are minimal, since minerals in municipal or well water supplies almost immediately coat the inside of the pipe, but lead will eventually find its way into the environment.

Tools required for pipe soldering include a blowtorch (typically propane), wire brushes, a suitable solder alloy and an acid paste flux, typically based on zinc chloride. Such fluxes should never be used on electronics or with electronics tools, since they will cause corrosion of the delicate electronic part.

Soldering defects

Soldering defects are solder joints that are not soldered correctly. These defects may arise when solder temperature is too low. When the base metals are too cold, the solder will not flow and will "ball up", without creating the metallurgial bond. An incorrect solder type (for example, electronics solder for mechanical joints or vice versa) will lead to a weak joint. An incorrect or missing flux can corrode the metals in the joint. Without flux the joint may not be clean. A dirty or contaminated joint leads to a weak bond. A lack of solder on a joint will make the joint fail. An excess of solder can create a "solder bridge" which is a short circuit. Movement of metals being soldered before the solder has cooled will make the solder appear grainy and may cause a weakened joint.

Soldering defects in electronics can lead to short circuits, high resistance in the joint, intermittent connections, components overheating, and damaged circuit boards. Flux left around integrated circuits' leads will lead to inter-lead leakage. It is a big issue on surface mount components and causes improper device operation as moisture absorption rises, In mechanical joints defects lead to joint failure and corrosion.

Soldering processes

  • Wave soldering
  • Reflow soldering
  • Infrared soldering
  • Induction soldering
  • Ultrasonic soldering
  • Dip soldering
  • Furnace soldering
  • Iron soldering
  • Resistance soldering
  • Torch soldering
  • Silver soldering/Brazing

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