subject: You Can Learn Perfect Soldering Or Be Burned! [print this page] When making your own solar panel a critical factor will be the various connections within the panel itself and the terminals that take the energy to where you need it. If these, usually soldered, connections are poor then you are at best, reducing the amount of power available and at worst introducing a fire hazard into something that you shouldn't really need to touch for 10 or 20 years!
I used to run a 3 day training school in both a Japanese & a Korean electronics company using a wire grid using stretched, tinned copper wire and students would learn by soldering at each crossing point ... excellent practise.
Overview
Soldering is the process utilised to connect two (or more) metal items together. Used in electrical and electronic assembly to join wires or to mount components onto a circuit board. Soldering provides an excellent conductive joint. The process is also used in plumbing for the jointing of copper pipes.
Soldering is accomplished by quickly heating the metal parts to be joined and then applying a flux and a solder to the mating surfaces. The molten solder flows into the joint by capillary action. The finished soldered joint forms a metallurgical bond between the parts, forming an excellent electrical connection between wires and a strong mechanical joint between the metal parts.
The flux is a chemical cleaner which prepares the hot surfaces for the molten solder. Heat is supplied with a soldering iron or other means. The solder is a low melting point alloy of non-ferrous metals.
Multicore Solder
Solder is a metal or metallic alloy used, when melted, to join metallic surfaces together. The most common alloy is some combination of tin and lead. Certain tin-lead alloys have a lower melting point than the parent metals by themselves.
The most common alloy used for electrical & electronic work is 60/40 lead/tin alloy with a melting point of 230 degrees C (460 degrees F). Another common alloy is 63/37 which melts at 183 degrees C (361 degrees F). This alloy is called eutectic; it is the only mixture that melts at a point rather than over a range. This reduces the plasticity.
Multicore solder is so called because it is a solder wire with the flux in a "core" running down the centre of the wire. To use multicore solder wire, the surfaces to be joined should already be clean rust, dirt and grease. Flux can also be applied by other means. Flux only cleans oxides off the surfaces to be soldered. It does not remove dirt, soot, oils, silicone, etc. The grease from your fingers will degrade surfaces to be soldered so such contact should be minimised. Although the surfaces to be soldered may look clean, there is always a thin film of oxide covering it. For a good solder bond, surface oxides must be removed during the soldering process using flux.
Soldering Iron
A soldering iron or gun is a hand-held electrical piece of equipment usually with a copper 'tip' heated by an internal element to the required & thermostatically controlled temperature. The 'tip' or 'bit' is the important part. It transfers heat (thermal energy) from the element to the solder connection. In most soldering iron tips the base metal is copper or some copper alloy because of its excellent thermal conductivity. The conductivity determines how fast heat can be transferred to the components being jointed.
The tip is plated to prolong its useful life. This plating aids the 'wetting' process and prevents the solder from 'sticking' to the tip. The tip should always be wiped on a wet sponge prior to its application to the joint. It should always be 'tinned' i.e. have a thin coating of solder when left in its holder.
'Wetting' Process
When the hot solder comes in contact with a copper surface, a metal solvent action takes place. The solder dissolves and penetrates the copper surface. The molecules of solder and copper blend to form a new alloy, one that's part copper and part solder, called 'intermetallic'. This solvent action is called 'wetting' and forms the intermetallic bond between the parts. 'Wetting' can only occur if the surface of the copper is free of contamination and from the oxide film that forms when the metal is exposed to air. Also, the solder and surface of the work need to have reached the proper temperature.
As heat is applied to the connection, this wetting characteristic can be seen visually ...if the molten solder quickly wicks up the sides of the component on contact, the wetting characteristic is considered good. If the solder is flowing or spreading quickly through or along the surface of the components being jointed, the wetting is also characterized as good.
Although the surfaces to be soldered may look clean, there is always a thin film of oxide covering it. For a good solder bond, surface oxides must be removed during the soldering process using flux.
Soldering
It is good practise to 'tin' the two surfaces to be soldered first be ensuring that each has a coating of solder and the flux core will remove any oxidisation and prepare the surfaces to be jointed, also improving the transfer of heat by melting when the iron is applied and 'bridging' any physical gaps.
The soldering iron tip should be applied to the joint ensuring that a good thermal contact is made i.e. that heat is being evenly applied to both parts to jointed in equal measure. This is particularly important where one of the parts is smaller or its insulation is close and could be burnt.
The idea is to get both of the parts being joined to a temperature slightly above the melting point of the solder before applying the solder. When the solder is applied, it must be pushed into the joint of the two components and not on the soldering iron tip ... the solder will always melt when applied to the tip of the soldering iron, it's purpose is to join the two components and that is where it should applied. If the temperature is correct you will see the solder 'flow' onto the two components itself ...this is the 'wetting' process.
The amount of solder applied should be sufficient to fully cover the jointing area; the aim should be to have a slightly concave surface. You should be able 'see' the outline of the joined parts through the solidified solder joint.
Because solder, particularly the 60/40 alloy, has an amount of 'plasticity' in its molten state it is important to make sure that the parts cannot move until the solder has solidified.
Recognizing a Good Soldered Connection
A soldered joint that looks good generally is. A good soldered joint should have a smooth, bright & shiny surface. (It is assumed that the solder wire being used is not matt finish).
A joint that has a matt finish indicates that the joint is 'dry', in that it hasn't wetted properly therefore there is possibly no intermetallic bond and the parts may be easily pulled apart.
A joint that has an 'eggshell' or 'orange peel' look means that the parts were moved during the 'plastic stage of the solder before it hardened properly.
Small solder balls around the joint are an indication of too much heat before the solder was applied and a burnt appearance indicates too much after the solder was applied and has burnt the flux.
A bulbous joint where you can see no outline shape of the joint indicates too much solder.
