In a PCB, soldering is necessary and very frequently used to connect components pins electrically in the electronic assemblies. It is a conductive substance which is quite solid on the room temperature and has low melting point. The solder is melted over the component pin and component footprint pad that makes a joint between the two. Despite of electrical connections the soldering joint supports both mechanically. Moreover, a low electrical resistance of and inductance of the soldering material should also low.
The solder or soldering wire is available in different brands, material compositions and forms. It is used very frequently for electronic circuits but the other application is in plumbing. Both applications have different material composition. The solder is molten by using a soldering iron whose tip heats upto a temperature well beyond the melting temperature of the solder. For large scale PCB assembly a solder tub is used which is heated over an elevated temperature to melt the solder. The solder is in melting condition within the tub.
Basically solder is a compound of two metals tin and lead. The basic composition is 60/40 solder, i.e., made of 60% tin and 40% lead metals.Its melting point is 200°C. It is available in wire form or sticks or pellet form. Most of the times soldering wire is used. Obviously, to melt this composition the temperature of the soldering iron tip or bath should be higher. The other composition is 63/37 i.e., 63% tin and 37% lead. Its melting temperature is 183°C which is slightly lower than the 60/40 composition. The main advantage is not the lower melting temperature but the low risk of cold joints.
The Non-eutectic 60/40 solder, make a semi-solid state between liquid to solidification stage. The problem occurs when a joint is moved during solidification stage. The joint becomes slightly broken that calls a cold solder joint. The eutectic 63/37 solders do not create this semi-solid state and the risk of developing a cold joint is reduced. A single cold joint in the entire PCB assembly makes it useless at a later stage.
A solder wire is a bendable, silver colored wire wound over reels which are available in different diameters. The common diameters of the solder wire are 18 and 22 SWG (Standard Wire Gauge). The higher the wire gauge number the thinner is the wire and vice versa. Thick wire i.e., 18 SWG is suitable for big soldering pad components such as DIP components and interconnect wires. The 22 or 26 SWG wire is a finer gauge wire and is better for SMD and fine pitch components.
Fig. 1:- A 60/40 (tin / lead) ratio 500 gram reel of 22 SWG solder wire.
Fig. 2:- A 63/ 37 composition ratio 500gram reel of 1.22mm core thickness
Fig. 3:- A lead free 500gram reel of 1.0mm diameter wire
A good soldering wire has a flux inside the soldering wire that is called a core. The core improves the electrical contact between pins and strengthens the mechanical joints. There are two types of cores: the rosin core and the acid core. The acid cores are developed for plumbing applications while rosin cores are developed for soldering of electronic assemblies.
Another type of soldering wire originated after introducing the EU RoHS compliance standards is the lead-free solder wires. The lead-free wire is introduced because the lead is dangerous for health. It is good for human health although have a higher melting point. Figures 1, 2, and 3 shows different types of soldering wires available in the market.