The fluxes used in wave soldering are essential – without them, many surfaces would solder only with considerable difficulty. The residues that fluxes leave will be more or less acceptable cosmetically, depending on the amount of flux supplied, the material used and the soldering conditions. Whether the residues present a long term reliability hazard will depend on whether or not they have been designed to be cleaned.

Flux changes when in contact with the atmosphere, due both to the loss of solvents and to oxidation. This can have a negative impact on its effectiveness as a wetting agent, and lead to residues that are thicker and/or more difficult to remove.

Flux choice is important, as this affects:

• Whether the flux base is rosin, a synthetic resin substitute, or an organic acid
• The solids content
• Whether solvent or water is used as a carrier.

The first of these affects the nature of the residue and whether cleaning is necessary; the second and third impact on the choice of fluxer and preheat systems.

The solder material impacts both on the strength of the joint and on the melting performance, where the liquidus point and the fluidity of the liquid solder are important.

63% tin solder (PB37A) is arguably not the best for every application, but is most commonly used because of its ready availability.

Adding 2% of silver would give some benefits, but the vastly increased price does not result in significant strength benefits, given that wave-soldered joints have a substantial safety margin.

Solder mask can affect the process in a number of ways as shown in the bulleted list under Solder mask. Key elements in material choice are good adhesion to the board surface and a suitable surface finish to avoid solder balling problems.

Whilst we have deliberately considered only the key materials at this stage, there are other potential problems relating to materials:

• The board metallisation is important, because a surface which has poor solderability or is contaminated can fail to solder or may dewet after soldering, in both cases producing unacceptable joints
• Volatiles trapped within the board may result in blow holes and voids
• The design of the PCB may also result in less than acceptable yield
• Any components that are actually immersed in the liquid solder receive a substantial thermal shock, which may result in damage if they are not suitably specified
• The glue used to attach such components may fail.

Some comments on all these are contained in later sections.