Hot Air (Solder) Leveling (HAL/HASL)

Background
HAL / HASL is well-established and understood by both manufacturers and end users. For a while though it looked like it would go out of fashion as a result of the use of components that need a particularly even surface this treatment could not guarantee. However this has not happened and today we are seeing that a large part of the market still prefers PCBs with HAL/HASL.

Chemical aspects
Tin alloyed with copper is not identified by a code, so here it is necessary to check the precise content by looking at the relevant data sheets. SN100C - used by Elprint and developed by Nihon Superior in Japan - has been in use ince 1999 and in addition to tin contains 0.7%Cu and 0.05%Ni (nickel). Nickel improves flow and reduces the amount of oxidation (dross).

The equipment
The equipment which applies solder to a PCB is an automated process which dips it into liquid tin and which blows away the excess in a closed chamber. This means that the working environment for the operator of the machine is considerably better than it would be with semi-automatic machines where the operator is exposed to considerable amounts of gas flux.

The process

  • Pretreatment The copper to be covered with tin alloy is activated with the help of a mild acid cleaner. This removes oxides from the surface of the copper.
  • Flux Flux is applied to prevent further oxidation and to create the right conditions for the application of the solder.
  • Dipping The panel is dipped in the melt and any excess solder is blown away with the help of a flow of hot air applied by hot air knives at a high pressure. This also ensures that the deposits are of an even thickness.
  • Cleaning Removes the flux by washing it in hot water. At this point the panel is then ready for mechanical treatment.

Storage
The storage of PCBs that have been HASL treated varies according to the amount of tin that has been applied.

  • Glossy surfaces that are relatively thick can be soldered up to two years later.
  • Matt surfaces however that are significantly thinner should not be stored for more than a year.

A particular stress point is the joining between the solder pad and the hole. At the edge of the hole the thickness should not be lower than 1my. This limitation is a result of the copper migrating into the tin coating.