Generally, the SMT pads on a PCB are made of copper or tin. These metallic parts react to the oxygen and moisture present in the air and may develop a thin coating of metallic oxide on the surface, similar to rusting. This oxide layer has less electrical and thermal conductivity compared to the metal and hence soldering to the oxidized surface becomes very difficult. The oxidation on the pad reduces the contact rating of the solder contacts, thus leading to faulty soldering. Solders applied won’t properly stick to the surface of the pad, therefore resulting in loose joints.
The main causes of SMT pad oxidation are:
- Contaminated caused by improper handling - if the operators touch the pads with bare fingers, it is possible to transfer oils and dirt to the pads.
- Humidity - it can aggravate the oxidation process.
Oxidation of SMT pads can be avoided or removed by
• Circuit boards, if not used, must be kept in airtight containers with desiccants.
• Circuit boards should not be kept in open air unless they are in use.
• PCB should be handled carefully and less often. They should be held only at the edges and the person should wear gloves.
• The surface of SMT pads should be cleaned with good quality abrasives, and flux (as below picture) must be applied before soldering to remove oxides.