Wafer coating is a process by which the semicondutor wafer has a filler metal (usually solder) deposited over its surface, thus achieving metallurgical bonding. The metal's melting temperature should be below 315 degrees C in order to have the process be classified as coating. The surface tension of the wafer is the driving force for the solder coating process. If the surface to be coated by the solder is not wetted, coating will not take place. A solder diffusion layer grows at the surface-solder interface, allowing the solder to spread through the surface during the coating process.
This is variation of the method of leadfinish. This is the process of applying a coat of metal over the leads of an integrated circuit. It is done for a number of different reasons. Firstly, it protects the lead against corrosion. Second, the coating protects the lead against abrasion. Third, it improves the solderability of the leads involved. Finally, coating is good as it improves the overall appearance of the leads.
There are, however, some problems that come with a lead coating and can result due to just this coating alone. Lead corrosion is perhaps the biggest problem that can occur due to an imperfect lead finish, so it is very important to avoid all imperfections. Another problem that can occur is poor solderability. This takes place when the solder is insufficiently wetted, usually due to contaminants, excess additives, or inadequate plate thickness.
It is important to be careful when using a coating, as many other problems can occur as well, though not as common. These include blistering, burns, dendrites, deposits, nodules, pitting, solder dullness, solder roughness, and tarnishing. With so many problems, wafer coating must be done very carefully and allows for little error.