Solder surface or PCB finish, Part 1: Function and properties

The soldering surface - also known as the end surface or PCB finish - is a thin metallic or organic layer that the PCB manufacturer applies to the copper surfaces of the component connections (solder pads). As natural copper tends to oxidize when exposed to air, soldering the components would be difficult or impossible. The soldering surface covers the exposed copper on the PCB and thus prevents the copper from corroding. In this way, the solderability of PCBs can be maintained within the guaranteed storage period under the defined storage conditions.

How does the surface coating process work?

The soldering surface is partially applied to the component connections by the PCB manufacturer after the etching process of the conductor pattern structure. Chemical metallization is used as the standard process for applying a solder surface. Prior to this, solder resist is partially applied to the surface of the PCB in the upstream process step. The solder resist only leaves the soldering and contact surfaces exposed and acts as a cover for the subsequent surface coating process.

It is important to know that the final surface only covers the soldering surfaces of the components as well as cooling surfaces and contact surfaces that are used by components. Despite the coating, copper remains the actual electrically conductive metal on the surface of printed circuit boards.

How do you choose the right soldering surface?

There is no universal solution for the "best soldering surface". All soldering surfaces have strengths and weaknesses. In Europe, the following end surfaces are the most common:

• Electroless nickel/gold (chem. NiAu, ENIG)
• Chemical tin (chem. Sn)
• OSP (Organic Solderability Preservative, Organic Surface Protection)
• Hot air leveling (HAL)
• Chemical silver (chem. Ag)

The following decision criteria should be taken into account when choosing the final surface:

• Number of soldering processes
• Electrical function test
• Bondability
• Suitability for connectors and press-fit technology
• Shelf life of the printed circuit boards
• Suitability for fine pitch applications

When deciding on a suitable soldering surface, it is important to take into account the application and processing of the PCB during component assembly. This is because the soldering surface can have a direct impact on the assembly process, quality, reliability and durability. An important prerequisite for the subsequent soldering process is a homogeneous surface for the component connections. Fine-pitch components in particular require a thin, planar coating on the solder pads.