Solder surface or printed circuit board finish, Part 1: Function and properties

Soldering surface Printed circuit boards

The solder surface, final surface or also called PCB finish, is a thin metallic or organic layer that the PCB manufacturer applies to the copper surfaces of the component connections (solder pads). The solder surface covers the exposed copper on the PCB and preserves the solderability within the guaranteed storage period of the PCBs under the defined storage conditions.

Natural copper tends to oxidize when exposed to air. Soldering the components would be difficult or impossible to do reliably. The soldering surface prevents corrosion of the copper and provides an easily solderable surface for the assembly of electronic components in the printed circuit board assembly.

Soldering surface covers only the connection surfaces for the components

The final surface covers only the soldering surfaces of the components as well as cooling surfaces and contact surfaces used by components. Despite coating, copper remains the actual electrically conductive metal on the surface of printed circuit boards.

The solder surface is partially applied to the component connections by the PCB manufacturer after the etching process of the conductor pattern structure. The standard process for applying a solder surface is chemical metallization.

Beforehand, solder resist is partially applied to the surface of the printed circuit board in the upstream process step. The solder resist leaves only the soldering and contact surfaces free and acts as a cover for the subsequent surface coating process.

Decision criteria for the selection of the soldering surface

In Europe, the most common solder surfaces are electroless nickel-gold (chem. NiAu, ENIG), electroless tin (chem. SN), OSP (Organic Solderability Preservative, Entek Plus), hot air leveling (HAL: Hot Air Leveling) and electroless silver (chem. Ag).

There is no universal solution for the “best soldering surface”. All solder surfaces have strengths and weaknesses. The decision criteria for the choice of soldering surface are the number of soldering processes, electrical function test, bonding capability, suitability for connectors and press-fit technology, the shelf life of the PCBs and suitability for fine-pitch applications.

When deciding on the appropriate solder surface, it is important to consider the application and processing of the PCB during component assembly. The solder surface can have a direct impact on the assembly process, quality, reliability and longevity. 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.

The table compares the properties of the solder surfaces offered by KSG.

Soldering surfaces
Comparison of the properties of solder surfaces for printed circuit boards

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