Electroplating Surface Grain Structures

Electronic components such as connectors, pins and contacts are electroplated for several reasons. Among the desired properties are hardness, brightness, electrical conductivity, and corrosion and wear resistance. These attributes are ultimately dependent on several factors including the plating process, the base material, the bath, and of course, the plating material.

These selections will also contribute to another important aspect, not often discussed: the grain structure of the plated surface. This is important since the grain structure will dictate the physical properties and performance attributes of the finished product. The best example to illustrate this principle: imagine a part that requires high brightness. To achieve this, it must have an extremely fine grain structure. Seems pretty simple, but of course, it’s much more complex than that thanks to physics.

What Determines Grain Structure?

How atoms are arranged determines the structure of an electrodeposit with most metal deposits being crystalline. The smallest atomic arrangement is referred to as a unit cell and is the building blocks of three-dimensional structures called crystals or grains.

There are basically four structures with electrodeposited metals. They are columnar, fibrous, laminar or banded and fine-grained. Metal coating exhibiting the best properties typically are fine-grained which are harder, smoother, brighter, and stronger, but not as ductile as columnar-grained deposits.

During the plating process, a metal ion near the workpiece or cathode picks up electrons and becomes an atom that will attach at a location (kink site) on the existing crystal arrangement. Atoms can encounter other atoms before finding a kink site and form a new layer which can become a new grain.

Plating Bath is a Factor...

The plating bath formulation plays a large role in determining the finish’s grain structure. For instance, fine grain structures are achieved by using more complex ion solutions, such as cyanide. Regardless of the bath mixture, the formulation must be tightly monitored and controlled. Consistency is absolutely critical. Contamination, for instance, could lead to structural malformations that will ultimately lead to plate failure due to cracking.

Additives such as brighteners and other organic compounds tend to limit grain size. It’s the surface structure of the deposit that determines brightness. Protrusions and depressions in the deposit do not vary much and readily reflect light. A dull deposit has larger columnar grains with deep depressions that do not reflect light as well.

Image Source: Ashworth MA et al. The effect of electroplating parameters and substrate material on tin whisker formation. Microelec-tron Reliab (2014), http://dx.doi.org/10.1016/j.microrel.2014.10.005