Comparing Bright Tin Plating to Dull Tin Plating
Appearance
The most noticeable difference between the two processes is an aesthetic one. As the names suggest, dull tin plating produces a matte finish with a dull, white surface. Bright tin plating, on the other hand, results in a finish that is more shiny and reflective. While bright tin plating may be more visually appealing, it is worth noting that it can become discoloured under exposure to high temperatures, tarnishing to a brown or black colour, while dull tin plating is far less prone to discolouring under similar conditions.
Composition
While both types of tin plating are essentially 100% pure tin, the deposit also picks up small amounts of the organic additives used in the plating solution, measured as carbon content. Dull tin plating has fewer additives than its bright tin counterpart (around 0.005% to 0.05% by weight, compared to 0.1% to 1.0% by weight for bright tin). Bright tin also has a smaller grain size than dull tin. This leads to a more uniform surface, which is what gives it its shiny appearance.
Solderability
If you are plating components for reflow soldering under high temperatures (between 250°C and 260°C), then matte tin plating is ideal for your needs. It is also suitable for use with both tin-lead and lead-free solders. As mentioned above, bright tin often discolours under higher temperatures and the material stress it incurs can lead to it becoming brittle.
Whiskering
Whiskers are small, conductive, thin fibres of metal, microscopic in size, that can grow from tin plated surfaces. Whiskering is more common with bright tin plating, owing to its small grain size, and whiskers can bridge neighbouring conductors, causing short circuits and electrical failures. Matte tin can often reduce the risk of whiskering in many applications.
Other considerations
When comparing matte and bright tin plating, other factors may also influence the most suitable finish for the component and its intended application. For instance, coating thickness can affect durability, solderability over time, cost, and the risk of the base material becoming exposed through wear or forming.
The base material is also important. For example, tin plated directly onto copper or copper alloys can form intermetallic layers over time, particularly at elevated temperatures. Where this could affect solderability or long-term performance, a nickel barrier layer may be specified between the base material and the tin coating. This is particularly relevant for brass and other copper alloys containing zinc, where an underplate can help limit diffusion into the tin deposit.
