For environmental and safety reasons, the electronics industry has been seeking to greatly reduce, and eventually eliminated lead (Pb) in components, electrical and electronic equipment.
The RoHS (Reduction of Hazardous Substances) directives enforce restrictions of six toxic materials ( including Lead, Mercury and Cadmium) to achieve this goal. Tin (Sn), and tin-compounds, have been the prime low-cost replacement for Pb.
This article focuses on the use of Tin as a plating (also referred to as finish or coating) for electrical connector applications. In a separate article, we’ll discuss using gold and silver as options for Pb replacement.
A finish is applied as a top-coat to electrical connectors to provide corrosion protection of the underlying metal and to reduce the insertion / extraction forces required to mate / remove a physical connection.
The plating results can be characterized as “Matte” or “Bright” / “Shiny”. One method to understanding these two types of plating is through a series of “compare and contrast” questions. You can also find more details in one of the few books published on the topic: Tin and Solder Plating in the Semiconductor Industry
Should I use parts with a Matte finish or a Bright finish?
There are trade-offs to be considered with either finish, and the decision depends on your requirements and preferences. Parts finished with Bright tin provide a shiny, aesthetically pleasing surface finish. It also provides somewhat lower coefficient of friction for mating with other parts, boards and connectors as compared to Matte tin. For example, press-fit interconnects often use Bright tin plating to reduce the required insertion force which in turn can reduce damages of the plated through-hole in the printed circuit board (PCB). Conversely, Matte tin finish parts are better suited for solder reflow processes, and suffer less stress effects from elevated temperatures. Matte tin will also retain its original color under high temperatures, compared with Bright tin.
What is the visual appearance of Matte and Bright plating?
Matte has more of a dull, white surface, whereas Bright is more shiny and reflective. The Bright finish may also discolor towards a brown or black color when exposed to high temperatures such as those used in reflow soldering processes.
A example of two heat sinks is shown below. On the left is a Matte Tinned TO-200 (p/n V5640b-T) and on the right is a Bright Tinned TO-225 (V-1100-SMD/B). with Matte on the left and Bright on the right.
(Images courtesy of connector manufacturer Assmann WSW Components)
What is the composition of Matte and Bright plating?
Both types are basically 100% pure tin (Sn). The differences in the chemical make-up are from the amount of co-deposit organic materials in the tin. Carbon is the predominate organic material used in plating and finishes. Matte tin contains relatively few additives, in the range of 0.005% – 0.05% by weight, and most typically around 0.015%. Bright tin has about 10 times more organic materials, in the range of 0.1% – 1.0% by weight, and most typically around 0.15%. The additives contribute to how the grains and boundaries form in the material.
How thick is the finish?
This depends on the product and usage. For electronic components and connectors, the plating is typically in the 150 micro-inch to 450 micro-inch thickness. This will vary with the manufacturer and the specific component.
If the materials are so similar, why is one dull in appearance and the other shiny?
The Matte tin has a larger grain size than Bright tin. This is nominally around 1um – 9um diameter for Matte and in the sub-micron range of 0.1um for Bright. The smaller grain size presents a more uniform surface for reflecting light, giving a shiny appearance. Below is a representation of the grains in the finish, with Matte on the left and Bright on the right.
Which is better for soldering?
Matte tin plating is suitable for lead-free reflow solder processes which has higher temperatures, typically in the range of 250C – 260C. It can also be used with all existing lead-alloy and lead-free solders and pastes, and is fully backward compatible with leader solder processes. Bright tin may discolor at the higher temperatures, and may also suffer more material stress during the reflow process.
What are the concerns regarding formation of Tin Whiskers?
Whiskers are microscopic, very thin fibers of metal which can grow from tin plated surfaces. These can grow for relatively large distances and can create short circuits between electrical components causing electrical failures. The material stress in the plating process is a root cause of whisker formation and growth.
Any final recommendations?
Many electronics suppliers are recommending using Matte finish when possible due to its better suitability with the lead-free reflow solder processing, less likelihood of whisker grow and that its appearance does not discolor under high temperatures.
To learn more about the solder reflow process, we suggest this text from Dr. Lee: Reflow Soldering Processes