Measuring Intermetallic Phase Growth in Lead Free Solder Joints
Accurate intermetallic phase growth measurement requires bevel-free metallographic polishing, etching chemical selection, and kinetic modeling to set supplier liability limits.
Immersion Tin Finish
Meaning
Chemical displacement processes used to deposit a thin layer of pure tin onto copper printed circuit boards provide a solderable surface finish while preventing oxidation during storage. This immersion tin finish is a popular choice for high density interconnects and flat surface requirements because it provides a very uniform and thin coating. Unlike hot air solder leveling, which can result in uneven thickness, the immersion process relies on a self-limiting chemical reaction where tin ions in a solution replace copper atoms on the board surface.
The resulting layer is typically around one micrometer thick and provides an excellent base for lead free soldering. It is particularly valued in the automotive industry for its compatibility with press-fit connectors and its ability to withstand multiple reflow cycles. However, the use of pure tin introduces specific reliability challenges, such as the growth of intermetallic compounds and the potential for metallic whiskers over time.
Deposition Process
Production of the coating involves a series of closely monitored chemical baths that clean the copper and then deposit the tin layer. The printed circuit board is first treated with an acid cleaner and a micro-etch to ensure the copper surface is active and free from contaminants. It is then submerged in the immersion tin solution, which contains stannous salts, an acid, and organic additives to control the grain size and morphology.
The reaction stops once the copper is completely covered, ensuring that the thickness is consistent across the entire board regardless of the pad size. This level of uniformity is essential for the placement of small components like 0201 resistors and fine-pitch ball grid arrays. After the deposition, the boards are thoroughly rinsed and dried to remove any residual chemicals that could cause corrosion.
Whisker Growth
Spontaneous formation of thin, needle-like metallic structures from the surface of the pure tin layer is a known risk associated with this finish. These tin whiskers can grow to several millimeters in length and are capable of causing electrical short circuits between adjacent traces or pads. The growth is driven by compressive stresses within the tin layer, which can be caused by the formation of copper-tin intermetallics at the interface.
To mitigate this risk, manufacturers often include anti-whisker additives in the plating chemistry or use a thicker copper layer to reduce stress. While the industry has made significant progress in controlling this phenomenon, it remains a concern for long life products in the medical and aerospace fields. Regular inspection and stress testing are often required to ensure that the immersion tin finish will not develop dangerous whiskers during its service life.
Storage Life
Sensitivity to environmental conditions makes the handling and storage of boards with this finish a critical task for assembly factories. The immersion tin finish has a shorter shelf life than some other finishes because the tin continues to react with the underlying copper even at room temperature. This solid state diffusion gradually converts the pure tin into copper-tin intermetallic compounds, which are more difficult to solder.
If the board is stored for too long or in a humid environment, the solderability of the surface will degrade, leading to defects like non-wetting or weak joints. Most manufacturers recommend a shelf life of six to twelve months and require the use of moisture barrier bags with desiccant. Boards that have exceeded their shelf life may need to be re-tested or undergo a specialized cleaning process before they can be used in production.
Proper inventory management is therefore essential to prevent the waste of expensive printed circuit boards.