Immersion Tin Finish
27 October 2008
Immersion processes like silver and tin have an organic deposited as part of the process that reduces the oxidation that would be expected with a pure tin or silver surface.
Tin and silver dissolve into the solder and become part of the solder joint and with such a thin coating it’s relatively undetectable in the final joint structure. Both tin and silver are dissolved during soldering so the solder joins directly to the copper surface, just like copper OSP. On gold boards the solder forms a joint with the nickel after dissolving the gold.
Tin is a chemical displacement process directly on to a prepared copper surface which must be perfectly clean with no surface contamination from the solder mask coating. It’s extremely important that the surface of the copper is correctly prepared with no copper directly at the surface of the tin coating as this will decrease solderability very quickly. Copper and tin have a very dynamic reaction; the diffusion between the two has a dramatic impact on shelf life and solderability.
Just like silver, tin is mainly defined by the control of the printed board process although the assembly operation can have an impact on the solderability. The key to the process is eliminating the copper/tin intermetallic forming at the surface of the coating. Now with <1um coating and a less porous finish this provides a more stable coating for assembly. It is being used for boards that also incorporate press fit connectors as this does benefit the insertion forces with a degree of lubricity.
Immersion tin has been used in the industry for many years as a solderable coating but only for very limited printed board prototyping applications. Previously the solderability was very limited - measured in days rather than months. Recent developments have seen considerable improvements which are well defined in IPC 4554 “Specification for Immersion Tin Plating Printed Circuit Boards”. Control of the thickness is an issue and cannot be measured with microsection analysis, however the use of X-Ray Fluorescence XRF has proved successful and its is documented in the IPC standard.
Concern has been shown over “Tin Whisker” formation and, yes, it can occur, but through proper plating controls this should not occur. The finish is not ideal for low resistance mechanical contacts or for wire bonding like some alternative coatings. Like any finish that is silver or tin in colour, people like it as it blends in with the solder even if it is not coated with a solder during reflow or wave soldering.
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