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Finishing coating: Immersion Tin

Imm Sn (immersion tin) is one of the main finishing coatings corresponding to the requirements of the European Union RoHS directive. It is an alternative to HASL.

The basic technological thickness of the tin layer is 1 μm (Imm Sn 0.5−0.8 micron and a barrier protective layer 0.08−0.1 micron).
The coating requires delicate handling — gloves should be used for assembling. When making solder mask jumpers (less than 5 mm), corrosion may occur. During the application process, it is not recommended to stop up the holes on the one side, to use peeling masks and lead-containing solders.

Immersion tin, unlike immersion gold/silver, is less widespread and is rarely used in the production of printed circuit boards. In addition, such a finish contributes to an increase of intermetallic compounds and the occurrence of short circuits on the landing areas relating to crystalline whiskers.

Features of technology development

At the beginning of the development of immersion tin technology intermetallic compounds, Cux Sny, significantly limited the production process. At that time, the solderability disappeared after 14−30 days with the application of coating. This was caused by the rapid spread and absorption of a thin tin layer by intermetallic compounds.

These nuances were eliminated by the application of the barrier layers — organic polymer compounds with good electrical conductivity and catalytic properties between tin and copper.

Tinning process

The process of applying immersion tin is like Imm Ag. The only difference is the time interval of exposure of the printed circuit board in the tin bath (up to 15 minutes). In addition, the technology is complemented by the stages of washing the electrolyte and testing the quality of solderability of the finishing coating (including X-ray fluorescence methods). The duration of the tinning process: 35 min.
— The possibility of using lead free methods. Immersion tin provides good wettability with lead-free methods for production of a protective finishing coating and has a simple precipitation process.

— Smoothness and flatness of the surface, due to them there is a possibility of soldering components of small size and BGA, insertion of dismountable pins (at the pressing-in stage) and printed circuit boards with a slight pitch.

— The durability of the finishing coating. In case it is properly stored, the solderability is kept up to 8 months.

— Good solderability indexes, they are considerably greater than that of ENIG (in relation to lead-free solders). Multiple temperature changes are possible.

— Insignificant probability of crystalline whiskers formation (compared to pure tin finishing coatings), due to small thickness of tin layer.

— Price affordability — the coating does not have layers of expensive noble metals.
Technology opportunities
Period of storage, months
Solderability quality
Multiple solderability
Availability of gold unwelding process compatibility
Availability of aluminum unwelding process compatibility
Finishing coating thickness, μm
Optimum pitch of landing areas, mm
Process temperature characteristics, ˚С
Torsion, bending and dimensional stability problems
Complanarity state of landing areas
Surface cleanness state
Corrosion resistance index
Visual defects
Qualitative risks
Solder reliability index
Compatibility with lead free methods
Index of the environmental damage, safety for the human body
Cost characteristics