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Finishing coating: immersion silver

Immersion silver (Imm Ag) is a special type of finishing coating resulting from the precipitation of a thin silver layer on a copper surface of the contact areas. It is suitable for the production and assembly of prototypes/serial batches of microwave products.

Forming a special silver-tin-lead alloy on the copper layer during the soldering process, the material allows to make reliable connections for BGA components. Features: high surface conductivity and no nickel.
— Price affordability. Since the total thickness of the coating does not exceed 0.5 μm, the costs when applying immersion silver are insignificant.

— Durability. The service life of Imm Ag exceeds that of immersion gold and tin. With proper storage and barrier protective layers, the shelf life increases up to 12 months.

— High solderability characteristics. The application of different types of multiple soldering is possible. The finishing coating is suitable for boards with a small pitch, small/BGA components. It is possible to use multiple soldering cycles.

— Attractive appearance. The surface of printed circuit boards becomes smooth and shiny.
Finishing coating Imm Ag requires delicate handling: it is necessary to use gloves and preserve the integrity of special packaging. At the stages of assembly of printed circuit boards, there are technological limitations on the range of operating regimes — there is no possibility of one-sided stopping up the holes, the use of peeling off masks. Loss of colour and the formation of microvoids, caused by the violation of the mechanical integrity of the solder joints (like black contact areas) are liable to occur.

Reasons for color distortion
During assembling, storing or soldering the coating, the change the original color is possible. This feature does not contribute to the change in the technological operation characteristics and is caused by the influence on the composition of the immersion silver of sulfates/chlorides contained in the air.
Overheating of the protective organic layer, excess light exposure during coating, fat fingerprints — all this also causes the tarnishing of Imm Ag. To prevent the appearance of yellowness, the finishing coating is additionally worked with antioxidants.

Silvering process
The process of producing Imm Ag coating is like ENIG: the stage of cleaning a printed circuit board → chemical copper plating → immersion in a bath with silver (for 1−4 minutes). In this case, a preservative antioxidant organic compound is applied with silver to prevent oxidation and displacement of the silver layer. The average time of formation of a uniform Pb/Sn/Ag coating on the copper surface is 35 minutes.
Note that the thickness of the finishing coating can be adjusted using X-ray fluorescent equipment. There is the possibility of the application of immersion silver is possible in horizontal/vertical way.
Technology opportunities
Period of storage, months
Solderability quality
Multiple solderability
Availability of gold welding process compatibility
Availability of aluminum welding process compatibility
Finishing coating thickness, μm
Optimum pitch of landing areas, mm
Process temperature characteristics, ˚С
Torsion, bending and dimensional stability problems
The state of the coplanarity of the contact pads
Complanarity state of landing areas
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