Immersion Tin (ImSn) PCB Finish

Immersion Tin (ImSn) is a metallic coating of ‘Tin’, which is deposited directly over the top copper layer of a PCB using an electroless chemical bath process. PCB finishes are used to protect the copper tracks and pads on printed circuit boards while in storage. Immersion Tin forms a dense uniform coating (0.8 to 1.2 micrometers) with good hole wall lubricity. Its appearance is usually White, so it is also called ‘White Tin’.

Immersion Tin is not a usual displacement process; it is self-limiting and depends on the exchange between Copper and Tin. It can only take place in the presence of thiourea, which is a carcinogen, with health implications, so the process needs to be highly controlled.

Advantages of Immersion Tin Surface Finish:

• Shelf-life: 6 months
• Easy to rework (but rework should be limited)
• Due to its planarity (flatness), suitable for fine-pitch/ BGA/ small components placement
• Not expensive
• Good for backplane panels, which are assembled using press fit pin insertion
• Offers good solderability after exposure to various thermal cycles 

Disadvantages of Immersion Tin Surface Finish:

• Not durable, sensitive to handle
• Tin Whiskering: Tin Whiskering is an internal defect developed during ‘Tin Deposition’. Due to this, some material protrudes out of the structure, creating whiskers. These ‘Whiskers’ grow outwards and create a shorting problem with the adjacent connections.
• Environmental concerns with Thiourea (carcinogen) used during processing
• Not good for PTH