Immersion Tin PCB Surface Finish

When it comes to the surface finish of printed circuit boards, a variety of methods can be employed. However, these techniques might prove puzzling for newcomers, especially concepts like “spray tin,” “immersion silver,” and “immersion tin” that can easily cause confusion. This article aims to provide a comprehensive exploration of the immersion tin process, shedding light on its intricacies and delving into the reasons behind its selection as a surface treatment technology.

What Is Immersion Tin Finish And Its Principle

The immersion tin is a widely employed technique in the PCB surface finish that serves both as a protective measure for the metal traces. This process involves immersing the PCB’s metal surface in a chemical solution containing tin, resulting in the formation of a uniform and thin tin layer on the metal surface. This layer offers protection while also simplifying subsequent PCBA processes, such as SMT assembly, and enabling more precise chip processing.

The immersion tin surface finish is sensitive and may affect the overall operation of the board if not handled properly. In the initial stages of this technology, concerns regarding tin whiskers and tin migration arose after immersion, presenting potential risks. Manufacturers have addressed these challenges by introducing organic additives to the tin plating solution, leading to a granular tin layer structure that overcomes these issues. This improved process demonstrates excellent thermal stability and solderability, making it environmentally friendly and replacing lead-tin alloy electroplating.

In the process, a chemical bath is used to deposit a tin metal coating onto the copper layer’s surface through electroless plating. The immersion tin thickness is approximately 1 to 3 microns, contributing to enhanced hole wall lubrication. Unlike a conventional displacement process, this process is autoreductive and relies on an exchange between tin and copper. The presence of thiourea, a carcinogenic substance with health implications, is a crucial aspect of this process and requires stringent control.

The working principle of immersion tin plating is to alter the chemical potential of copper ions to generate tin ions in the plating solution, initiating a chemical substitution reaction—an electrochemical process.

Reduced metal is deposited onto the copper substrate to create a tin coating. Metal complex compounds adsorbed onto the tin immersion layer catalyze the reduction of tin ions to metallic tin. Consequently, tin ions continue to be reduced to metallic tin based on this equation:

                                       2Cu + 4TU + Sn2 → 2Cu+(TU)2 + Sn

Tin immersion represents a valuable method for treating the copper surface of PCB. The outcomes of this process do not contaminate solder and do not generate additional polyester layers on solder mask surfaces. This technology is applicable to hole structures and sizes as well as connector areas of various board configurations.

Chemical reaction principle of PCB immersion tin

Immersion Tin Plating Process

immersion tin process

  1. Initial Cleaning: During this stage, the circuit board is immersed in a cleansing solution to eliminate impurities, grease, and dust particles that might have accumulated during the manufacturing process.
  2. Micro-Etching: The utilization of an acid solution for micro-etching enhances the surface texture of the circuit board, bolstering the adhesion capability of copper and tin.
  3. Pre-Dipping: Prior to the immersion tin PCB finish process, a rinse with an acid solution is performed to prevent copper surface oxidation and establish a robust foundation for the subsequent immersion tin procedure.
  4. Tin Immersion: The circuit board is immersed into a solution containing tin metal cations, initiating an electroless reduction sequence. Within this process, tin ions precipitate the reduction of copper metal, forming a uniform and thin layer of tin.
  5. Post-Cleaning: To eliminate any residual electroplating salt, it is recommended to employ a warm water wash, mitigating the risk of leaving unsightly marks upon board drying.
  6. Board Drying: Placing the board within an oven or utilizing forced air drying expels surface moisture that might have been absorbed.

Should You Opt for Immersion Tin Plating Process

  • In comparison to alternative processing techniques, the immersion tin plating process offers distinct advantages, particularly in mitigating issues associated with PCB black pad occurrence.
  • The immersion tin coating ensures ample clearance and establishes dependable connections for components, rendering it highly compatible with delicate components like microchips, QFN packages, and BGAs.
  • The utilization of more economical materials enhances its cost-effectiveness, especially when employed for large-scale manufacturing.
  • The tin layer exhibits a consistent and level structure, effectively safeguarding the copper layer against oxidation, making it exceptionally well-suited for diverse industrial applications.
  • This process aligns with RoHS compliance standards by employing fewer chemicals during its execution.
  • Furthermore, its rework capability adds to its appeal, enabling retention for extended periods ranging from six months to a year.
  • It is noteworthy, however, that while immersion tin is suited for PTH, the formation of tin plugs within PTH pores during processing could potentially impact connection and signal pathway quality.
  • During the deposition of tin, there is a possibility of generating tin whiskers that extend to the periphery, potentially leading to unintended short circuits. For this reason, TechSparks strongly advises conducting necessary inspections subsequent to the processing phase.

Immersion Tin VS Immersion Silver 

CharacteristicHot Dip SolderingElectroless Tin Plating
Process PrinciplePCB immersed in molten tinElectrochemical reaction in electrolyte
Coating ThicknessRelatively thick, usually >1μmThin, can be controlled at 0.1-1μm
Surface SmoothnessGenerally lower, may have depressionsTypically good, smooth surface
SolderabilityGood, suitable for through-hole solderingGood, suitable for surface-mount assembly
Corrosion ResistanceRelatively low, prone to oxidationDecent, higher corrosion resistance
Solder DriftSignificant drift possibleMinimal drift, precise positioning
CostRelatively lowHigher
Environmental ImpactRequires waste solution and emissions treatmentRelatively more environmentally friendly
Application RangeSuitable for general electronic productsSuitable for high precision and demanding products

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