Why Are Copper Busbars Required to Be Plated with Matte Tin?

Why Are Copper Busbars Required to Be Plated with Matte Tin?

In our impression, tin plating is usually shiny. However, when used in battery packs, copper busbars are often specified to be plated with matte tin. So, what is matte tin, and why is it used?
I. Purpose of Tin Plating on Copper Busbars
The base material copper of busbars used in power battery packs and electrical equipment is prone to oxidation in air, forming CuO/Cu₂O oxide layers. This leads to a sharp increase in contact resistance, and the oxide layer also reduces welding reliability.
The functions of tin plating mainly include:
  1. Anti-oxidation: The dense tin layer formed after plating isolates air, preventing oxidation of the copper base material, thereby extending the service life of the copper busbar.
  2. Stable Low Resistance: Tin has excellent electrical conductivity, and its surface oxide film (SnO₂) is easily broken, allowing it to maintain a low contact resistance state over the long term.
  3. Ease of Welding and Assembly: Tin has a low melting point (only 231.9°C), facilitating the welding and fixation of busbars to terminals.
  4. Corrosion Resistance: The tin layer protects the copper base material from corrosion, making it suitable for complex environmental conditions such as humidity and salt spray.
Copper Busbar Topic: Why Are Copper Busbars Required to Be Plated with Matte Tin?
In our impression, tin plating is usually shiny. However, when used in battery packs, copper busbars are often specified to be plated with matte tin. So, what is matte tin, and why is it used?
I. Purpose of Tin Plating on Copper Busbars
The base material copper of busbars used in power battery packs and electrical equipment is prone to oxidation in air, forming CuO/Cu₂O oxide layers. This leads to a sharp increase in contact resistance, and the oxide layer also reduces welding reliability.
The functions of tin plating mainly include:
  1. Anti-oxidation: The dense tin layer formed after plating isolates air, preventing oxidation of the copper base material, thereby extending the service life of the copper busbar.
  2. Stable Low Resistance: Tin has excellent electrical conductivity, and its surface oxide film (SnO₂) is easily broken, allowing it to maintain a low contact resistance state over the long term.
  3. Ease of Welding and Assembly: Tin has a low melting point (only 231.9°C), facilitating the welding and fixation of busbars to terminals.
  4. Corrosion Resistance: The tin layer protects the copper base material from corrosion, making it suitable for complex environmental conditions such as humidity and salt spray.
II. Necessity of Matte Tin Plating
  1. Bright Tin Pros and Cons:
    • Advantages: Bright tin plating involves adding brighteners to the plating solution, resulting in a finer crystallization process and smaller grain size (generally <2μm). The coating surface is bright, smooth, with excellent solder wettability, and is cost-competitive. Suitable for scenarios where appearance matters.
    • Disadvantages: Due to the brighteners, the bright tin coating contains more organic substances, leading to poorer solderability. It is prone to tin whisker formation under high temperatures. Excessive tin whisker growth can cause short circuits in the busbars, affecting the safety of the power battery pack.
  2. Matte Tin Pros and Cons:
    The matte tin coating surface has a matte or satin effect, with relatively rough crystallization and larger grain size (generally 4-5μm).
    • Advantages: Matte tin offers better solderability and tin whisker resistance compared to bright tin. Due to its coarser crystallization, it has better high-temperature resistance, helping to maintain structural integrity under heat. The coarser grains result in a larger actual contact area, leading to lower contact resistance, better wear resistance, and stronger anti-deformation capability. The risk of tin whiskers is significantly reduced, making it more suitable for the vibrating conditions during new energy vehicle operation and power battery scenarios requiring high connection stability and safety.
    • Disadvantages: The coating surface is matte, with lower gloss and relatively rough crystallization. The cost is relatively higher.
Copper Busbar Topic: Why Are Copper Busbars Required to Be Plated with Matte Tin?
In our impression, tin plating is usually shiny. However, when used in battery packs, copper busbars are often specified to be plated with matte tin. So, what is matte tin, and why is it used?
I. Purpose of Tin Plating on Copper Busbars
The base material copper of busbars used in power battery packs and electrical equipment is prone to oxidation in air, forming CuO/Cu₂O oxide layers. This leads to a sharp increase in contact resistance, and the oxide layer also reduces welding reliability.
The functions of tin plating mainly include:
  1. Anti-oxidation: The dense tin layer formed after plating isolates air, preventing oxidation of the copper base material, thereby extending the service life of the copper busbar.
  2. Stable Low Resistance: Tin has excellent electrical conductivity, and its surface oxide film (SnO₂) is easily broken, allowing it to maintain a low contact resistance state over the long term.
  3. Ease of Welding and Assembly: Tin has a low melting point (only 231.9°C), facilitating the welding and fixation of busbars to terminals.
  4. Corrosion Resistance: The tin layer protects the copper base material from corrosion, making it suitable for complex environmental conditions such as humidity and salt spray.
II. Necessity of Matte Tin Plating
  1. Bright Tin Pros and Cons:
    • Advantages: Bright tin plating involves adding brighteners to the plating solution, resulting in a finer crystallization process and smaller grain size (generally <2μm). The coating surface is bright, smooth, with excellent solder wettability, and is cost-competitive. Suitable for scenarios where appearance matters.
    • Disadvantages: Due to the brighteners, the bright tin coating contains more organic substances, leading to poorer solderability. It is prone to tin whisker formation under high temperatures. Excessive tin whisker growth can cause short circuits in the busbars, affecting the safety of the power battery pack.
  2. Matte Tin Pros and Cons:
    The matte tin coating surface has a matte or satin effect, with relatively rough crystallization and larger grain size (generally 4-5μm).
    • Advantages: Matte tin offers better solderability and tin whisker resistance compared to bright tin. Due to its coarser crystallization, it has better high-temperature resistance, helping to maintain structural integrity under heat. The coarser grains result in a larger actual contact area, leading to lower contact resistance, better wear resistance, and stronger anti-deformation capability. The risk of tin whiskers is significantly reduced, making it more suitable for the vibrating conditions during new energy vehicle operation and power battery scenarios requiring high connection stability and safety.
    • Disadvantages: The coating surface is matte, with lower gloss and relatively rough crystallization. The cost is relatively higher.
III. Differences Between Bright Tin and Matte Tin
Comparison Dimension
Bright Tin
Matte Tin
Appearance
Mirror bright, highly reflective
Matte, non-reflective, uniform and fine surface
Crystal Structure
Fine, dense grains, orderly arrangement
Coarse grains, no obvious directional arrangement
Key Performance
Excellent solder wettability, outstanding appearance advantage
Excellent wear resistance, vibration resistance, low tin whisker risk
Application Scenarios
Static connections with low vibration and appearance requirements (e.g., fixed busbars inside equipment)
Power connections requiring long-term stable operation under high vibration
IV. Process Differences
The basic processes for both (degreasing → acid pickling → pre-tinning → main plating → post-treatment) are essentially the same. The main differences lie in the additive system and post-treatment.
  1. Bright Tin:
    • Brighteners containing sulfur organic compounds and nitrogen heterocyclic compounds are added to the main plating bath. These act via adsorption to refine grains and promote orderly arrangement, forming a bright coating. Current density is typically 1-3 A/dm².
    • Post-treatment like polishing and cleaning is usually required to remove minor defects and scratches, improving brightness and smoothness.
  2. Matte Tin:
    • Matte Tin: Defoamers (like fatty acid derivatives) and grain refiners are added to the main plating bath. These inhibit grain growth and disrupt directional alignment, forming a matte coating. Current density (2-5 A/dm²) is slightly higher than for bright tin, promoting denser crystallization.
    • Complex post-treatment is generally not required. After plating, simple cleaning suffices.
V. Key Test Items:
  1. Coating Thickness: Tin layer thickness for power battery busbars is typically required to be 5-15μm, usually tested by X-ray fluorescence spectrometer.
  2. Adhesion: Must pass bend test (180° bend without coating peeling) and tape peel test. Depending on coating thickness, cross-cut test or pull-off test may be used.
  3. Corrosion Resistance: Neutral salt spray test typically required for 48-96 hours. After testing, there should be no rust, exposure of copper base, or coating blistering.
  4. Tin Whisker Test: High-temperature aging at 120°C for 1000 hours, observe under microscope for no significant tin whiskers.
  5. Solderability: Solder dip test at 235°C for 5 seconds, requiring wetting area ≥90%.
  6. Contact Resistance: Tested by four-point probe method. Initial contact resistance should be ≤ TBD mΩ (according to project requirements), with no significant increase after aging.
VI. Summary
In conclusion, bright tin plating for copper busbars emphasizes appearance and static connections, while matte tin is more suitable for application scenarios like bolted connections of copper busbars within power battery packs.
Today's article and reference materials have been uploaded to the Knowledge Planet.

 

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