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PCB surface finishes is a dynamic, demanding, and ever-evolving domain. With all the immersion-based coatings available in the market, printed circuit board manufacturers have produced consistent quality circuit boards. 

Immersion coated PCB surface finish uses a chemical process that involves depositing a flat metal layer on copper traces. The coating’s flatness makes it an ideal choice of surface finish for boards with small components. 

Immersion Tin is the least expensive of all types of coatings. It is very economical; however, it does come with certain shortcomings. One of the main drawbacks of immersion Tin is, once you deposit the tin onto copper, it will begin to tarnish. This also means if you wish to avoid low quality solder joints, it is imperative to do your soldering activity within 30 days. 

In case you are expecting higher production volumes, it is not an issue. Additionally, if you are manufacturing large batches of printed circuit boards quickly, you can avoid the tarnishing phenomenon. 

However, in you are not producing high volumes, it is probably better for you to choose an alternative surface finish such as immersion silver. 

Immersion Tin as an Alternative

As the name explains, this surface finish uses a very thin metallic layer of tin over the copper layer of your circuit board. Immersion Tin is a lead-free alternative for PCB surface finish and is capable of producing consistent flat surfaces, which is cost-efficient as well as solders well. 

The appearance of the thin layer is typically white. Therefore, this type of immersion finish is also known as “white tin” in the industry. PCB manufacturers apply this coating on copper via an Electroless chemical bath. 

The immersion Tin PCB surface finish protects the copper from oxidation. This protection lasts the entire shelf-life of the printed circuit board, which is another benefit of immersion Tin finish. 

However, it does require extremely careful handling throughout the circuit board assembly process. Due to this process being prone to damage caused during handling, it is better to establish an entire process that can help minimize PCB handling errors and mishaps.

Another problematic aspect of using immersion Tin is the strong affinity copper and tin have for each other. It ends up allowing the two metals to diffuse into one another, which may result in the formation of small strands known as “tin whiskers.”

These tiny strands of diffused tin can cause shorts in the circuits and reduce your PCB’s solder joints quality. This can adversely impact the performance and shelf life of your printed circuit board. 

Advantages and Disadvantages of Immersion Tin

Like any other finishes, immersion Tin PCB surface finish has its own sets of benefits and shortcomings. Let us have a look at them. 

Pros

  • It is a lead-free surface finish
  • It is highly reliable and produces fine quality
  • It produces planar or flat surfaces
  • One of the most cost-effective PCB surface finishes available in the market
  • It is a top choice when it comes to “Press Fit Pin Insertion”
  • It is re-workable

Cons

  • It is not a viable solution for PTH
  • It uses Thiourea in the process, which is a known carcinogen 
  • It is not ideal for multiple re-flow and assembly processes 
  • Known for the formation of tin whiskers
  • It could cause damage to solder mask
  • The immersion coating can be easily damaged due to handling mishaps and errors 
  • It is difficult to measure the thickness of the surface finish 

Final Thoughts

Although immersion Tin is a good alternative for lead-free surface finish, you will require decent storage conditions for the circuit boards. Additionally, it is best not to store them for more than six months, or else you will start noticing the formation of tin whiskers. 

If you have any questions about what you’ve just read or anything else related to your PCB assembly, feel free to reach out to us today. We’re here to help!

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Nova is a full-service printed circuit board assembler located in Denver, Colorado.