Fresh PCB Nuggets: Pros and Cons of the 6 Most Common Surface Finishes

NCAB_HarryKennedy.jpgA surface finish can be defined as a coating, either metallic or organic in nature, that is applied to a PCB to ensure the solderability of the metal underneath. Most of the surface treatment dissolves into the solder paste or wave solder during the soldering process, and the solder joint is forming between the solder and the copper. One exception is ENIG/immersion gold, where the solder dissolves the thin layer of gold and forms a joint with the underlying nickel phosphorous alloy.

NCAB_0720_F1.jpgThere are only two different types of surface finishes for PCBs: organic and metal. Here are the six most common finishes on the market:

  1. ENIG
  2. HASL leaded
  3. HASL lead-free
  4. Organic solderability preservatives (OSP)
  5. Immersion Sn
  6. Immersion Ag

OSP is more like a lacquer, which prohibits oxygen from attacking the copper underneath. All others are coating metals, and they may be applied using one of two different methods: electroless or immersion.

Electroless systems work in the same fashion; they use a reducing agent inside the bath itself. This means that the metal thickness increases during the whole period that the PCB is in contact with the solution.

An immersion systems process uses a chemical displacement reaction to deposit a metal layer onto the exposed metal surface of the PCB. The base metal donates the electrons that reduce the positively charged metal ions present in the solution. The immersion layer will continue to grow; however, as the thickness of the deposit increases, the rate of deposition falls. Therefore, the process is self-limiting.

Comparing both types and all available options can quickly demonstrate the relative benefits or drawbacks. Typically, the decisive factors when it comes to selecting the most suitable finish is the end application, the assembly process, and the design of the PCB itself.

For each of the six most common surface finish, these tables illustrate the pros and cons (Tables 1–6).

NCAB_0720_T1.jpg
Table 1: ENIG pros and cons.

NCAB_0720_T2.jpg
Table 2: HASL leaded pros and cons.

NCAB_0720_T3.jpg
Table 3: HASL lead-free pros and cons.

NCAB_0720_T4.jpg
Table 4: OSP pros and cons.

NCAB_0720_T5.jpg
Table 5: Immersion Sn pros and cons.

NCAB_0720_T6.jpg
Table 6: Immersion Ag pros and cons.

Conclusion
There is much more to understand about surface finishes. Each of the six surface finishes I mentioned has a different process for application, handling recommendations, storage conditions and time, baking requirements, thickness requirements, masking limitations and design considerations and concerns. Tables 1–6 are simply an overview. We always recommend discussing surface finishes with your PCB supplier as early as possible in the design stage.

Harry Kennedy is a field application engineer at NCAB Group.

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2020

Fresh PCB Nuggets: Pros and Cons of the 6 Most Common Surface Finishes

07-23-2020

There are only two different types of surface finishes for PCBs: organic and metal. Harry Kennedy describes the pros and cons of the six most common finishes on the market.

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02-12-2020

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Fresh PCB Concepts: Create PCB Fabrication Notes for Success

01-09-2020

When NCAB Group receives Gerber files for an order, there are times Harry Kennedy cringes because there are missing pieces of information necessary to make sure the PCB is made with zero defects. As we come back to work for the New Year, Harry shares some key factors he wishes customers would consider before they send their files.

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2019

Fresh PCB Concepts: Designing a PCB for Telecom Applications

12-12-2019

Jeff Beauchamp and Harry Kennedy discuss PCBs for telecommunication applications, including key factors to consider, such as design and material considerations. They also recommend involving your PCB supplier at the time of design to help ensure manufacturability at the lowest possible cost.

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Fresh PCB Concepts: The Current Material Situation

11-11-2019

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10-23-2019

When faced with critical time-to-market situations, it is all too easy to say, “It doesn’t matter because this is just the prototype; we can fix this later.” However, if the design is perfected from the beginning, cost savings can be applied, and manufacturability can be ensured. Perhaps most importantly, the design can be adapted with reliability in mind, leaving a seamless transition from prototype to production. How do we get it right from the start?

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10-02-2019

When you’re designing a PCB, it’s standard to call out FR-4 material, but you could be holding yourself back or even exposing your board to risk by not knowing more about PCB materials. Let’s take a small look into why. What Is FR-4, exactly? Harry Kennedy of NCAB explains.

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Fresh PCB Concepts: Qualities of Medically Applied PCBs

08-26-2019

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