5 Types of PCB Surface Finishes: Is One of Them Right For ...
Choosing the surface finish is an essential step in the design of your printed circuit boards.
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The PCB surface finish helps protect copper circuitry from corrosion. It also provides a solderable surface for your components. There are a number of factors to consider, including:
- The components you use
- Your expected production volume
- Your requirements for durability
- Environmental impact, and
- Cost
Below are 5 types of PCB surface finishes, along with their benefits and drawbacks.
Finish Type #1 – Hot Air Solder Leveling (HASL)
“Hot Air Solder Leveling” is the least expensive type of PCB surface finish.
It is widely available and very economical. The board is dipped in molten solder and then leveled off with a hot air knife. If you are using through-hole or larger SMT components, HASL can be effective. However, for SMT components smaller than 0805 or SOIC, it's not ideal.
The surface is not completely level, which can cause issues with smaller components. The solder used is typically Tin-Lead, meaning it isn't RoHS compliant. If reducing lead is crucial, consider choosing lead-free hasl instead.
Advantages:
- Excellent solderability
- Inexpensive / Low cost
- Allows large processing window
- Long industry experience / well-known finish
Disadvantages:
- Difference in thickness/topography between large and small pads
- Not suited for < 20mil pitch SMD & BGA
- Bridging on fine pitch
- Not ideal for HDI products
Finish Type #2 – Lead-Free HASL
Lead-Free HASL is similar to standard HASL, but with an obvious difference – it doesn’t use Tin-Lead solder.
Instead, Tin-Copper, Tin-Nickel, or Tin-Copper-Nickel-Germanium may be used. This makes Lead-Free HASL an economical and RoHS compliant choice. But like standard HASL, it's not ideal for smaller components.
For boards with smaller components, immersion coatings might be a better choice. They are slightly more expensive but more suitable for this purpose.
Advantages:
- Excellent solderability
- Relatively inexpensive
- Allows large processing window
- Multiple thermal excursions
Disadvantages:
- Difference in thickness/topography between large and small pads
- High processing temperature – 260-270 degrees C
- Not suited for < 20mil pitch SMD & BGA
- Bridging on fine pitch
Finish Type #3 – Immersion Tin (ISn)
All immersion coatings involve a chemical process.
A flat layer of metal is deposited on the copper traces. This flatness makes it ideal for small components. Among immersion coatings, tin is the least expensive. However, it has some drawbacks.
The main drawback is that after tin is deposited onto the copper, it begins to tarnish. To avoid lower quality solder joints, you must do soldering within 30 days.
If you are expecting high production volumes, this may not be an issue. Using large batches of boards quickly can prevent tarnishing. For lower production volumes, immersion silver might serve you better.
Advantages:
- Immersion finish = excellent flatness
- Good for fine pitch / BGA / smaller components
- Mid-range cost for lead-free finish
- Press fit suitable finish
- Good solderability after multiple thermal excursions
Disadvantages:
- Very sensitive to handling – gloves must be used
- Tin whisker concerns
- Aggressive to solder mask – solder mask dam shall be ≥ 5 mil
- Baking prior to use can have a negative effect
- Not recommended to use peelable masks
Finish Type #4 – Immersion Silver (IAg)
Unlike tin, immersion silver does not react with copper in the same manner, but it tarnishes when exposed to air.
This necessitates anti-tarnish packaging during storage.
Proper packaging ensures solderability for 6-12 months. However, once the PCB is removed from packaging, it needs solder reflow within a day. For a longer shelf life, gold plating is a better option.
Advantages:
- Immersion finish = excellent flatness
- Good for fine pitch / BGA / smaller components
- Mid-range cost for lead-free finishes
- Can be reworked
Disadvantages:
- Very sensitive to handling/tarnishing/cosmetic concerns
- Special packaging required
- Short operational window between assembly stages
- Not recommended to use peelable masks
Finish Type #5 – Electroless Nickel Immersion Gold (ENIG)
Electro gold flash plating involves a thin layer of gold over electroless or electrolytic nickel.
This plating is hard and durable, offering a long shelf life, often lasting years. However, its durability and longevity make it more expensive compared to other finishes.
Advantages:
- Immersion finish = excellent flatness
- Good for fine pitch / BGA / smaller components
- Tried and tested process
- Wire bondable
Disadvantages:
- Expensive finish
- Black pad concerns on BGA
- Can be aggressive to solder mask – larger solder mask dam preferred
- Avoid solder mask defined BGAs
Choose the Right Finish for Your PCBs
Remember that in selecting your PCB finish, you need to consider component types, production volume, durability requirements, environmental impact, and cost. By weighing these factors, you can confidently pick the best option.
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