PCB Surface Finishes Breakdown
PCB surface finishes on a bare board may seem like an insignificant decision at the buying step of your product development but understanding the variations and their intricacies may prevent problems down the line.
The surface finish of a printed circuit board does mainly two things. First, it helps protect the copper circuitry from corrosion. Secondly, it creates a solderable surface for your PCBA components. As such, there are a few items to consider, which include:
- Which components you are utilizing
- Your PCB’s expected production volume
- Your board’s requirements for environmental and general durability
- Environmental impact
- Budget
The 5 most common types of PCB surface finishes are discussed below along with their pros and cons.
Hot Air Solder Leveling (HASL)
The most commonly utilized PCB surface finish is HASL or “Hot Air Solder Leveling”. This is because it has been around forever and is the most economical. However, new technologies are making better surface finishes the same if not very close in pricing to HASL.
In this process, the PCB is dipped in molten solder and then leveled off with a hot air knife, hence the name. If your board is using through-hole or large SMT components, HASL can work with no problem. However, if your board utilizes SMT components smaller than 0805 or SOIC, it may not be an ideal surface finish.
This surface finish is not completely flat, so it can cause issues with smaller components. The solder utilized in this process is normally Tin-Lead. That means that it also isn’t RoHS compliant. If your project requires RoHS or your company wants to reduce the amount of lead you use, Lead-Free HASL may need to be spec’d.
Pros:
- Very good solderability
- Inexpensive / Lower cost
- Allows for a large processing window
- Long industry experience / well-known finish
Cons:
- Not a flat surface as well as discrepancies in thickness/topography between large and small pads
- Not suited for smaller more dense components (< 20mil pitch SMD & BGA)
- Bridging on fine pitch
- Not ideal for HDI products
Lead-Free HASL
Lead-Free HASL is similar to regular HASL, but without using Tin-Lead solder.
Lead-free HASL instead uses Tin-Copper, Tin-Nickel or Tin-Copper-Nickel Germanium in their process. This allows Lead-Free HASL to be an economical and RoHS compliant choice. Similar to standard HASL though, it is not ideal for smaller SMT components.
PCBAs that utilize high density/fine-pitch components may be better off using immersion coatings. They are sometimes slightly more expensive but are more suitable for this purpose and may lead to less problems down the line.
Pros:
- Very good solderability
- Inexpensive / Lower cost
- Allows for a large processing window
- Long industry experience / well-known finish
- Multiple thermal excursions
Cons:
- Not a flat surface as well as discrepancies in thickness/topography between large and small pads
- High processing temps in the 260-270 degrees C range
- Not suited for smaller more dense components (< 20mil pitch SMD & BGA)
- Possible bridging on fine pitch
Immersion Tin (ISn)
In ISn PCB fabrication, a chemical process is utilized.
This process requires a flat layer of metal being deposited onto the copper traces. The flatness of the coating makes it good for small components. Tin is one of the budget friendly types of immersion coating. Although it is an budget friendly choice, it has some drawbacks.
The largest issue is that after the tin is deposited onto the copper it begins to tarnish. As a result, if you want to avoid low quality solder joints, you need to have your components soldered on within 30 days.
In high volume production, this may not be a problem. If you are using up a large amount of boards quickly, tarnishing can also be avoided. However, if production volume is low or you want to keep bare boards in stock, it may be smarter to utilize a coating like immersion silver.
Pros:
- Soldering areas are flat
- Suitable for fine pitch / BGA / smaller components
- Reasonable price for lead-free finish
- Press fit suitable finish
- Good solderability even after multiple thermal cycles
Cons:
- Sensitive to handling – gloves should be utilized
- Tin whisker issues
- Aggressive to solder mask – solder mask dam shall be ≥ 5 mil
- Baking prior to use can diminish solderability
Immersion Silver (IAg)
So on one hand, immersion silver does not react with copper the way that ISn does. On the other hand, it tarnishes when it is exposed to air. As a result, all IAg PCBs should be stored in anti-tarnish packaging when in storage and while handling.
When these kinds of PCBs are stored in proper packaging, they can be reliably soldered for 6-12 months. However, once the PCB is removed from its packaging, it will need to go through solder reflow within a day. Shelf life can be improved with gold plating.
Pros:
- Flat surface
- Great for small, fine pitch, and BGA components
- Mid-range cost for lead-free finish
- Boards can be reworked
Cons:
- Possible handling/tarnishing problems
- Special packaging to prevent tarnishing
- Small window of optimal solderability during assembly process
Electroless Nickel Immersion Gold (ENIG)
My personal favorite PCB surface finish is ENIG. The process of electro gold flash plating utilizes a thin layer of gold over electroless or electrolytic nickel.
Gold plating is hard and durable. This allows it to have a long shelf life, possibly lasting for years from a good fabricator. The material, process, and reliability tend to make ENIG boards more expensive than the other finishes. However, we’ve seen many board houses begin to move a large portion of their processes to ENIG making it the same price if not very close to the others.
Pros:
- The most flat surface finish
- Good for small, fine pitch, and BGA components
- Reliable fabrication process and procedures
- Wire bondable
Cons:
- Sometimes a more expensive finish
- Black pad concerns with BGA
- Signal Loss (RF)
- Best to avoid solder mask defined BGA’s
Choose the Right PCB Surface Finishes for your Projects
Surface finish of a PCB is a crucial decision that should be taken into account prior to fabrication. Considering things like component types and production volume will be key in a smooth assembly process. Durability, environmental impact, and cost may also be factors to take into account and discuss with your team. By taking a holistic picture of your needs, only then can you pick the right surface finish for your PCBs.
Vinatronic has over 30 years of experience helping companies pick which surface finishes work best with their requirements. There are also less common surface finishes that Vinatronic is familiar with and can discuss with you those options if none of these work. Feel free to reach out if you have any fabrication, assembly, or design questions.
