Printed circuit boards are a maze of intricate interconnections terminating at exterior metal pads or in holes designed to accommodate component leads soldered onto or into them. However, the plated copper pads and holes need an interface to facilitate the soldering process and protection against oxidation and contamination. To provide this protective interface, PCB manufacturers rely on surface finishes applied to the board during fabrication. Two of the most common choices of surface finishes are hot air solder leveling (HASL) and electroless nickel immersion gold (ENIG). Consider the following highlights:
Considerations for HASL and ENIG |
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| Hot Air Solder Leveling (HASL) | Electroless Nickel Immersion Gold (ENIG) |
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Beyond the surface, there are additional considerations between hot air solder leveling and electroless nickel immersion gold and how they compare. With a better understanding of these two surface finishes, electronics manufacturers can make the right choice between HASL vs. ENIG for their products.
Hot Air Solder Leveling Surface Finish
A circuit board undergoing hot air solder leveling will proceed through the following steps:
- Technicians clean and apply flux to the PCB surfaces.
- A vertical dip or horizontal spray coats the PCB in molten solder.
- Hot compressed air jets (“air knives”) remove the excess solder.
Due to its thickness, HASL doesn’t give the best uniformity in its application on a circuit board. However, passing PCBs through a horizontal process rather than a vertical one produces a smoother surface finish due to the natural effects of gravity. Smaller components and fine-pitch parts are also not recommended for HASL-finished boards due to the thickness of the surface finish. Equipment manufacturers also need to be aware that due to the heat involved during the HASL application, circuit boards may experience delamination and other thermal stress-related issues.
HASL is a common choice thanks to its simplicity and low cost, making it a dependable surface finish. However, with the industry moving away from lead-based solder, the HASL process has lost popularity to other RoHS-compliant processes such as ENIG. This trend contrasts with the development of lead-free hot air solder leveling (LF-HASL), which is swiftly gaining popularity among manufacturers.
Electroless Nickel Immersion Gold Surface Finish
The process for applying an ENIG surface finish to a printed circuit board is much more complex and involved than the HASL process:
- The PCB is pre-treated with cleaning, micro-etching, and activation chemicals. Each of these steps is critical as a mistake can cause problems down the line, resulting in many scrapped boards.
- Electroless nickel is applied to the PCB through immersion to provide a barrier between the copper and the gold, preventing the gold from diffusing into the copper. Once completed, technicians rinse off excess nickel.
- Technicians immerse the PCB in a gold solution bath, where the interaction with the nickel deposits a thin layer of gold on the PCB.
- Finally, a wash rinses excess gold, and the PCB dries before a final inspection.
ENIG provides a flat finish of uniform thickness, excellent corrosion protection, and great solderability to the circuit board. This surface finish is perfect for small devices, fine-pitch parts, and wire-bonded parts and can go through multiple reflow cycles for assembly steps requiring repeated passes.
However, due to the nickel and gold and the multiple processes, ENIG is a more expensive process than other surface finishes like HASL. Electroless nickel immersion gold is also difficult to inspect visually during manufacturing due to the nickel and is not suitable for high-temperature applications. Additionally, ENIG is susceptible to “black pad,” a defect that can occur during its application that will hinder the solderability of the finished circuit board.
HASL vs. ENIG And Deciding The Most Appropriate Process
Hot air solder leveling surface finish has been in use for a long time, and it is a proven and reliable process. It is widely available, reworkable, and less expensive than ENIG. However, with its lead-based solder, HASL is not RoHS compliant. The lead-free version of HASL is necessary to avoid this limitation, but it may not be as readily available from all circuit board fabricators. HASL is also not recommended for fine-pitch components.
Electroless nickel immersion gold, on the other hand, is extensively used in aerospace, medical, and other industries that require high-reliability circuit boards. It is lead-free, making it an obvious choice for environmental concerns and RoHS requirements, and components of any size and pin-pitch are solderable to it. However, ENIG has a higher cost.
There are many other considerations to remember when selecting the best surface finish for a circuit board. The engineering team at VSE is ready to answer your questions and help you decide between HASL vs. ENIG surface finishes.
