A circuit board that will be surface-finished with hot air solder leveling will go through the following steps:

·  Technicians clean and apply flux to the PCB surfaces.

·  Molten solder coats the PCB after dipping it vertically in a solder bath or spraying it horizontally.

·  Once coated, hot compressed air jets (colloquially known as “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, again 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.

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HASL has been used for a long time thanks to its simplicity and low cost, making it a dependable surface finish. But with the industry moving away from lead-based solder, the HASL process lost popularity to other RoHS-compliant processes such as ENIG. This trend reverses with the development of lead-free hot air solder leveling (LF-HASL), swiftly gaining popularity among manufacturers. Next, we’ll look at electroless nickel immersion gold to see how it stacks up when comparing HASL vs. ENIG.

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 of the PCB and the gold, preventing it 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 when compared to 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, Deciding Which Process is Best

Hot air solder leveling surface finish has been in use for a long time, and it is a proven and reliable process. It is also widely available, reworkable, and less expensive than ENIG. However, with its lead-based solder, HASL is not RoHS compliant. To avoid this limitation, the lead-free version of HASL is necessary and 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, a higher cost associated with using ENIG exceeds HASL.

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