Both HASL and ENIG are common types of PCB finishes. Together with immersion tin, immersion silver, OSP and other methods, they help increase the longevity and high performance of the board.
Let's focus on HASL and ENIG, which are the widely used methods you will most likely have to deal with in PCB manufacturing. You will review the technical details, advantages and disadvantages of both procedures.
Hot Air Solder Leveling (HASL) is a widely used and affordable surface finish for PCBs. The process involves coating a PCB with molten solder, then using a hot air knife to remove excess solder and smooth out the surface.
HASL is not RoHS compliant due to the use of tin, but a lead-free version is available. It is a good choice for high-reliability applications and the production of low to medium-cost electronics.
Some benefits of HASL include excellent solderability, availability at most PCB shops, a large processing window, and ease of rework and repair. It also has a good cost-value ratio, and exposure to high temperatures during the process can serve as a test for copper circuits.
However, HASL has some limitations. It creates an uneven surface, can make components sensitive to thermal shock, and copper pads are prone to solder bridging. It is not available for Plated Through Hole (PTH) designs, not ideal for fine pitch components, and has restrictions on extremely thin or thick plates. In addition, lead is included in the basic version and lead-free HASL is less powerful than traditional methods. It also doesn't work with row bindings and is not perfect for cases where tight tolerances are critical.
If you require high-reliability components with low scrap rates or are using Fine-Pitch technology, other surface finish options should be considered.
The Electroless Nickel Immersion Gold (ENIG) surface finish is a widely-used option, commonly found in high-end PCBs. It is popular in industries like telecommunications, consumer goods, aerospace, military, and medical, especially in applications like SMT, lead-free soldering, and BGA packages. ENIG is applied to the copper pads by a chemical process that involves electrolysis. The copper layer is first covered by a nickel layer, followed by a thin immersion gold layer.
ENIG provides a flat surface that fills in all unevenness, making it ideal for plated through hole applications, fine pitch components, and smaller components. It is a well-tested process and can be compatible with aluminum wire bonding. The finish is also lead-free and has a long shelf life, making it ideal for both high-end consumer electronics and professional-grade equipment.
However, ENIG is prone to black pad syndrome and is one of the most expensive finishes. It can also be aggressive to soldering, so solder resist dams are preferred. The finish is not reworkable or repairable, making it vulnerable to damage by ET and susceptible to dropout (RF). The process is complex and requires experienced staff, and therefore, not a cost-effective solution.
In summary, ENIG is a popular surface finish for high-end PCBs in various industries, providing a flat surface, lead-free and ideal for plated through hole applications, fine pitch components, and smaller components. However, its high cost, vulnerability to black pad syndrome, and difficulty in repair and rework make it less ideal for cost-effective solutions.