Either ENIG or OSP are the best PCB finishes simply because they provide the flattest surface possible. HASL has a bumpy surface, while immersion silver poses corrosion issues and shelf life limitation. Both are not recommended as a result.
ENIG or gold is the best finish, not only because it has a flatter finish, but also because it is considerably more robust during BGA rework. Plus, it is cost-effective today, although a while back gold was slightly more expensive. Today, gold and HASL are about the same price.
Factors to consider when choosing a surface finish are:
- Shelf life
- Global availability
Not all finishes are equal, aside from cost, function is a primary concern not
merely for the topography for fine pitch devices but also for loss characteristics.