Open The Pod Bay Doors, HASL
Does anyone use HASL (Hot Air Surface Leveling) anymore? It's also known as HAL.
Prior to the RoHS days, HASL was probably the most common surface finish. You can get it lead free, but most boards seem to use immersion silver or ENIG (Electroless Nickel Immersion Gold). HASL has traditionally come at a lower cost than those other two finishes, but immersion silver can generally be found at the same price now.
One of the chief disadvantages of HASL these days, is the lack of planarity on the surface. (Note the bumps on the BGA land pattern in the image on the right.) With thru-hole or large components, an uneven surface doesn't matter so much. With the increasingly smaller BGAs and QFNs, however, surface irregularities can cause big problems.
Both Immersion silver and ENIG have nice flat surfaces. OSP (Organic Surface Preservative) has a pretty flat surface too, but it's not used much except in high volume consumer goods or specialized applications.
Oh, the pain! Save me, William.