Werner is quite correct but only mentioned the metallurgy, not the
ramifications for processing.

HASL is a quite obnoxious process in terms of board stress during
application, but most non-aggressive board constructs can handle it: 
The panel is plunged into a vat of molten solder and then withdrawn
while being blown off with very hot air.  The deposits are lumpy, and
start giving problems for smaller parts.  

Fused SnPb is just convection heated to slightly above the melting
point--no thermal shock.  Also the deposits are extremely even, so very
fine pitch work is no problem.  However, the normal way of achieving
Fused SnPb is to use the plated SnPb for the final etch resist.  This
means that if you have a board with a solder mask, then the SnPb will be
under the mask, which will greatly reduce its effectiveness.

Wayne Thayer

>>> [log in to unmask] 5/6/2007 9:39 pm >>>
Hello,
 
I come across QML vendors that state their finish system : HASL, Fused
Solder, while some only states : HASL. I know that HASL stands for "Hot
Air Solder Leveling", what then is fused solder? There must be some
difference between them - what are they? 
 
Thanks and regards,
Wee Mei

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