Hi All -

HASL is a great process for thru hole and surface mount assembly of .050"
pitch and greater.  It has a number of limitations that come into play with
fine pitch and ball grid array in particular.

First, the ability to precisely control solder coat thickness is at the mercy
of the product itself.  Much of the overall variation in thickness, solder
crown height and solder crown position on the pads is a product of turbulence
in the airflow promoted by the surface topography of the circuit board.
Additionally, the board has a tendency to warp and twist slightly during the
process throwing in yet another set of variables.  No matter how good the
process equipment, and how stable the process, there is a relatively broad
range of performance which must be expected from HASL.

As previously mentioned, if the airflow is cranked up high enough to perfectly
level the pads (stripping the crowns to nil), you often end up with a product
that has little more than a copper /tin intermetallic layer left.
Solderability of this type surface is marginal at best, even with aggressive
fluxes.   It's storage life is unpredictable, even with moisture barrier bags,
etc.

For these reasons, many of us are looking seriously at alternate finishes,
including the white tins, immersion gold over nickel, flash gold over nickel,
etc.

To the best of my knowledge, those specifications which listed specific
thicknesses for solder coating have largely been abandoned in favor of a
solderability requirement as noted in IPC-6012 and ANSI/J-STD-001.

Regards - Kelly

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