Steve, the reason that there is controversy regarding HASL thickness
specs is because the sites or lands to be soldered are different sizes.
The process reality is that geometry of the individual sites is the
primary influence on thickness control in Hot Air Leveling.

A brief overview may help.  A clean, fluxed copper surface is dipped in
molten solder (460f approx.) while the metal is still molten it is
withdrawn thru a pair of air knifes.  Each of the individual sites on
the circuit now coated with liquid eutectic metal will offer more or
less resistance to a fixed air pressure from the knifes.  For example; a
ground plane say .250" x .250" will offer less resitance to the air
leveling than a .015" x .070" QFP site that is adjacent to it.
Consequently, the QFP site will be slightly thicker and have less pull
away at the leading edge or what we call, (surface tension phenomena).

The process is repeatable given a consistent geometry.
The parties need to agree on a particular site to be measured. (critical
QFP lands make an ideal candidate.)
The location on those sites to be measured. (near the center)
The method used for measurement (XRF) x-ray flourescence

Envoila, we can now issue a spec that everyone can live with.
(somewhere around 200 microinches +/- 100 microinches)

The IPC could be more specific than "coverage and solderable" but, it
will require some cataloging of common sites.

The above, im sure was sufficiently boring as to have run off the faint
of heart.  Now, perhaps you can help me answer a question that has made
me crazy for some time;

If the goal here is pure planarity, i'm having a problem with quibbiling
over microinches on the solder coating when, the compnents can be +/-
.004"! and this is after we gob on... what, .005" of solder paste?

Can you put this into perspective for me?

Hope this helps, and thanks in advance

Mark Simmons, V-Score Central

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