Eddie,
Concerning the thickness issue, as you declared, it is difficult to
provide a clearly defined requirement. Varying the air knife
pressure, heat, speed, and air volume most often requires more than
setting a particular gage. If thickness could be controlled, there
would be smooth surface topography. What thickness spec do we apply
- one for the peaks and another for the
valleys?
Many suppliers do not understand fluxes or how to determine their
capability. Many do not understand thermal shock/stress, solder
temperature requirements, dwell times, and contact areas. All have
problems with excess Cu oxidation after rinse/microetch - before
introduction to a process not always capable of removing such
oxidation preventing wetting to be effected. Many more do not
provide adequate training for personnel attempting to manage an
often unmanageable process - and on it goes.
I didn't say it could not be done. I said it varies day to day, etc.
This means it is difficult to do. This means some suppliers manage
processes better than others - as with all PCB fab processes. Even a
"good" supplier can have a "bad" day because the process doesn't
lend itself to controllability. HASL is a soldering process and many
suppliers do not understand soldering.
As for alternative coatings, immersion Au over immersion Ni provides
the best solder surface. Its problems exist as excessive phosphorous
intermetallic formations in the Ni that may, or may not, cause
solder joint failure in time, under stress.
Soldering is defined as a process in which two metal surfaces are
metallurgically joined, using a specified solder medium (metal
filler with a melting point below 800 degrees F.). The process is
effected by "wetting" the surfaces to be joined requiring neither
diffusion nor intermetallic growth. The effect of this process is
called a solder joint.
I focus on surfaces, difussion, and intermetallic growth. First,
surface is defined as an object's area having no depth. Second, if
intermetallic growth is excessively effected (as it often is in
HASL) the surface definition is violated. This is true when a
supplier applies excessive heat over extended time, as an example,
thereby causing excessive intermetallic formations. Though not
required for soldering, the effect often is caused and when
excessive, expecially over time, solderability becomes poor - to say
the least.
I am happy you have a good supplier providing long shelf life and
good solderability. So many suppliers do not. Therefore,
solderability issues, not SIR issues, are primary and must be
resolved.
There is so much more to the equation, but if you're happy, I cannot
argue. I only suggest there may come a day when you will suffer poor
or impossible solderability because of HASL. And on it goes. Anyway,
soon we'll all be faced with lead free soldering - by law.
Happy soldering,
Earl Moon
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