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Hot air solder leveling in the lead-free era
! !
Figure 3. Potential coplanarity issue with a HASL
finish.
Figure 4. Paste bleed due to problem of stencil gasket-

ing on non-planar HASL coating.
Figure 2. Voids in laminate resin due to excessive likelihood of damage is being further
exposure to heat.
reduced as more heat-resistant grades of
laminate are introduced to accommodate
reflow with enough surviving to the second
stage of reflow, if a final stage of wave
in July 2006, the view that was most
other requirements of lead-free processing.
soldering is required it is often difficult to
widely expressed was that the HASL finish
The variation in the solder coating
meet barrel-fill criteria.
would disappear as the industry moved
thickness of a properly applied lead-free
The immersion silver finish dissolves dur-
to lead-free technologies4. The justifica-
HASL finish is typically about half that of
ing the soldering process so that all it can
tion for this prediction was problems with
the tin-lead HASL finish it replaces. And
do is provide protection from oxidation for
the HASL process that were claimed to
the coplanarity has been found to be good
the underlying copper. The solderability
include:
enough for the finish to be used on boards
with large area array components.
of the boards with this finish is therefore
• Damage to the printed circuit
And on the issue of heat and fume,
dependent on the quality of preparation
board by the additional thermal
modern HASL lines have been designed
of the copper before the application of the
excursion (Figure 2).
to provide operating conditions consistent
finish, and so is vulnerable to variation in
• Coplanarity problems caused by
with those of any other equipment in a
the control of the pre-etch process. Since
pad to pad variation in the coating
modern printed circuit board shop (Figure
the deposit is formed mainly by a chemi-
thickness (Figure 3).
5).
cal displacement process, the thickness
• Stencil gasketing problems during of the coating, and hence the degree of
paste printing associated with the
hAsl alternatives
protection that it can provide, is limited.
variation in coating thickness and
If the problems attributed to the HASL
Although it is claimed not to affect solder-
‘mushroom cap’ profile of the coat-
process were indeed the reason for not
ability, the silver is vulnerable to tarnishing
ing on small pads (Figure 4).
using this finish, it would have disappeared
in polluted atmospheres. Additional coat-
• The heat and fumes associated
long before the implementation of lead-
ings can be applied to prevent tarnishing
with the process that were consid-
free since the printed circuit board finishes
if there is concern about the cosmetic
ered out of place in a modern PCB
now promoted on the basis of their being
appearance of the board, but in any case it
shop.
lead-free—OSP, immersion silver, immer-
is generally recommended that boards with
In practice these potential problems have
sion tin and electroless nickel/immersion
an immersion silver finish be stored in
not been realized to the extent expected,
gold (ENIG) were already available. While
sealed packaging.
and lead-free HASL has been in wide-
all of these finishes found some applica-
As a consequence of its successful pro-
spread use, initially in Europe, since 2002,
tion prior to the need to eliminate lead
motion, immersion silver has enjoyed the
with many millions of boards successfully
from electronic assemblies, the HASL fin-
status of the preferred lead-free finish in
soldered and put into long-term service.
ish maintained its dominant market share.
the US market and so has been exposed to
Data collected by the author’s company
All of the mentioned alternatives offer
a wider range of operating conditions than
indicates that there are more than 400
a degree of coplanarity that cannot be
had been the case before lead-free imple-
lead-free HASL lines currently in com-
matched by even the best HASL finish,
mentation. One phenomenon to emerge
mercial production around the world, with
but the process maintained its dominance
from that experience is catastrophic ‘creep
more being commissioned every month to
because none of the alternatives offer a
corrosion.’ This phenomenon occurs when
meet the growing demand. This number
solution that is problem-free. For many ap-
the assembled board is exposed to humid
has grown by 45% since the last quarter
plications, HASL offers the best compro-
atmospheres polluted with sulfur and is
of 2006. To have achieved that growth, an
mise, particularly when robust solderability
driven by the electrochemical cell set up be-
average of more than five new lines would
is the main requirement.
have been commissioned each month, and
The protection that can
that number is consistent with the practi-
be provided by the
cal experience within the industry over
thin organic film
that period. More than half of these lines
of the OSP finish,
are in Europe, but about a quarter are now
albeit chemically
in China as the volume of boards ordered
bonded to the cop-
with this finish by European and American
per, is limited
OEM increases.
particularly under
tween
Unless the panel is passed through the
conditions of heat,
the silver and
HASL process more than twice and/or a
humidity and air
the small areas of copper
solder temperature higher than necessary is
pollution. Even
if solderability is
!
used the expected, damage to the laminate
has not been observed, even with CEM-1,
good enough for
Figure 5. Modern automatic in-line horizontal HASL line (photo courtesy
CEM-2 and FR-2 grades of laminate. The
the first stage of
Cemco/FSL Ltd).
www.globalsmt.net Global SMT & Packaging – March 2009 – 11
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