Hot air solder leveling in the lead-free era
a
Figure 7. Cross-section of an ideal HASL finish.
!
SN100CL HASL coating
providing complete coverage of
Interfacial intermetallic
the intermetallic layer
compound layer
b
Figure 8. Schematic representation of the cross section of an ideal HASL finish.
jority of cases, but confidence in the finish
a
c
has been damaged by a small but persistent
incidence of catastrophic joint failures
associated with the phenomenon known
as ‘black pad.’ A further complication is
that phosphorus, an unavoidable constitu-
ent of the electroless nickel deposit, can
react with the tin, copper and silver that
are the common constituents of lead-free
Figure 6. Typical lead-free HASL finish on through-
solders to form at the solder/substrate
hole (a), QFP pads (b) and through-hole (c). interface a complex intermetallic com-

pound that make the joint vulnerable to
b
inevitably exposed near the edge of the
impact loading. The gold dissolves quickly
solder resist on solder-mask-defined pads
5
.
in molten solder so that the joint is formed
The
by wetting of the underlying nickel so that
immersion tin process matches
the HASL process in forming a layer of
the solderability of the finish is ultimately
intermetallic at the tin-copper interface,
determined by the quality of that nickel
although by solid state diffusion rather
surface. As the product of a chemical dis-
than by reaction with molten solder. Un-
placement process, the gold coating is very
like immersion silver, it is not vulnerable
thin. The protection that it can provide to
to tarnishing and does not need special
the underlying nickel is limited and very

storage. However, as with immersion
dependent on process control. The very
Figure 9. Dewetting of the solder coating.
silver, the deposit is formed by a chemical
good solderability of freshly etched nickel
displacement process so that the thickness
can be quickly lost if the immersion gold
is limited. The intermetallic layer contin-
finish does not have the integrity to protect
a defined area with the coating stretched
ues to grow in storage and can completely
it from oxidation.
thinner at corners.
The main thing that can go wrong with
consume the tin with consequent loss
characteristics of a hAsl finish
the finish is for there to be areas where the
of solderability. The growth rate of the
Because, as explained earlier, a HASL
coating is too thin (Figure 9). This problem
intermetallic is greatly accelerated during
finish is in a way already one half of a
is a sign that the coating has not properly
a thermal excursion so that solderabil-
solder joint, the visual criteria for qual-
wetted the copper substrate or that there
ity problems can be encountered in the
ity are essentially the same as for a solder
was not enough solder left on the pad
second stage of double-sided reflow or in a
joint. The surface should be smooth and
to maintain a stable film over the whole
final stage of wave soldering.
bright with no evidence of non-wetting or
area. Whatever the cause, in the areas
As well as being perfectly flat, the
dewetting (Figure 6). In cross-section, the
where the solder coating is not sufficient
electroless nickel/immersion gold finish is hard
ideal finish would look something like that
to fully cover the intermetallic layer there
and sufficiently wear resistant for use on
in Figure 7, with the features highlighted
will be a high probability of non-wetting
edge connectors that require only a limited
in the schematic diagram of Figure 8. It is
and/or dewetting in subsequent soldering
number of insertion/withdrawal cycles.
clear from the cross-section that surface
processes.
This finish can also accept wire bonding.
tension plays a major role in determining
There are several reasons why satisfac-
The finish delivers excellent solderability
the profile of the finish with a character-
tory wetting might not be achieved:
and high reliability joints in the great ma-
istic ‘mushroom cap’ tending to form on
12 – Global SMT & Packaging – March 2009 www.globalsmt.net
Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55