Hot air solder leveling in the lead-free era
!
"#$%&'!
a
()*&'+&*,$$(-!-#+.#/)%!
-#..&'!
Figure 13. Solderability is lost when the intermetallic
layer has grown through to the surface.
b
lead-free hAsl alloys
As in other processes involving solder, the
tin-lead eutectic is the benchmark against
which other solders are judged. The Sn-
SN-37Pb SnCuNiGe SAC305
!
!
37Pb alloy can deliver a HASL finish that
!
!
!
!
is smooth and bright and reasonably even Figure 14. (a) As HASL’d. (b) After 500h at 125˚C. The nickel-stabilized intermetallic grows more slowly.
and there is no doubt that it is the eutectic
character of the tin-lead that is the major
!
factor in making that possible. Because of
the way it solidifies in a single stage with
a sharply defined melting point a eutectic
has high fluidity close to its melting point
and does not suffer from the shrinkage
effects that detract from the appearance of
Figure 15. Surface tension forces mean coating tends to be thicker on smaller pads.
the finish left by alloys that exhibit non-
eutectic behavior.
It is not a coincidence, therefore, that
the solder most widely used in the lead-free
HASL process is also the only lead-free
alloy that comes close to matching the eu-
tectic behavior of the Sn-37Pb. That alloy
is based on the tin-copper eutectic but with
an addition of nickel at a specific level that
promotes eutectic behavior
6
(Figure 11).
This alloy also incorporates an addition
of germanium to control oxidation in the
liquid and solid state.
This SnCuNiGe alloy has two other
!
properties that make it particularly suited
to the HASL process, a low rate of copper
Figure 16. Comparison of HASL coating thickness (Fellman7).
dissolution and a stable interfacial inter-
metallic. all the tin (Figure 13). The slow growth of alloy is approximately halved to about 35-
Since the copper pads of the printed the intermetallic layer (Figure 14) means 40°C. The main implication of this smaller
circuit board are exposed to molten solder, that solderability is retained even through process window is that temperatures have
it is important that not too much copper extensive thermal excursions. to be held much closer to the set point
is lost by dissolution during the process. than needed to be the case for the tin-lead
The nickel addition that promotes eutectic equipment requirements and HASL process. Machine manufacturers
behavior in the tin-copper eutectic also process parameters have responded to this challenge by in-
stabilizes the intermetallic layer that forms The lead-free HASL process has to operate creasing heater capacity and insulation of
on the surface of the copper so that it acts within the constraint that applies to all the solder pot and upgrading temperature
as a barrier to further dissolution. Figure lead-free processes—the melting point of control.
12 shows the results of an experiment to the alloy is higher than that of the tin-lead The temperature that has to be reached
compare the rate of dissolution of the solder that it replaces, but because of limits for wetting of the copper to occur is much
SnCuNiGe alloy with the tin-lead bench- on the temperature/time profiles to which the same for lead-free solder as it is for tin-
mark and ‘SAC305’, the Sn-3.0Ag-0.5Cu some materials can be exposed, process lead solder, so the primary determinant of
alloy that has been widely promoted as the temperatures have to be kept as close as the process temperature is the heating rate
default lead-free solder. possible to those used with Sn-37Pb. required to get the surface of the board to
Even for a HASL finish that has been With a melting point of 227°C, 44°C the melting point in the shortest practica-
correctly applied, the solderability is even- higher than that of Sn-37Pb, this con- ble time. The heating rate is proportional
tually lost when the intermetallic layer has straint means that the process window for to the temperature difference between the
grown to the point where it has consumed lead-free HASL with the most widely used board and the solder so that the solder
14 – Global SMT & Packaging – March 2009 www.globalsmt.net
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