Hot air solder leveling in the lead-free era
Mean coating highest Lowest Number of
Pad size " Pitch " thickness reading reading readings
0.027 dia 0.05 4.93 7.19 2.45 78
0.018 dia 0.04 9.07 16.85 4.32 60
0.025 dia 0.05 7.45 10.51 5.49 75
0.020 dia 0.05 8.72 15.34 6.15 87
0.025 x 0.020 N/A 4.95 6.57 3.12 40
0.020 dia 0.05 4.8 9.83 2.53 104
Figure 17. Thickness of SnCuNiGe coating on BGA
0.030 dia 0.05 5.39 8.08 2.98 76
pads (courtesy Cemco/FSL Ltd).
0.030 dia 0.05 5.04 6.91 2.61 140
Average 6.29 10.16 3.71
Table 1: Typical range of thickness of an SnCuNiGe HASL finish on BGA pads.
that will depend on where the set tempera- increase solder temperature to hold the
ture is measured. Typically a temperature copper in solution. The copper level of the
setting similar to that for the solder bath solder bath can also be reduced by partially
is used. draining the pot and replenishing with a
Because it depends on the particular low-copper top-up alloy.
characteristics of the machine, it is not pos-
sible to be specific about air knife settings coating thickness
such as air pressure, angle, proximity and The profile of the solder coating in a
offset, but they differ from those that have HASL finish is determined by the volume
often been used with tin-lead solder to the of solder left on the pad after the board
extent that the objective should be to leave has passed through the hot air knife and
Figure 18. Minimum coating thickness on the
enough solder on each pad to form the surface tension forces. As illustrated in
shoulder of through holes.
complete coverage exemplified in Figure 8. Figure 15, because of the way in which
Because of the unevenness of the tin-lead surface tension forces operate there is a
finish, the practice developed of blowing tendency for the coating to be thicker on
temperature might be set higher than
most of the solder off to leave a finish that smaller pads.
it need be to get wetting just to get faster
is as flat as possible. With lead-free solder, Perhaps it is because of the higher
heating to that melting point. The heating
that can leave insufficient free (unreacted) surface tension of lead-free solder that the
rate is also determined by the effectiveness
solder to fully cover the intermetallic coating that it forms in the HASL process
of heat transfer across the interface so that
layer with a consequent negative effect on is both thinner and more uniform than
another feature of vertical HASL machines
solderability, particularly after exposure to that formed by tin-lead solder in similar
designed for use with lead-free solder is
a solder paste reflow circumstances
7
. (Figure 16)
faster circulation of the solder over the
A typical range of thickness of an Sn-
panel. The flow rate of solder across the
Managing the composition of the CuNiGe HASL finish on BGA pads (Figure
board is already high in the horizontal pro-
solder bath 17) is given in Table 1.
cess and because it is in-line a preheating
As with the tin-lead HASL process, the Unfortunately the surface tension that
stage can be easily included so that a lower
copper of the pads tends to dissolve in the contributes to a more uniform coating
solder temperature setting is possible.
solder. The level of copper in the solder thickness also tends to pull the coating
Single-sided and simple double-sided
bath can quickly rise to more than 1%. thin on the shoulder of plated through
boards can be HASL’d with a solder bath
As the copper content rises, that liquidus holes (Figure 18). However, if a sufficient
temperature of 260°C but to get the
temperature of the solder rises, which volume of solder is left on the board, a
surface of a heavier multilayer board to
means that there is a tendency for the coating thickness of at least 1.5 µm can be
wetting temperature in a reasonable time
needle-like crystals of Cu6Sn5 intermetal- maintained, and that is sufficient to ensure
the solder bath temperature might have to
lic to precipitate from the melt. These good through hole solderability even after
be set as high as 280°C. However, the tem-
crystals have a higher density than lead-free several lead-free reflow cycles.
perature actually reached by the laminate
solder, and tend to sink to the bottom
during its few seconds of exposure to this
of the solder pot from which they can be Whisker susceptibility
temperature is much less than that, so the
dredged. However, they also form in cold Since the solder coating is applied in the
integrity of the laminate is not compro-
spots, which are usually at the surface of molten form, it is free of the residual com-
mised.
the solder. Excess copper can often be pressive stress that is acknowledged as the
The temperature of the air impinging
removed by lifting these accumulations out primary driving force for whisker growth
the surface of the panel as it emerges from
of the bath. Since the rate of dissolution of in electrodeposited coating. Testing to the
the molten solder has to be above the melt-
copper decreases with the increasing cop- JESD22A121 specification, “Measuring
ing point of the solder, and the setting on
per content of the solder, some operators Whisker Growth on Tin and Tin Alloy
a particular machine required to achieve
let the copper levels rise as far as 1.2% and Surface Finishes,” commissioned by the
16 – Global SMT & Packaging – March 2009 www.globalsmt.net
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