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Hot air solder leveling in the lead-free era
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Figure 10. Schematic representation of the vertical
HASL process.
• The copper surface was not pre-
Figure 11. Comparison of the appearance of the SnCuNiGe alloy with Sn-37Pb.
pared to the point where complete
wetting (with no subsequent
dewetting) could be achieved with
the flux used with the solder tem-
perature and contact time to which
the panel was exposed during the
HASL process.
• The adjustment of the hot air
knives was such that not enough
solder was left on the pads to per-
mit full coverage of the pad surface
under the influence of surface
tension forces.
If the solder coating is not sufficient to
provide a layer of free, unreacted solder
over the intermetallic layer, solderability is
quickly lost. Although the presence of the
interfacial intermetallic provides confirma-
tion that wetting has occurred, the inter-
metallic loses solderability when exposed
Figure 12. Comparison copper dissolution rates of HASL alloys.
to the atmosphere.
When such problems are noted, the
solution can usually be found simply by wetting. The principle is similar in the horizon-
taking the countermeasures implied by the In the vertical process (Figure 10), a tal process, but instead of the board being
foregoing diagnoses. panel of boards is manually mounted on immersed in a bath of solder, it is flooded
a frame which moves vertically down- with solder applied through nozzles or
The lead-free hAsl process ward into the solder bath, holds for an between rollers on the top and bottom
One of the advantages of the HASL optimized immersion time, typically 2-3 of the panel before being passed between
process is its simplicity. The cleaned and seconds, and then raises the panel through hot air knives located above and below
fluxed printed circuit board is brought into the operating air knives that are adjusted the board. Because it is an in-line process,
contact with molten solder for sufficient to clear through holes and vias while leav- horizontal HASL can be easily integrated
time to achieve wetting, then excess solder ing an adequate thickness of solder on all with other in-line processes from the
is blown off with air knives—planar jets areas where a solderable finish is required. pre-etch to washing and drying to create
of air at a temperature above the melt- The board is removed from the jig as the a continuous line such as that in Figure 5.
ing point of the solder. When the solder solder cools and solidifies and is placed on With the in-line preheating stage, the time
coating has solidified, the board is passed the conveyor of the washer/dryer. The op- required for exposure to solder can be re-
through a washer to remove flux residues erator can quickly inspect for finish quality duced to a matter of a fraction of a second
and dried. Air cooling ensures the smooth- during this handling. with a corresponding increase in per hour
est brightest finish, and for the favored The great majority of HASL lines are throughput.
SnCuNiGe nothing is gained in regard to of vertical design and quite capable of ap- The horizontal process has an advan-
minimization of the intermetallic thickness plying the finish to a high quality standard. tage in that the finish is more uniform in
by water or oil quenching. Although most lines require manual thickness. In the vertical process, there is a
It is immediately apparent from visual loading and unloading of the panel, fully tendency for the solder to move toward the
inspection whether the process has been automated vertical lines are available that lower edge of the pad under the influence
successful, but XRF techniques and/ can deliver rack-to-rack performance of gravity, whereas in the horizontal pro-
or cross-sectioning can be used to check incorporating fluxing and preheating and cess gravity has no effect on the distribu-
the coating thickness and the quality of subsequent washing and drying. tion of the solder over the surface. Global SMT & Packaging – March 2009 – 13
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