A clearer road ahead

YIELD I TESTING
Figure 5: Red features
are voids in the solder
joining the metal heat
sink to the underside of
a concentrator cell
The numerous large and small features are voids
in the adhesive. Like delaminations, they block
heat flow and cause overheating.
In a concentrator cell, overheating is even more
dangerous than in other cell types; the cell shown
here is likely incapable of providing reliable longterm
service.
Imaging all sides and features
Figure 5 is also a concentrator solar cell, but it has
been imaged from the bottom side - that is, the
transducer scanned the external surface of the
metal heat sink beneath the cell.
The heat sink is attached by a layer of solder, and
the purpose of imaging was to look for voids in the
solder, since voids are very efficient blockers of
heat and can easily cause the cell to overheat and
fail. The echoes were gated just above and just
below the solder layer.
The red pseudocolor features of various sizes are
all voids - air bubbles introduced when the solder
was fluid. They occupy 11.86% of the area, a
dangerously large heat-blocking area for such a
thermally sensitive system. The red voids also tell
something about their own formation. When, in
making a second image, the gating was changed
to collect echoes only from the deeper part of the
solder, no new voids appeared.
Also, the voids that are red in Figure 5 appeared
as dark acoustic shadows, because they were
blocking the ultrasound returning from the bottom
of the solder layer. All of the voids were therefore
at the interface between the solder and the heat
sink. Use of a new mode developed by Sonoscan
can, simultaneously with imaging, map the
thickness of the internal solder layer.
Acoustic microscopes have numerous techniques
and modes that succeed in imaging and analyzing
internal anomalies in solar cell assemblies.
The data from acoustic micro imaging is a
powerful tool for achieving long-term reliability.
© 2012 Angel Business Communications.
Permission required.
36 www.solar-international.net I Issue IX 2012