Impact of Pad Layouts and Solder Volume on Self-Alignment of Micro Solar Cells

In micro-concentrating photovoltaics (micro-CPV), tiny solar cells
$(< 1\text{mm}^{2})$
are assembled on a circuit board on glass. To mount thousands of dies per square meter, a high throughput process is required which still yields alignment accuracy in the order of 10
$\mu\mathrm{m}$
. We developed a micro-CPV module technology where micro cells are accurately mounted directly on a circuit board on glass using a high throughput pick and place process combined with self-alignment based on the restoring force due to surface-tension of the liquid solder. In this work, we study how the self-alignment accuracy is influenced by the pad layout, solder volume and initial position of the die. We show that self-alignment significantly benefits from an induced motion, whether induced by initial displacement of the die or by flowing of molten solder along tracks. Furthermore, we found that lower solder volumes lead to higher alignment accuracies for dies on rectangular pads, whereas the dependence on solder volume is lower for pads with connected tracks where the solder flows out. Overall, alignment accuracies below 15
$\mu\mathrm{m}$
are demonstrated despite initial displacements up to 150
$\mu\mathrm{m}$
. Thus, self-alignment using surface tension of the liquid solder is suitable for micro-CPV.