Development of a Low Voiding Solder Attach Process for Photovoltaic Cell Assemblies

Abstract

The level of competition for a share of the burgeoning solar cell market naturally drives companies towards low-cost, high-volume subcontractors. But the stringent operating requirements in a concentrated photovoltaic (CPV) application are more consistent with the process capabilities of lower volume manufacturers. Specifications for attach of PV cells as large as 1 cm2 require the high thermal conductivity of a solder connection with minimal voiding in order to prevent current-crowding effects and thermal hotspots. These design aspects are most easily achieved through the use of lower volume, more labor-intensive batch processes such as vacuum reflow, often using expensive alloys such as 80Au20Sn. Crane Electronics in Redmond, WA, has developed an in-line solder reflow process for PV cell attach that uses standard, inexpensive solder alloys to address large die, low-voiding applications. This paper discusses the initial design and subsequent incremental improvement of that process, with supporting x-ray and yield data.