Editor's note: These series of articles are actually a collaboration of efforts from Chris Jakubiec, Robert Strittmatter, and Alex Lidow, Efficient Power Conversion Corporation.
Examining eGaN® Field Reliability
Efficient Power Conversion (EPC) Corporation’s enhancement-mode gallium nitride (eGaN®)
FETs and integrated circuits (ICs) are finding their way into many end user applications such as LIDAR, wireless charging, DC-DC conversion, RF base station transmission, satellite systems, and audio amplifiers.
Field reliability is the ultimate metric that corroborates the quality level of eGaN® FETs and ICs that have been deployed in customer applications. In our first installment we provided an overview of eGaN® FET field reliability which included 6 years of volume production shipment, and greater than 17 billion total device hours recorded. A subsequent calculated Failure In Time (FIT – failures in 109 hours) of approximately 0.24 FITs shows excellent field reliability performance to date.
eGaN® FET electrical performance as well as reliability has been significantly enhanced by eliminating the package that traditional MOSFETs have been surrounded by. Removing the package around the die results in less variables that potentially contribute to field reliability issues such as mold compound, substrates, bond wires, and die attach materials. Additionally, eGaN® FETs are resistant to moisture related issues of packaged devices, as they are well protected by passivation layers.
EPC performs a thorough root cause analysis of all returned field failures. As of the writing of our first installment (February 2016) 127 field failures have been investigated. Of the 127 field failures, 37 devices passed electrical testing with no anomalies detected, and were classified as good units. Figure 1 below shows the breakdown of analyzed failures grouped into root cause categories. The next three sections will go into more details describing the types of field failures that have been analyzed, as well as some recommendations to prevent such issues in the field.
Field failure breakdown by root cause category
Device assembly and handling accounted for the highest number of field returns, with 75 units recorded in this category.
Improper control of the amount of solder paste and flux released during assembly, together with inadequate rinsing and curing of the flux made up 36 of the field failure units in this category. Flux that has not been properly rinsed and dried can accumulate in the areas between the solder balls, and has the potential to form dendrites which can create conductive leakage paths and lead to device failures. For high reliability assembly it is advised to rinse all residual flux underneath the die, and perform a high temperature dry curing step before applying any power to the device. Figure 2 shows dendrites connecting two terminals within uncured flux.
eGaN® FET showing dendrite formation due to residual flux.
Die tilt issues can arise due to poor stencil design resulting in uneven solder paste volume release over the PCB pads. Vibration during assembly, reflow profile, and PCB solder mask design are other factors that can contribute to tilted die.