Editpr's note:Alex Lidow and Robert Strittmatter also collaborated in the creation of this article.
eGaN Technology Physics of Failure
The first three installments in this series covered field reliability experience and stress test qualification of Efficient Power Conversion (EPC) Corporation’s enhancement-mode gallium nitride (eGaN) field effect transistors (FETs) and integrated circuits (ICs). Excellent field reliability that was documented is the result of applying stress tests covering the intended operating conditions the devices will experience within applications. Of equal importance is understanding the underlying physics of how eGaN devices will fail when stressed beyond intended operating conditions (e.g. datasheet parameters and safe operating area). This installment will take a deeper dive into the physics of failure centered around thermo-mechanical reliability of eGaN wafer level chip-scale packages (WLCSP).
EPC FETs and ICs are offered in various wafer level chip-scale packages. The advantages of delivering power FET devices in WLCSP are faster switching speeds, lower on-resistance, lower thermal resistance, improved reliability, and lower cost. Traditional plastic molded power device packages include leads and pads to provide connections to the printed circuit board (PCB), whereas solder bumps and solder land grid arrays are used in EPC chip-scale packages. This evolution to a significantly higher performing and lower cost power package is accompanied by a need to understand the thermo-mechanical capability of solder mounted eGaN devices, which in turn can provide high board-level reliability in the field. Figure 1 shows examples of EPC eGaN devices in solder land grid array (a) and solder bump (b) chip-scale packages.
Figure 1: (a) EPC2007C chip-scale package solder land grid array.
(b) EPC2032 chip-scale package solder bump.
EPC chip-scale devices are without plastic encapsulation, bond wires, and die attach materials included in traditional power packages. With the elimination of these package interfaces that can fail under thermal stresses, the main area of focus in the chip-scale package is the die to PCB solder joint reliability.
Solder joint stress comes from temperature fluctuations within the die, solder joint, and PCB. More specifically, the global coefficient of thermal expansion (CTE) mismatch between the three attaching materials accounts for the majority of solder joint stress. During temperature excursions above and below ambient, the die (CTE
3 ppm/o C) and typical FR4/FR5 PCB (CTE ∼ 10-16 ppm/o C) substrates will expand and contract at different rates. As the die and PCB move at different rates under the influence of temperature, a shear strain will develop in the solder joint connecting the two.
Generally, after many temperature cycles the repeated effect of the shear strain on the solder joint will result in fatigue and ultimately fractures (see Figure 2 for example). In addition to degraded or complete loss of signals, the fractured solder joints can result in higher die temperatures due to the loss of the conductive thermal dissipation path to the PCB traces. A local CTE effect within the solder joint also exists, however it is not the dominant fatigue mechanism and will not be examined here.
Figure 2: Cracked solder joints resulting from intermittent operating life stress testing.
Thermo-Mechanical Stress Testing & Modeling
As previously described in the 3rd installment of this blog, there are two main industry accepted stress test methods to evaluate thermo-mechanical reliability. Temperature cycling (TC) is done in an unbiased condition by cycling the ambient temperature between two levels (typical conditions for qualification of EPC devices -40o C to +125o C). Intermittent Operating Life (IOL) stress test uses increased cyclic power to heat the device junction temperature (TJ) to a predefined level, and subsequently lowers the power to return the junction temperature to the starting condition (a typical cycle might be Δ TJ = 100o C). EPC is using both TC and IOL stress tests to evaluate eGaN device thermo-mechanical capability. Cumulative stress tests results including TC and IOL, were also included in the previous installment.