2019 Reports

QFN Power Cycle Reliability Testing
Authors: Michael Gaynes, Michael Meilunas
The Quad Flat No-Lead Package (QFN) is a low cost, near chip scale package with electrical and thermal performance advantages due to short wirebond length and an exposed copper lead frame. However, the no-lead perimeter lands on the bottom of the package result in a low stand-off solder interconnect that has shorter fatigue life compared to traditional quad flat packs with leads.

In Information Technology hardware applications, during periods of low operational traffic, chip power is conserved by going to an idle state. Consequently, many power-on/idle cycles accrue. Unlike traditional thermal cycle testing, a power-on state heats the chip internally and heat is transferred from the chip to its surrounding packaging materials that include the solder joints. Moreover, power-on cycles accumulate in the tens of thousands of cycles.

Power cycle testing was completed on a 12 x 12 mm QFN package where the target power-on chip temperature was 130 ⁰C resulting in a corner solder joint temperature of 80 ⁰C. The QFN package was solder attached to a 3 mm thick printed circuit board on sites with and without thermal vias. The solder voiding between the QFN lead frame thermal pad and thermal pad on the PCB has typically >40% as expected from industry experience. Finite element analysis was used to show that only a small percent of solder area coverage of the thermal pad is needed to benefit from the thermal vias in the PCB which reduce thermal resistance significantly compared to no thermal vias. The solder connection from the QFN thermal pad to a PCB thermal pad is only of benefit if thermal vias are present.  Twelve QFNs were power cycled out to >31,000 cycles. Three parts failed with typical bulk fatigue in the corner solder joints. Only one of these fails was relevant. A runaway chip temperature on the other QFNs negated any reliability assessment. However, the one relevant failure occurred before the chip temperature increased inordinately and therefore, indicates a reliability concern and the need for continued study.
Key words: QFN, power cycle