Fatigue of Solder Interconnects in Microelectronic Assemblies under Random Vibration

Abstract

This study investigates vibration fatigue durability of solder interconnects of surface mount electronic IC packages under harmonic and random excitation, through accelerated stress testing and physics-of-failure (PoF) simulations. The solder material of interest is Sn3.0Ag0.5Cu. The test vehicle consists of several HVQFN (Heat-sink Very-thin Quad Flat-pack No-leads) components soldered onto a Printed Wiring Board (PWB). The fatigue damage accumulation rates under ED random vibration excitation is found to be much higher than under harmonic vibration excitation, for equivalent RMS responses of PWB. To understand the reasons for such discrepancy we use FEA-based modeling. For simplicity of modeling we use a simple PWB with a simple Leadless Ceramic Resistor (LCR) component. We use a multiscale FEA modeling approach where the global analysis provides insights into the PWB modal response under the given excitation and can be conducted in the frequency-domain or in the time-domain. The local FEA model allows us to establish a transfer function between PWB curvature (flexural strain) in the vicinity of the critical component and equivalent strain in the critical region of the critical solder joint. al analysis is conducted to investigate the PWB mode shapes and natural frequencies, and they are compared with the measured strain time history, to understand the modal participation factors under both harmonic and ED random vibration excitation. The failure data is found to correlate well with the modal participations. Multiscale finite-element (FE) simulations are conducted to assess the dynamic response of the critical interconnects. This is done with two steps, first using a FE global analysis in the frequency domain for the global response consisting of all participating modes. Participation factors for individual modes are estimated from the global response, using numerical filtering methods. Inverse FFT is used to convert the frequency-domain response into the time-domain response histories for each dominant mode. In the second step, dynamic FE local analysis is conducted to obtain the transfer function between the PWB flexure and the strain in the critical solder interconnect for each dominant mode. Finally, rainflow cycle counting and Coffin-Manson fatigue model are used to estimate durability model constants and to compare fatigue damage accumulation rates under harmonic versus ED random vibration excitation.