Durability of electronic assemblies under vibration (Part I: sinusoidal vibrations)

Electronic equipment used in the field is often exposed to harsh vibration loading, which can lead to the failure of electronic assemblies due to high-cycle fatigue of the solder interconnects. For Surface Mount Technology (SMT) components, the out-of-plane displacement of the circuit board induces the highest stresses in the solder joints. Furthermore, the vibrations experienced can be either sinusoidal or random in nature:

Examples of sinusoidal and random vibration signals

Unlike thermal cycling, where cracks typically initiate and propagate within the solder bulk, vibration loading can lead to multiple failure modes. This is primarily because the mechanical loads acting on the solder interconnects during vibration are significantly different, and the associated strain rates are much higher:

Typical failure modes observed in solder interconnects after vibration exposure (example of a Ball Grid Array (BGA) package with solder balls) 

The multitude of failure modes makes it difficult to develop simple analytical models for assessing durability under vibration. For example, when assessing durability under thermal cycling, modified Coffin-Manson models can be used, which account for shear strain in the solder joints as the fatigue criterion. This is relevant because, at a first order, shear forces are the primary contributors to solder joint fatigue, with cracks typically observed in the solder bulk. However, for vibration, the failure criteria cannot focus solely on the solder joints, as failure modes can also occur in other, closely located areas. An indirect but effective criterion is the local PCB strain near critical solder joints, which can be easily measured with a strain gauge or calculated using finite element modeling (FEM) simulations:

Methodologies for assessing local PCB strain: measurement with a strain gauge (left) and calculation using Finite Element Modeling (right)

Several publications have investigated the durability of SAC305 electronic assemblies under sinusoidal vibrations, considering different test boards (PCB stack-ups and SMT packages), each instrumented with strain gauges near the critical solder interconnects to measure local PCB strain. Sine dwell tests were conducted at the natural frequency of each board until failure to measure the number of cycles to failure corresponding to a specific local PCB strain value. This data was then used to plot the High Cycle Fatigue (HCF) curve for a specific solder alloy composition, namely SAC305:

Durability plot for SAC305 solder joints under sinusoidal vibration

Although the data points are scattered, which is normal and expected for high-cycle fatigue of electronic assemblies, there is a clear trend indicating that local PCB strain can serve as an indirect and effective criterion for assessing the durability of SAC305 electronic assemblies subjected to vibrations. The curve obtained from the sinusoidal vibration test is a prerequisite for evaluating durability under more complex vibration loading conditions.

An important step is conducting a failure analysis to identify the different failure modes induced by sinusoidal testing. As expected, cracks are observed at various locations, highlighting the complexity of the mechanical loading on solder interconnects:

Failure modes in SAC305 solder interconnects after sinusoidal vibration exposure

Since the HCF curve accounts for several failure modes, it is considered a conservative curve and can, therefore, be used for design purposes.

In Part II, we will explore how to use this HCF curve, along with specific fatigue damage models, to predict the durability of electronic assemblies subjected to random vibrations.

[1] J-B. Libot, "Méthodologie d’évaluation de la durée de vie des assemblages électroniques sans plomb en environnements thermique et vibratoire", PhD thesis, Institut National Polytechnique de Toulouse, 2017.

[2] B.M. Paquette, "Harmonic Vibration Testing of Electronic Components Attached to Printed Wiring Boards with SAC305 and Eutectic", Master's thesis, University of Maryland, 2010.

[3] Y. Zhou, "Harmonic and random vibration durability investigation for SAC305 (Sn3.0Ag0.5Cu) solder interconnects", PhD thesis, University of Maryland, 2008.

[4] C.Choi, A. Dasgupta, " Fatigue of Solder Interconnects in Microelectronic Assemblies under Random Vibration", XVII International Colloquium on Mechanical Fatigue of Metals, Procedia Engineering 74, pp.165-169, 2014.