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Home

Dynamic Response Determination of an Electronic Printed Circuit
Board

Tuesday, 01 July 2014

Understanding the mechanical reliability of a PCB in an electronic
system is an important part of assessing the reliability of the entire
system.

John Deere Electronic Solutions, Fargo, North Dakota; and John Deere
India Pvt Ltd., Maharashtra, India

Most of today’s automotive electronic systems are composed of two major
mechanical elements: an equipment chassis or enclosure, and a printed circuit
board (PCB) assembly. The PCB is composed of laminated copper and FR-4 glass
epoxy. These systems often operate in severe vibration environments for extended
periods without failing. The vibrations transmitted throughout the PCB induce
strains in the connectors, components, and most importantly, the solder joints
attaching the components to it.

Damping plays a key role in dynamic response prediction. It is of great
importance in dynamic design of electronic control units (ECUs), especially for
response prediction and vibration control. Dynamic response of a structure is
determined by dynamics characteristics of the structure and external loads.
Resonance plays a key role in the dynamic response of a structural system. Two
critical parameters of resonance are resonance frequency and damping ratio,
which are determined by mass, stiffness, and damping characteristics of the
system.

The objective of this work is to understand and predict the dynamic behavior
of the system using various mechanical tests and simulations. The material
properties of the PCB are obtained from standard tensile testing based on ASTM
D638. The modal response of the model, including modal frequencies and mode
shapes, is acquired through testing. Next, the same model is also analyzed by
finite element simulations to determine the same dynamic properties. Thus, the
results obtained through both approaches are compared.

With an effort to understand the force response behavior of the system, a
swept sine vibration test is carried out on the same model using specified
vibration level as an input excitation, and results in response amplitude curves
at specified locations. This same scenario is simulated with finite element
analysis (FEA) using commercial software assuming different damping ratios.
These simulation results are compared with the testing results for deducing the
appropriate damping ratio. As different kinds of electronic components (like
transformers, capacitors, or chips) are mounted on both sides of the PCB using
solder joints, adhesive, etc., various complexities are encountered while
modeling them for analysis. For this reason, simple PCBs without any components
are used in this work.

Finite element analysis is the most frequently used, effective, and versatile
computational tool for analysis. In this study, commercially available software
is used for modeling and analysis of the bare PCBs. The common analysis
procedure includes geometry modeling, discretization or mesh generation using
suitable elements, defining materials to different parts, applying loads,
obtaining a solution, and reviewing results. The board is modeled as isotropic
material, and is fixed at four corner locations. The model is composed of
higher-order quadratic elements; about 8,000 elements are used. The Lanczos mode
extraction method is used in this study because it is the recommended method for
medium to large models.

One of the common reasons for the experimental modal analysis is to acquire
the modal parameters by testing directly on the physical system, and to
correlate/verify with those from FEA and validate the FE model. These modal
parameters are determined via data curve-fitting algorithms from a set of
frequency response functions (FRFs) obtained from measurements taken at various
points throughout the structure. The FRFs are acquired by exciting the PCBs,
usually by a random motion shaker or an instrumented impact hammer, and
measuring the response by means of an accelerometer. Sufficient numbers of
points are considered all over the structure for the efficient frequency
response measurements.

The FRFs measured are analyzed to obtain the modal parameters. Operating
vibrations can be divided into three main classes: swept sine vibrations, random
vibrations, and mechanical shocks. Applying a sine vibration to a product can
assist in finding out the potential weaknesses in its design. This is done by
exposing the product to a desired range of frequencies, one frequency at a time.
This frequency range is generally selected to cover the entire operating range
that the product might see in actual use, but the intensity of the vibration may
be increased over real-world expectations. In swept sine vibration analysis, all
loads, as well as the structure's response, vary sinusoidally at the specific
frequency. A typical harmonic analysis results in the response (usually
displacements or accelerations) of the structure to cyclic loads over a
frequency range. “Peaks” are identified from response graphs and then reviewed
at those peak frequencies.

The modal parameters of the PCBs obtained through the FEA software and
experimental methods show good correlation. Further, swept sine vibration
analysis is carried out using finite element software on the same model using
specified vibration level as an input excitation. The same scenario is
replicated in vibration testing, and the results are compared for deducing the
damping ratio, which is found to be 9%.

This work was done by Rahul Jagdale and Prashant Bardia of John Deere
India Pvt Ltd., and Mark Schmaltz of John Deere Electronic Solutions. The full
technical paper on this technology is available for purchase through SAE
International at http://papers.sae.org/2013-01-2799.

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