Online proceedings - EDA Publishing Association

7-9 October 2009, Leuven, Belgium
Crack Tip Localization of Sub-critical Crack Growth
by Means of IR-Imaging and Pulse Excitation
D. May 1 , B. Wunderle 1,3 , R. Schacht 1,2 , B. Michel 1
1 MicroMaterials Center Berlin, Fraunhofer IZM, Berlin, Germany Email: daniel.may@izm.fraunhofer.de
2 Hochschule Lausitz (FH), Senftenberg, Germany
3 TU Chemnitz, Germany
Abstract- Taking the advantage of the thermo-elastic effect, while
using an infrared camera system, the mechanical stress could be
made visible. The stress concentrations at the tip of a subcritical
crack growth could clearly be detected. Through the observation
during the periodic loading of a CT-specimen the crack growth
rate can be determined. For this purpose, a specially developed
loading stage will be presented. It is now possible to have further
investigation in material class of polymers, which is very
important in the field of system integration. First promising
results will be presented.
I. INTRODUCTION
Today’s need for fast and reliable technology development
including further miniaturization, functionality and complexity at
lower cost is a trend that will continue [1]. To assure reliability for
advanced packages presupposes the understanding and description
of failure mechanisms and to be able to apply them to large models
[2]. For e.g. power packages delamination of the die-attach or
encapsulant is known to be a problem which needs optimization
[3]. Then for e.g. System-in-Package solutions, the number of
interfaces and thus possible delamination sites has increased a
situation where numerical lifetime prediction methods also face the
challenge of availability of critical parameters for materials and
material pairings. These parameters in general are given as energy
release rates for fracture-mechanical treatment and depending on
process conditions, moisture and temperature. These values are
usually had to be measured because they are not readily available
from manufacturer’s datasheets.
Modern Finite Element (FE) tools allow the programming of
routines to calculate energy release rates as failure criterion for a
possible crack in a rather short time [4]. However, the bottle neck is
a rapid and yet accurate determination of critical and undercritical
data for crack growth.
In the past, many different setups have been proposed to study
crack growth ([5,6]). Most of them need complicated methods to
determine the crack length under external loading conditions in
respect to the specimen geometry and the real process conditions.
As for most designs the precise crack length is needed for an
energy release rate and phase angle determination from finite
element simulations, here a new method is proposed using the
transient thermal response of the specimen under different loading
conditions by infrared thermography [7].
The IR thermography is becoming increasingly important for
non-destructive testing of microelectronic components and
structures on the chip, package and board level. Besides the use of
pulse thermography for determining the temperature of thermal
conductivity according to Parker [8] or for detection of
delaminations in micro-electronic thin-film assemblies, and the
degradation of electrical vias in printed circuit boards [9,10] the
lock-in thermography is also used to detect shorts in integrated
circuits. Müller [11] has shown that the lock-in thermography can
be used in investigation of fracture and cracking behavior of steel
components using the thermo-elastic effect [12]. To investigate in
crack propagation in solids, among others, a so called Compact
Tension (CT) specimen is used. If the subcritical crack growth is
examined the sample with strain amplitude less than the critical has
to be loaded periodically. During the experiment the length of the
growing crack must be measured by appropriate methods.
These experiments are carried out, in specially designed testing
equipment. Due to the high costs often only a few of such
machines are available. Therefore long experimental periods are
being developed for a material such as to be characterized under
different temperatures and loads.
To gain even more quickly usable material parameters, e.g. at an
early product development phase and to make a choice of materials
a design of a novel strain apparatus should be as simple and yet as
precise as necessary to carry out several parallel investigations.
This work focuses on the adaptation of the stress analysis on
materials of micro-electronics (e.g. in the system integration very
important material class of polymers).
A. Theoretical Basics
In the early 19th Century, J. Gough [13] observed that the
temperature of a polluted materials changes. The correlation
between mechanical stress and temperature change has J. Thomson
[12] in 1853 first recognized in theory and published. According to
Stanley Chan and Biot [14, 15] the link between material
parameters, state of stress of the sample and the temperature change
can be described as follows:
∆
·
··∆ (1)
where T is the absolute temperature, α is the coefficient of
thermal expansion, ρ the density, c p , heat capacity (at constant
pressure) and Δ (σ1 + σ2 + σ3) the change in the sum of principal
stress (hydrostatic stress).
Here with positive values of the principal stresses (tensile stress)
a cooling and with negative values (compressive load) a warming
take place in the material.
With the development of modern infrared measurement in the
©EDA Publishing/THERMINIC 2009 91
ISBN: 978-2-35500-010-2