402
S. Shi et al.
representing elastic portion at the given cyclic loading frequency and strain level. The
loss modulus measures the energy dissipated as heat representing the viscous portion
(i.e. plastic deformation portion). Since the CTE of all materials used in a substrate is
not the same, material modulus is one of the key parameters in determining internal
stress level in the PWB or substrate. High internal stress level is detrimental to
substrate quality and reliability and often causes substrate warpage, delamination
and cracks.
Toughness is the ability of a material to absorb energy through deformation
without fracturing. It is a parameter used to measure the ability of a material can
deform before its rupture under a given strain rate. Since material fracture is one
of the major failure modes in microelectronic packaging, toughness is an important material mechanical property to consider in material selection, especially for
the applications where the substrates need to go through large bending or cyclic
deformation in the later assembly process or field use. Solder mask material selection for flex substrate applications is a good example. Because material toughness
strongly depends on particular application, measurement of material toughness is
often performed in-house.
Flexural strength represents the maximum stress that a material can sustain before
its fracture and is often measured by so-called three-point bending method where
a material sample is supported at the ends and loaded in the center. The flexural
strength sets maximum total stress level (internal stress plus external stress) limit that
a material can bear from manufacturing through product life time. In real applications,
the maximum designed stress level on any material in a substrate or PWB is much
lower than its flexural strength. The material flexural strength is readily available
from substrate or PWB material handbooks. IPC-4101 specifies the minimum flexural
strength of various materials for substrate applications.
Material density is considered when substrate weight becomes a factor such as in
aerospace applications. For cored substrates, the core weight is the main portion of
the total weight. Therefore, the selection of core material is the main path to meet
the weight target.
The decomposition temperature (T d ) is an intrinsic chemical property of a material. In the context of organic substrates, at a pre-defined heating rate the decomposition temperature refers to the temperature at which the organic resin system starts
to break down. There are multiple ways to characterize the decomposition temperature of a material. Thermo-gravimetric analysis (TGA) is the most commonly used
technique by which the material weight loss is measured over a period of time at a
constant temperature or over a range of temperature at a constant heating rate. T d is a
very important chemical property for a base material as it determines the maximum
process or use temperature of a fabricated substrate. Above T d , the organic material
starts to lose its strength, toughness, and adhesion. This property did not cause much
concern before the RoHS requirements came into effect, because most of organic
base materials can sustain eutectic tin/lead solder reflow temperatures. However,
significant attention is now paid to this property during material selection, as leadfree solder reflow temperatures begin to adversely impact the substrate reliability for
some organic base materials.
S. Shi et al.
representing elastic portion at the given cyclic loading frequency and strain level. The
loss modulus measures the energy dissipated as heat representing the viscous portion
(i.e. plastic deformation portion). Since the CTE of all materials used in a substrate is
not the same, material modulus is one of the key parameters in determining internal
stress level in the PWB or substrate. High internal stress level is detrimental to
substrate quality and reliability and often causes substrate warpage, delamination
and cracks.
Toughness is the ability of a material to absorb energy through deformation
without fracturing. It is a parameter used to measure the ability of a material can
deform before its rupture under a given strain rate. Since material fracture is one
of the major failure modes in microelectronic packaging, toughness is an important material mechanical property to consider in material selection, especially for
the applications where the substrates need to go through large bending or cyclic
deformation in the later assembly process or field use. Solder mask material selection for flex substrate applications is a good example. Because material toughness
strongly depends on particular application, measurement of material toughness is
often performed in-house.
Flexural strength represents the maximum stress that a material can sustain before
its fracture and is often measured by so-called three-point bending method where
a material sample is supported at the ends and loaded in the center. The flexural
strength sets maximum total stress level (internal stress plus external stress) limit that
a material can bear from manufacturing through product life time. In real applications,
the maximum designed stress level on any material in a substrate or PWB is much
lower than its flexural strength. The material flexural strength is readily available
from substrate or PWB material handbooks. IPC-4101 specifies the minimum flexural
strength of various materials for substrate applications.
Material density is considered when substrate weight becomes a factor such as in
aerospace applications. For cored substrates, the core weight is the main portion of
the total weight. Therefore, the selection of core material is the main path to meet
the weight target.
The decomposition temperature (T d ) is an intrinsic chemical property of a material. In the context of organic substrates, at a pre-defined heating rate the decomposition temperature refers to the temperature at which the organic resin system starts
to break down. There are multiple ways to characterize the decomposition temperature of a material. Thermo-gravimetric analysis (TGA) is the most commonly used
technique by which the material weight loss is measured over a period of time at a
constant temperature or over a range of temperature at a constant heating rate. T d is a
very important chemical property for a base material as it determines the maximum
process or use temperature of a fabricated substrate. Above T d , the organic material
starts to lose its strength, toughness, and adhesion. This property did not cause much
concern before the RoHS requirements came into effect, because most of organic
base materials can sustain eutectic tin/lead solder reflow temperatures. However,
significant attention is now paid to this property during material selection, as leadfree solder reflow temperatures begin to adversely impact the substrate reliability for
some organic base materials.