14 Fundamentals of Advanced Materials …
403
Flammability is another chemical property of a base material. This property is
significant when high power density is needed for a substrate or PWB to meet the
application needs. The Underwriters Laboratories (UL) classifies the flammability
properties as 94 V-0, 94 V-1, or 94 V-2. 94 V-0 represents the lowest level of flammability category of a material. The definition of these classifications can be found in
UL website.
Moisture absorption primarily results from the chemical nature of cured organic
resins. Most of the reinforcement and conductor materials in a PWB or substrate do
not absorb noticeable moisture. For resin systems commonly used in substrates, the
moisture absorption ranges from 0.1% of FR-4 epoxy to ~0.5% of BT/epoxy blend
and cyanate ester [3]. Moisture absorption is an important property of base materials
for substrate applications, especially for high temperature lead-free substrate applications. The absorbed moisture in substrate mainly has two effects on substrate reliability during package assembly process. One is that the absorbed moisture can weaken
the interfacial adhesion between resin to reinforcement or conductor through hydrolysis of chemical bonds or inter-molecular attraction reduction by water molecule like
plasticization effect. The other is so-called the “pop-corning” effect during solder
reflow. The absorbed moisture molecules can be accumulated in very tiny pores in
the cured resin or on the interfaces. The moisture vaporizes and generates high vapor
pressure during solder reflow. Both of the effects can result in material cracks and
interfacial delamination.
Chemical resistance is the ability of a material to retain its original property from
solvent attack or chemical reaction when a material contacts solvents and chemicals during the substrate manufacturing process and the subsequent microelectronic
assembly processes. These chemicals and solvents include plating and etching baths,
liquid dielectric materials, and defluxing solvents. The methylene chloride absorption
test is most commonly used for laminate chemical resistance assessments [3].
The dielectric constant or permittivity is a property of an electrical insulating material (a dielectric) and is equal to the ratio of the capacitance of a capacitor filled with
the given material to the capacitance of an identical capacitor in a vacuum without the
dielectric material. Dielectric constant is used to measure the ability of a material to
store an electric charge. When an electric field is applied onto a dielectric material,
dielectric polarization occurs. Positive charges are displaced from its equilibrium
positions toward the negative side and negative charges are displaced toward the
positive side. This process generates the electrical potential energy and stored in the
material. The stronger the dielectric polarization, results in higher dielectric constant.
Dielectric polarization is closely related to the molecular polarity of a material. Under
the same electric field, the higher the molecular polarity is, the higher the dielectric
polarization is. Polymers with symmetric structure such as polytetrafluoroethylene
(PTFE) and polyethylene (PE) have lower molecular polarity. Dielectric constant
varies with frequency, humidity, and temperature and requires careful control in its
measurement. Since dielectric constant can distort electrical signal or voltage drops, it
is a critical material parameter in selecting base materials, especially for applications
that requires high frequency, such as telecommunication and avionics.
403
Flammability is another chemical property of a base material. This property is
significant when high power density is needed for a substrate or PWB to meet the
application needs. The Underwriters Laboratories (UL) classifies the flammability
properties as 94 V-0, 94 V-1, or 94 V-2. 94 V-0 represents the lowest level of flammability category of a material. The definition of these classifications can be found in
UL website.
Moisture absorption primarily results from the chemical nature of cured organic
resins. Most of the reinforcement and conductor materials in a PWB or substrate do
not absorb noticeable moisture. For resin systems commonly used in substrates, the
moisture absorption ranges from 0.1% of FR-4 epoxy to ~0.5% of BT/epoxy blend
and cyanate ester [3]. Moisture absorption is an important property of base materials
for substrate applications, especially for high temperature lead-free substrate applications. The absorbed moisture in substrate mainly has two effects on substrate reliability during package assembly process. One is that the absorbed moisture can weaken
the interfacial adhesion between resin to reinforcement or conductor through hydrolysis of chemical bonds or inter-molecular attraction reduction by water molecule like
plasticization effect. The other is so-called the “pop-corning” effect during solder
reflow. The absorbed moisture molecules can be accumulated in very tiny pores in
the cured resin or on the interfaces. The moisture vaporizes and generates high vapor
pressure during solder reflow. Both of the effects can result in material cracks and
interfacial delamination.
Chemical resistance is the ability of a material to retain its original property from
solvent attack or chemical reaction when a material contacts solvents and chemicals during the substrate manufacturing process and the subsequent microelectronic
assembly processes. These chemicals and solvents include plating and etching baths,
liquid dielectric materials, and defluxing solvents. The methylene chloride absorption
test is most commonly used for laminate chemical resistance assessments [3].
The dielectric constant or permittivity is a property of an electrical insulating material (a dielectric) and is equal to the ratio of the capacitance of a capacitor filled with
the given material to the capacitance of an identical capacitor in a vacuum without the
dielectric material. Dielectric constant is used to measure the ability of a material to
store an electric charge. When an electric field is applied onto a dielectric material,
dielectric polarization occurs. Positive charges are displaced from its equilibrium
positions toward the negative side and negative charges are displaced toward the
positive side. This process generates the electrical potential energy and stored in the
material. The stronger the dielectric polarization, results in higher dielectric constant.
Dielectric polarization is closely related to the molecular polarity of a material. Under
the same electric field, the higher the molecular polarity is, the higher the dielectric
polarization is. Polymers with symmetric structure such as polytetrafluoroethylene
(PTFE) and polyethylene (PE) have lower molecular polarity. Dielectric constant
varies with frequency, humidity, and temperature and requires careful control in its
measurement. Since dielectric constant can distort electrical signal or voltage drops, it
is a critical material parameter in selecting base materials, especially for applications
that requires high frequency, such as telecommunication and avionics.