14 Fundamentals of Advanced Materials …
413
foils used in substrates is 0.5 oz, 1 oz, 2 oz, and 3 oz, corresponding to thickness of
approximately 18 μm, 36 μm, 70 μm, and 140 μm, respectively. Use of thin Cu foils
of 0.25 oz and 0.375 oz thickness is emerging [3]. Since the copper foil thickness
directly impacts current density, the total substrate thickness, substrate warpage, and
etching productivity, it is usually the first parameter to be considered in substrate
manufacturing.
The surface roughness of copper foil impacts the copper foil adhesion to the
resin system, electrical performance, and manufacturing capability of trace width.
Higher surface roughness foils generate a larger surface area and more mechanical
bonding sites. Therefore it usually results in higher adhesion strength. However,
higher frequency electrical signals propagate near the surface of the conductor. An
electrical signal needs to travel farther on a rougher surface and results in more
signal attenuation. With very thin dielectric layer sandwiched in-between two copper
foils, the surface roughness of the copper foil causes capacitance and resistance
variation from location to location, and this can distort the signal as well. Copper foil
surface roughness also impacts copper etching time. In order to ensure the etching
completeness, the etch time must be set up long enough to ensure the thickest location
is etched away. This results in trapezoidal circuit traces and reduces the capability
of making fine-line circuits in the substrate.
Surface treating copper foil is done for the purpose of achieving specific needs.
Coupling agent treatments are used to increase the adhesion between copper foil
and resin systems, and help prevent copper surface oxidation and contamination.
Bonding treatments, or nodularization is done by plating copper or copper oxide
nodules to the surface of copper foil to increase the surface area. The purpose of
this treatment is mainly to achieve good adhesion to high performance resin systems
such as BT, cyanate ester, and polyimide resin systems. Thermal barrier treatment
is conducted by coating a several hundred angstroms thick of zinc, nickel, or a
brass layer over copper foil nodules. Their purpose is to prevent or retard the bond
degradation between copper foil and resin systems during laminate, substrate, and
microelectronic package manufacturing. Passivation and antioxidant coatings are
very thin layers of chromium-based or organic-based coatings on the both sides of
the copper foil, preventing oxidation during storage and lamination. The coating is
usually removed prior to substrate and PWB manufacturing [3].
The mechanical properties of copper foil that are distinguishable among different
grade of copper foil are tensile strength and elongation. These two properties are
mainly determined by grain size, orientation, and structure from the manufacturing
process. Readers may refer to IPC-4562 specification or get detail technical data
from suppliers. Usually, electrodeposited copper foil has higher tensile strength but
lower elongation percentage than rolled-wrought copper foil.
The purity and resistivity of copper foils is also specified in IPC-4562. The
minimum purity of electrodeposited copper foil without treatment but with any silver
counted as copper is 99.8%. The minimum purity for rolled-wrought copper foils is
99.9%.
The main purpose of integrating other metal alloys onto copper foil is to make
buried resistors. A resistive metal alloy is first coated onto the base copper foil. The
413
foils used in substrates is 0.5 oz, 1 oz, 2 oz, and 3 oz, corresponding to thickness of
approximately 18 μm, 36 μm, 70 μm, and 140 μm, respectively. Use of thin Cu foils
of 0.25 oz and 0.375 oz thickness is emerging [3]. Since the copper foil thickness
directly impacts current density, the total substrate thickness, substrate warpage, and
etching productivity, it is usually the first parameter to be considered in substrate
manufacturing.
The surface roughness of copper foil impacts the copper foil adhesion to the
resin system, electrical performance, and manufacturing capability of trace width.
Higher surface roughness foils generate a larger surface area and more mechanical
bonding sites. Therefore it usually results in higher adhesion strength. However,
higher frequency electrical signals propagate near the surface of the conductor. An
electrical signal needs to travel farther on a rougher surface and results in more
signal attenuation. With very thin dielectric layer sandwiched in-between two copper
foils, the surface roughness of the copper foil causes capacitance and resistance
variation from location to location, and this can distort the signal as well. Copper foil
surface roughness also impacts copper etching time. In order to ensure the etching
completeness, the etch time must be set up long enough to ensure the thickest location
is etched away. This results in trapezoidal circuit traces and reduces the capability
of making fine-line circuits in the substrate.
Surface treating copper foil is done for the purpose of achieving specific needs.
Coupling agent treatments are used to increase the adhesion between copper foil
and resin systems, and help prevent copper surface oxidation and contamination.
Bonding treatments, or nodularization is done by plating copper or copper oxide
nodules to the surface of copper foil to increase the surface area. The purpose of
this treatment is mainly to achieve good adhesion to high performance resin systems
such as BT, cyanate ester, and polyimide resin systems. Thermal barrier treatment
is conducted by coating a several hundred angstroms thick of zinc, nickel, or a
brass layer over copper foil nodules. Their purpose is to prevent or retard the bond
degradation between copper foil and resin systems during laminate, substrate, and
microelectronic package manufacturing. Passivation and antioxidant coatings are
very thin layers of chromium-based or organic-based coatings on the both sides of
the copper foil, preventing oxidation during storage and lamination. The coating is
usually removed prior to substrate and PWB manufacturing [3].
The mechanical properties of copper foil that are distinguishable among different
grade of copper foil are tensile strength and elongation. These two properties are
mainly determined by grain size, orientation, and structure from the manufacturing
process. Readers may refer to IPC-4562 specification or get detail technical data
from suppliers. Usually, electrodeposited copper foil has higher tensile strength but
lower elongation percentage than rolled-wrought copper foil.
The purity and resistivity of copper foils is also specified in IPC-4562. The
minimum purity of electrodeposited copper foil without treatment but with any silver
counted as copper is 99.8%. The minimum purity for rolled-wrought copper foils is
99.9%.
The main purpose of integrating other metal alloys onto copper foil is to make
buried resistors. A resistive metal alloy is first coated onto the base copper foil. The