14 Fundamentals of Advanced Materials …
405
a parameter describing surface texture of a real surface. Surface roughness is often
expressed by both deviations amplitude and spatial frequency. The larger the deviation is, the rougher the surface is. Surface roughness is an important parameter as it
impacts adhesion between build-up layers in the substrates. For thin substrate applications, high surface roughness can result in short circuits or electrical breakdown
during substrate manufacturing, testing or field use. For high frequency RF applications, high surface roughness generates longer signal travel distance with varied
capacitance from location to location. This results in signal distortion and attenuation.
Surface energy or surface tension quantifies the amount of energy required to break
down intermolecular and interatomic bonds to create a new surface. Compared to
the molecules or atoms inside a bulk material, the molecules or atoms at the surface
are normally at higher energy state and relatively less stable. Surface energy is a
key parameter affecting liquid wetting on solid surface and ultimately impacting
adhesion strength of two materials. Higher surface energy of a solid material usually
results in better wetting. For a surface that has been exposed to the air for long time,
the surface often has absorbed many molecules from the ambient and the surface
energy has been reduced noticeably. Different kinds of cleaning methods are used to
regenerate the fresh surface to increase the surface energy and help the increase of
adhesion strength. Some of the cleaning methods include solvent cleaning, plasma,
etching, and blasting.
Surface resistivity is determined by the ratio of DC voltage drop per unit length
to the surface current per unit width. It is an intrinsic material property describing
how well the insulating material opposes the current flow on the material surface.
The measured surface resistivity of a material strongly depends on the environmental
conditions such as humidity and temperature as well as the conductor configuration
and contact wellness on surface. Therefore, the measurement procedure needs to be
well controlled. Standards, such as ASTM Standard D 257-99 or ESD STM 11.112001, can be followed. This parameter is often considered when small pitch exposed
conductor traces are required.
Per ASTM D495 Plastic Test Standard, arc resistance is measured by the number
of seconds that a material resists the formation of a surface conducting path when
subjected to an intermittently occurring arc of high voltage, low current characteristics. The results at 3 mm thickness are considered representative of the material’s
performance in any thickness. Performance Level Categories (PLC) was introduced
by UL to avoid excessive implied precision and bias.
Cored substrates are the most popular substrate utilized in microelectronic packaging. A typical cored substrate is schematically presented in Fig. 14.1. The substrate
is composed of a core, build up layer(s), via, PTH, expose metal pad, solder mask,
and surface finish.
405
a parameter describing surface texture of a real surface. Surface roughness is often
expressed by both deviations amplitude and spatial frequency. The larger the deviation is, the rougher the surface is. Surface roughness is an important parameter as it
impacts adhesion between build-up layers in the substrates. For thin substrate applications, high surface roughness can result in short circuits or electrical breakdown
during substrate manufacturing, testing or field use. For high frequency RF applications, high surface roughness generates longer signal travel distance with varied
capacitance from location to location. This results in signal distortion and attenuation.
Surface energy or surface tension quantifies the amount of energy required to break
down intermolecular and interatomic bonds to create a new surface. Compared to
the molecules or atoms inside a bulk material, the molecules or atoms at the surface
are normally at higher energy state and relatively less stable. Surface energy is a
key parameter affecting liquid wetting on solid surface and ultimately impacting
adhesion strength of two materials. Higher surface energy of a solid material usually
results in better wetting. For a surface that has been exposed to the air for long time,
the surface often has absorbed many molecules from the ambient and the surface
energy has been reduced noticeably. Different kinds of cleaning methods are used to
regenerate the fresh surface to increase the surface energy and help the increase of
adhesion strength. Some of the cleaning methods include solvent cleaning, plasma,
etching, and blasting.
Surface resistivity is determined by the ratio of DC voltage drop per unit length
to the surface current per unit width. It is an intrinsic material property describing
how well the insulating material opposes the current flow on the material surface.
The measured surface resistivity of a material strongly depends on the environmental
conditions such as humidity and temperature as well as the conductor configuration
and contact wellness on surface. Therefore, the measurement procedure needs to be
well controlled. Standards, such as ASTM Standard D 257-99 or ESD STM 11.112001, can be followed. This parameter is often considered when small pitch exposed
conductor traces are required.
Per ASTM D495 Plastic Test Standard, arc resistance is measured by the number
of seconds that a material resists the formation of a surface conducting path when
subjected to an intermittently occurring arc of high voltage, low current characteristics. The results at 3 mm thickness are considered representative of the material’s
performance in any thickness. Performance Level Categories (PLC) was introduced
by UL to avoid excessive implied precision and bias.
Cored substrates are the most popular substrate utilized in microelectronic packaging. A typical cored substrate is schematically presented in Fig. 14.1. The substrate
is composed of a core, build up layer(s), via, PTH, expose metal pad, solder mask,
and surface finish.