PCB
Material
The PCB
Material is the dielectric or insulating material on which the circuits of the
design can be etched or printed. The PCB dielectric material will be available
in the following forms,
•
Laminate
•
Copper –Clad
•
Core
•
Pre-Preg
•
Copper foil
The Laminate and Pre-preg are the
base material, on which the copper will be placed.
Copper
cladding is the combination of both dielectric and copper, which means that,
copper has been claded on one side of the dielectric material.
Core is a
type of material; it has copper on its both sides
Copper foil is the entire copper which is being used on
layers
The PCB
Material Properties:
There are number of material properties which have to be
considered while choosing the martial for the product design and they are,
Dielectric
constant:
This property is a measure of the effect an insulating
material has on the capacitance of a conductor imbedded in or surrounded by
it. It is also a measure of the degree
to which an electromagnetic wave is slowed down as it travels through the
insulating material. The higher the
relative dielectric constant, the slower a signal travels on a wire,
 |
(velocity
of propagation)
|
 |
(dielectric
constant)
|
where
C = 3*108 meters per second, or about 30 cm/ns (12 in/ns)
E ' r = effective dielectric constant
Vp = velocity of propagation
C = 3*108 meters per second, or about 30 cm/ns (12 in/ns)
E ' r = effective dielectric constant
Vp = velocity of propagation
Glass Transition Temperature, Tg
All common laminate
resins exhibit changing temperature coefficients of expansion as temperature
increases. Glass transition temperature or Tg is the temperature at which the
temperature coefficient of expansion makes a significant change from a low
value to a much higher value. At Tg, all materials start expanding.
Loss tangent-
Loss tangent is a measure of how much of the electromagnetic
field travelling through a dielectric is absorbed or lost in the dielectric.
Dielectric Breakdown Voltage, DBV
Dielectric breakdown voltage is a measure of an insulator’s
ability to withstand the stress of high voltages placed across it. In general,
it can be seen that all of the commonly available laminates have at least 1000
volts per mil of thickness DBV. This
means that a 2 mil thick laminate can withstand a voltage stress as high as
2000 volts.
Moisture Absorption- All resins absorb some moisture or water when exposed to high
humidity environments. This absorption
affects the PCB in two ways. Water has a
relative dielectric constant of approximately 73. If a laminate absorbs a significant amount of
water the resulting relative dielectric constant of the combination will be
higher than the 4.1 used to calculate impedance and can cause impedance
mismatches. A more important effect of
moisture absorption is increased leakage current.
The most commonly used material is FR-4, which means Flame
retardant or Flame resistant. Some of the high speed materials are from Nelco,
Rogers & Getek.
