PCB Materials

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

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.