titanate ceramics can be used to increase the dielectric constant). The reinforcement type defines two major classes of materials - woven and non-woven. Woven reinforcements are cheaper, but the high dielectric constant of glass may not be favorable for many higher-frequency applications. The spatially nonhomogeneous structure also introduces local variations in electrical parameters, due to different resin/glass ratio at different areas of the weave pattern. Nonwoven reinforcements, or materials with low or no reinforcement, are more expensive but more suitable for some RF/analog applications. The substrates are characterized by several key parameters, chiefly thermomechanical ( glass transition temperature , tensile strength , shear strength , thermal expansion ), electrical ( dielectric constant , loss tangent , dielectric breakdown voltage , leakage current , tracking resistance ...), and others (e.g. moisture absorption ). At the glass transition temperature the resin in the composite softens and significantly increases thermal expansion; exceeding T g then exerts mechanical overload on the board components - e.g. the joints and the vias. Below T g the thermal expansion of the resin roughly matches copper and glass, above it gets significantly higher. As the reinforcement and copper confine the board along the plane, virtually all volume expansion projects to the thickness and stresses the plated-through
holes. Repeated soldering or other exposition to higher temperatures can cause failure of the plating, especially with thicker boards; thick boards therefore require high T g matrix. The materials used determine the substrate's dielectric constant. This constant is also dependent on frequency, usually decreasing with frequency. As this constant determines the signal propagation speed , frequency dependence introduces phase distortion in wideband applications; as flat dielectric constant vs frequency characteristics as achievable is important here. The impedance of transmission lines decreases with frequency, therefore faster edges of signals reflect more than slower ones. Dielectric breakdown voltage determines the maximum voltage gradient the material can be subjected to before suffering a breakdown. Tracking resistance determines how the material resists high voltage electrical discharges creeping over the board surface. Loss tangent determines how much of the electromagnetic energy from the signals in the conductors is absorbed in the board material. This factor is important for high frequencies. Low- loss materials are more expensive. Choosing unnecessarily low-loss material is a common error in high-frequency digital design; it increases the cost of the boards without a corresponding benefit. Signal degradation by loss tangent and dielectric constant can be easily assessed by an eye pattern .
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- Summer '15
- Test, Circuit Board, Printed circuit board