Editing Printed circuit board (section)

=== Key substrate parameters ===
The circuit-board substrates are usually dielectric composite materials. The composites contain a matrix (usually an [[epoxy resin]]) and a reinforcement (usually a woven, sometimes non-woven, glass fibers, sometimes even paper), and in some cases a filler is added to the resin (e.g. ceramics; 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 non-homogeneous structure also introduces local variations in electrical parameters, due to different resin/glass ratio at different areas of the weave pattern. Non-woven 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]], [[breakdown voltage|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<sub>g</sub> then exerts mechanical overload on the board components - e.g. the joints and the vias. Below T<sub>g</sub> 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 a matrix with a high T<sub>g</sub>.

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 [[phase velocity|signal propagation speed]], frequency dependence introduces phase distortion in wideband applications; as flat a dielectric constant vs frequency characteristics as is 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 (conduction, or arcing, through the dielectric).

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 engineering 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]].

Moisture absorption occurs when the material is exposed to high humidity or water. Both the resin and the reinforcement may absorb water; water also may be soaked by capillary forces through voids in the materials and along the reinforcement. Epoxies of the FR-4 materials are not too susceptible, with absorption of only 0.15%. [[Teflon]] has very low absorption of 0.01%. [[Polyimides]] and cyanate esters, on the other side, suffer from high water absorption. Absorbed water can lead to significant degradation of key parameters; it impairs tracking resistance, breakdown voltage, and dielectric parameters. Relative dielectric constant of water is about 73, compared to about 4 for common circuit board materials. Absorbed moisture can also vaporize on heating, as during [[soldering]], and cause cracking and [[delamination]],<ref>Sood, B. and Pecht, M. 2011. Printed Circuit Board Laminates. Wiley Encyclopedia of Composites. 1–11.</ref> the same effect responsible for "popcorning" damage on wet packaging of electronic parts. Careful baking of the substrates may be required to dry them prior to soldering.<ref>{{Cite journal |last=Lee W. Ritchey, Speeding Edge |date=November 1999 |title=A Survey and Tutorial of DIELECTRIC MATERIALS USED in the Manufacture of Printed Circuit Boards |url=http://speedingedge.com/PDF-Files/tutorial.pdf|journal=Circuitree Magazine}}</ref>