Dictionary of Engineering Terms
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Editor note; The alphabetic links above will switch to the main glossary, use this to switch back to the PCB Dictionary of terms
Thermal Resistance: The resistance of a substance to the conductivity to heat. In this case the amount of heat an area of copper pad on a FR4 board can dissipate without increasing its own temperature.
Thermal Resistance Graph
Note; the larger the pad area the smaller the increase in temperature, regardless of the thickness of the copper. Note the Red line is 1 oz copper, the blue line is 2 oz copper, and the black line is 3oz copper. The copper pad area is in square inches.
Normally the thickest copper is on the outer layers, while the inner board layers use a thinner layer of copper [less copper weight].
FR4: A type of Printed Circuit Board [PCB] material, Flame Retardant #4, which is self-extinguishing. FR4 is a very common material used in manufacturing of insulating laminate used with printed wiring boards. There is of course a classification of FR3, FR2, and FR1 material, having different characteristics.
Pad: A portion of a conductive pattern used as a soldering area. Also called a land. The metallized area on a substrate or on the face of an integrated circuit used for making electrical
connections.
Note that the military documents listed may or may not be valid, as DOD documents are pulled from service all the time, or folded into other documents. The DOD term for a with-drawn document is cancelled with no replacement.
FR4 is a common used substrate used for general purpose printed circuit boards and high speed digital circuit boards. But there are other dielectrics used for high speed circuit operation; although most of the other materials are known by there trade names and not from a reference in a standard or specification [PCB Board Materials]. However FR4 performs well up to a few GHz, so most high speed digital circuits will still work well using this material.
