Tom
11-18-2002, 03:39 PM
IPC Advanced Study Guide Page Reference: Page 251, Section 4.11
B. Dielectric Thickness - This is one of the 2 most important factors in determining impedance. As the dielectric thickness increases, the impedance of the trace will increase. With very thin dielectrics, a standard tolerance can create wide changes in impedance.
C. Core Material vs. Prepreg - In calculating impedance one should be aware that prepreg and core material, based on supplied thickness, will probably have a slightly different „Dr. Thus, the impedance value must be calculated for each layer utilizing the appropriate values.
D. Conductor Width - Conductor width is another important factor in determining impedance. As the conductor width increases, the impedance will decrease. A lot of times the conductor width cannot be varied much due to routing constraints, therefore it is the 2nd most important factor in determining impedance. Keep in mind that the narrower the conductor, with the same adjustment allowance, the more difficult it is for the PWB fabricator to meet the required impedance tolerance.
E. Conductor Thickness ¡V Conductor thickness is a variable in determining impedance, but it is less critical and has much less of an impact than conductor width or dielectric spacing or Er. As conductor thickness increases, the overall impedance will decrease.
It is also important to prevent reflections, or minimize their effect, in a transmission line is to match the impedance of the load or the driver to the impedance of the transmission line. A termination resistor is usually the method of choice. To match the load to the line, the resistor should be placed at the farthest load in the circuit and must be a value that is the same as the impedance of the line. To match the driver to the line, the resistor should be placed at the driver output. The resistor value should, when summed with the value of the driver¡¦s output impedance, equal the impedance of the line.
B. Dielectric Thickness - This is one of the 2 most important factors in determining impedance. As the dielectric thickness increases, the impedance of the trace will increase. With very thin dielectrics, a standard tolerance can create wide changes in impedance.
C. Core Material vs. Prepreg - In calculating impedance one should be aware that prepreg and core material, based on supplied thickness, will probably have a slightly different „Dr. Thus, the impedance value must be calculated for each layer utilizing the appropriate values.
D. Conductor Width - Conductor width is another important factor in determining impedance. As the conductor width increases, the impedance will decrease. A lot of times the conductor width cannot be varied much due to routing constraints, therefore it is the 2nd most important factor in determining impedance. Keep in mind that the narrower the conductor, with the same adjustment allowance, the more difficult it is for the PWB fabricator to meet the required impedance tolerance.
E. Conductor Thickness ¡V Conductor thickness is a variable in determining impedance, but it is less critical and has much less of an impact than conductor width or dielectric spacing or Er. As conductor thickness increases, the overall impedance will decrease.
It is also important to prevent reflections, or minimize their effect, in a transmission line is to match the impedance of the load or the driver to the impedance of the transmission line. A termination resistor is usually the method of choice. To match the load to the line, the resistor should be placed at the farthest load in the circuit and must be a value that is the same as the impedance of the line. To match the driver to the line, the resistor should be placed at the driver output. The resistor value should, when summed with the value of the driver¡¦s output impedance, equal the impedance of the line.