Tom
11-25-2002, 10:57 AM
IPC Advanced Study Guide Page Reference: Page 254, Section 4.11
Documentation of Impedance
A. Nominal impedance and it's tolerance should be specified in the fabrication notes. The industry standard is currently +/-10%. However, an open ended tolerance may be more cost effective and may perform better. e.g. As high as it can get but no lower than X, or as low as it can get but no higher than X.
B. The notes should include which layers have controlled impedance.
C. The notes should specify to which conductor width the controlled impedance applies.
D. The PWB fabricator should be allowed to adjust the dielectric thickness and/or the conductor width in order to meet process requirements and provide the needed impedance. The dielectric thickness requirements should be shown as a nominal figure for the impedance calculations, with limitations controlled by the overall thickness tolerance. Keep in mind that there could be minimal dielectric thickness requirements. Conductor adjustment tolerances should be stated in the notes.
Whose formula do you use?
The answer to this question is very simple - use the formula that your board fabricator uses. If one checks with different printed board manufacturing facilities, it turns out that each fabricator will have their own custom formulas optimized for their specific processes. There are many different impedance calculators available from many different sources, which can perform the necessary functions to get you in the ballpark. Be aware that the answers that the calculators will give , most likely, will not be the same answer the board manufacturer gets with their equations or calculators. If you find that all your fabricators are using the same method or tool to determine impedance, your best approach is to use that method or tool. Also, its very important to coordinate the printed board dielectric and stack-up requirements with the fabricator.
Layout Considerations
A. Termination Stubs End terminators should be connected to the conductor or component pin at or beyond the last load. Connecting terminations before the last load can create a stub, which can cause a signal reflection.
B. Vias A via adds an increase in capacitance and inductance per unit length of a conductor and can impact impedance if numerous vias are placed very close together.
C. Corners 90° corners add capacitance and lower inductance of the circuit. One would expect a 90° corner to affect impedance, causing a reflection. In reality, the length of the discontinuity is so short that 90° corners do not impact impedance enough to generate reflections large enough to be of concern.
Documentation of Impedance
A. Nominal impedance and it's tolerance should be specified in the fabrication notes. The industry standard is currently +/-10%. However, an open ended tolerance may be more cost effective and may perform better. e.g. As high as it can get but no lower than X, or as low as it can get but no higher than X.
B. The notes should include which layers have controlled impedance.
C. The notes should specify to which conductor width the controlled impedance applies.
D. The PWB fabricator should be allowed to adjust the dielectric thickness and/or the conductor width in order to meet process requirements and provide the needed impedance. The dielectric thickness requirements should be shown as a nominal figure for the impedance calculations, with limitations controlled by the overall thickness tolerance. Keep in mind that there could be minimal dielectric thickness requirements. Conductor adjustment tolerances should be stated in the notes.
Whose formula do you use?
The answer to this question is very simple - use the formula that your board fabricator uses. If one checks with different printed board manufacturing facilities, it turns out that each fabricator will have their own custom formulas optimized for their specific processes. There are many different impedance calculators available from many different sources, which can perform the necessary functions to get you in the ballpark. Be aware that the answers that the calculators will give , most likely, will not be the same answer the board manufacturer gets with their equations or calculators. If you find that all your fabricators are using the same method or tool to determine impedance, your best approach is to use that method or tool. Also, its very important to coordinate the printed board dielectric and stack-up requirements with the fabricator.
Layout Considerations
A. Termination Stubs End terminators should be connected to the conductor or component pin at or beyond the last load. Connecting terminations before the last load can create a stub, which can cause a signal reflection.
B. Vias A via adds an increase in capacitance and inductance per unit length of a conductor and can impact impedance if numerous vias are placed very close together.
C. Corners 90° corners add capacitance and lower inductance of the circuit. One would expect a 90° corner to affect impedance, causing a reflection. In reality, the length of the discontinuity is so short that 90° corners do not impact impedance enough to generate reflections large enough to be of concern.