DIFFERENTIAL IMPEDANCE CALCULATOR    Controlled Impedance high speed design for multilayer PCB's

© Copyright 1998 - 2005 In-Circuit Design Pty Ltd

 

The Differential Impedance Calculator is provided free to registered users. Please Register here

Instructions:
1. Select the number of substrate layers required.
2. You will then be presented with a table representing the suggested stack-up for that type of substrate.
3. Modify the variables to examine the effects on the trace Characteristic and Differential Impedance.

Number of physical board layers
4 6 8 10 12

 

Important: The trace separation should not be adjusted to alter the Differential Impedance - trace separation should always be kept to the minimum clearance specified by the PCB vendor.

 

Note:

1.      All dimensions are in MIL (thousands of an inch).

2.      The Dielectric Constant of FR4 material may vary by as much as 20% (4.2 to 5.2).

3.      The overall Dielectric Thickness (Cu to Cu) should total 62 MIL nominally.

4.      Variables unavailable for modification have no significant effect on the impedance of the traces.

5.      The default multilayer board stack-ups are taken from Advance Design for SMT, Barry Olney/AMC.

6.      The Impedance Calculator uses formulae derived from:

·        IPC-D-317 - Design Standard for Electronic Packaging Utilizing High Speed Techniques. 

·        EMC & the Printed Circuit Board - Montrose.

7.      Only Edge Coupled Differential Pairs are considered. No allowance has been made for Broad Side Coupling from adjacent layers. It is good practice to route adjacent layers orthogonal to each other in order to reduce any coupling that may occur.

8.   To reduce EMI, high frequency, fast rise time signals should be routed between the reference planes.

 

All care has been taken to ensure that the results are correct but no responsibility is taken for any errors.

 

If you prefer to do the calculations yourself - please use the formulae below.

 

Microstrip Differential Impedance

(for traces routed on an outer layer)

Unbalanced Stripline Differential Impedance

(for traces embedded between planes)

Zo =  87/Sqrt(Er+1.41)* ln(5.98H/(0.8W+T))

Zdiff = 2*Zo (1 – 0.48 e-0.96D/H)

Zo =  80/Sqrt Er* ln(1.9(2H+T)/(0.8W+T))

* (1 - (H / 4(H + C + T)))

Zdiff = 2*Zo (1 – 0.347 e-2.9D/B) 

 

where

W = trace width

T = trace thickness

H = distance to nearest reference plane

Er = dielectric constant

D = trace edge to edge spacing

C = signal layer separation

B = reference plane separation

 

Material Dielectric Constant
FR4 Fiberglass Epoxy 4.7
Teflon 2.2
Teflon Glass 2.5
Polyimide 3.5
Polyimide Glass 4.2

Relative Dielectric Constants of substrate materials