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Physical dimensions and material characteristics affect electrical properties like attenuation, propagation delay and impedance. Flex circuit designers and manufacturers must understand which physical tolerances require tight control and which ones are not as critical. Design changes which increase manufacturing yield, and simplify the design requires a complete understanding of the results.
Manufacturers must build controlled impedance flex circuits in most signal transmission applications. At low frequencies impedance control is not important because the tracks act as a resistor. The tracks, vias and pads, are largely invisible to the circuit.
High speed logic now cause the flex foil to function more like a wave guide. The speed of wave propagation becomes important, and variations or changes in the impedance could cause scattered or fringe signals. Since controlling impedance is so important with high speed logic, the flex circuit manufacturer must understand the effects of the manufacturing tolerance on impedance.
Impedance of a metal track on a board can be expressed as a function of flex dimensions.
For surface micro strip:
For strip line:
In these equations,
w is line width
t is the line thickness
h is the thickness of the dielectric
(Fig. 1 and 2)
Propagation delay, another quantity of importance can be expressed as follows for surface micro strip:
Instead of doing straight worst case analysis of tolerances, efforts are made to consider a number of variations and controlling specific processes to hit the desired result.
To look at other structural features, which influence electrical properties, other advanced techniques are used. They calculate the affects of magnetic fields around conductors.
One of the conditions which are monitored very closely by flex circuit manufacturers is conductor undercut, it is a fact of life, and fortunately does not have to be eliminated. It its’ effect can be understood, it can be included as a design parameter.
Generally, undercut becomes more important as geometries shrink. Fig. 3 illustrates how undercut alters the impedance of a centered strip line configuration.
On very wide lines undercut has relatively little effect on the electrical properties. At narrow line widths the effect on impedance becomes appreciable –about 5 ohms.
If a flex designer know the magnitude of this effect in advance it is possible to adjust the nominal line width to ensure a final desired impedance.
Most micro strip formulas in literature apply only to surface micro strip, yet designs often cover conductors with a dielectric layer.
In Fig. 4 the impedance of a buried micro strip line is compared to a surface micro strip line of the same width and ground plane separation.
Impedance decreases 5 to 15% depending on line width and ground plane separation.
To obtain performance one desires, careful consideration to the variables must be done.
For surface micro strip:
For strip line:
You can calculate how sensitive impedance and propagation delay are to physical parameter changes.
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Fig. 1 Shows how the variation in the dielectric thickness affects the impedance of the micro strip.
Fig. 2 Shows how the variation in the dielectric constant affects the propagation delivery of the centered strip line.
Figure 1: Impedance versus dielectric thickness for surface microstrip where w= 4mil, t= 1 and Σ=4.5.
Figure 2: Propagation delay versus dielectric constant for centered stripline. Σ r is relative dielectric constant
Figure 3: The effect on etchout on impedence for centered stripline.
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