Design Guidance

2022-11-08 BEST

Impedance Controlled in Flex Designs

A cross hatched reference plane has a significant impact on the impedance controlled value. Since cross hatching means the plane is no longer solid copper, but has a significant percentage of copper removed from it, the plane no longer provides 100% shielding to the signal traces. As such, the controlled impedance of the signal traces increases.

Impedance is a measure of how much the circuit impedes the flow of current. It is like resistance, but it also takes into account the effects of capacitance and inductance. Impedance is measured in ohms.

Impedance is very important to transmission lines and is used to determine the performance of a high-speed circuit. Impedance can be controlled with several different configurations including Characteristic, Differential, and Coplanar Impedance models.

The first type is a micro-strip configuration where a conductor is above a ground plane.

The second type is the strip line configuration where a conductor is running between two ground planes. The varying of conductor width, dielectric thickness’ and material selections controls the impedance. (Dielectric constant (DK value)).

The third type is the strip line are adjacent to each other in the same plane with tightly controlled width and spacing, ground plane optional.

Predictable electrical characteristics make flex circuits an ideal choice for high-speed signal transmission. Uniform spacing between conductors and grounds, continuous shield layers, and repeatable geometries are features that help control impedance and reduce crosstalk. And with flex circuits, you can eliminate connectors and other transitions that contribute to signal attenuation.

BEST TECH can provide tight tolerances on line width, spacing, and distance to ground layers in order to meet your impedance requirements. (According to IPC Standard, the normal tolerance is +/-20% and we can control line width/spacing it at +/-10%) Actual impedance will also depend on the circuit’s shape after installation.

Contact BEST TECH for advice on designing circuits to specific electrical characteristics.

• Microstrip - a single ground plane beneath the signal lines.

 microstrip

• Stripline - dual ground layers above and below the signal lines.

stripline

• Edge coupled differential pairs – traces are adjacent to each other in the same plane with tightly controlled width and spacing, ground plane optional.

 Edge Coupled Differential Pairs

 

Best Technology has a simple excel file that can calculate the impedance control and you can also send your Drawing, and stack it up so that we can calculate the exact impedance control using the professional software Polar Si9000. Normal tolerance of impedance control is +/-10% for values bigger than 50 ohm, and +/-20%for value <=50 Ohm.

• Rigid-flex/stiffened flex circuits with uninterrupted ground layers.

• Silver epoxy coating. Silver epoxy is applied to the outside of circuits and electrically connected to other layers via access holes in the cover coat. Silver epoxy shielding is more flexible than copper.

Impedance, on our flexible circuits, can be reliably reproduced time and time again because of the relatively low dielectric constant and the roll-annealed copper (RA Cu) structure.

Cross-hatching planes give poor electrical quality for controlled impedance and other applications. In some applications, a wide, solid strip under critical traces is acceptable.

The higher the copper percentage being removed in the crosshatch, the higher the increase in controlled impedance when compared to the solid copper plane.

A 50% reduction in copper due to cross-hatching may affect the controlled impedance as much as 7% -- 16% (when compared to the case of the solid plane).