Understanding Impedance: Key Points Explained Simply

Like rigid PCBs used in high-speed devices, flexible and rigid-flex circuit boards also need carefully controlled electrical pathways (impedance). They use the same target values – usually 50 ohms for single wires and up to 120 ohms for paired wires.

But achieving these values is trickier for flexible boards. Why? Because they must bend during use – something rigid boards don’t need to do. This bending requirement changes how we design the electrical pathways.

In this post, we’ll explore:

• Key design areas affected by impedance control
• Smart solutions to maintain flexibility while hitting electrical targets

Like rigid PCBs used in high-speed devices, flexible and rigid-flex circuit boards also need carefully controlled electrical pathways (impedance). They use the same target values – usually 50 ohms for single wires and up to 120 ohms for paired wires.

But achieving these values is trickier for flexible boards. Why? Because they must bend during use – something rigid boards don’t need to do. This bending requirement changes how we design the electrical pathways.

In this post, we’ll explore:

• Key design areas affected by impedance control
• Smart solutions to maintain flexibility while hitting electrical targets

olyimide flex boards use two copper types:

1. ED copper (Electroplated)
   • Texture: Rough surface
2. RA copper (Rolled & Annealed)
   • Texture: Smoother surface

Which to choose?
Depends on your bending needs:

• “Bend once” (permanent install): ED often works
• Repeated bending (like in moving parts): RA preferred

Why ½ oz copper is standard?
This thickness (≈17μm) gives the best balance:
Ultra-thin construction
Maximum bend flexibility
Good current capacity
Meets most design needs

High-speed design note: Current RA copper isn’t perfectly smooth. May cause “skin effect” issues at high frequencies (Where signals travel mostly on conductor surfaces).

Flexible PCB Core Materials: Two Types

Polyimide cores come in two styles, based on how copper sticks to the polyimide:

1. Adhesive Method (Older):
   • Uses glue (acrylic/epoxy) to bond copper to polyimide
   • Like sticking layers together with tape
2. Adhesiveless Method (Newer):
   • Polyimide is cast directly onto copper
   • No glue layer needed

Why Designers Prefer Adhesiveless:
Better for impedance control:

• Glue has different electrical properties , can disrupt signals
• Adhesiveless = consistent electrical behavior

Thinner & more flexible:

• Eliminates glue layer, extra-thin yet durable design

Setting Up Controlled Impedance in Your Design

The four main layer configurations used to get controlled impedance in flexible circuits are similar to those used in rigid PCB designs. They are: Embedded Microstrip, Edge-Coupled Embedded Microstrip, Symmetrical Stripline, and Edge-Coupled Stripline.

When it comes to a flex circuit design, the main difference between the Microstrip and Stripline configurations, aside from the fact that the Microstrip configurations don’t have shielding on one side, is the effect on the flex thickness. And this, in turn, affects the mechanical bend ability and reliability of the circuit.

If you want maximum bend capability, the 2-layer Microstrip is the best configuration. When you combine it with ½ OZ copper, a 0.002” flex core and 0.0005” Coverlays, after lamination, the finished flex thickness is about 0.006”. According to the general 10X thickness rule, this means the minimum bend radius is 0.060”.

The Stripline configurations, although offering shielding on both sides of the circuitry, substantially augment the flex thickness and impose limitations on the bend capabilities of the component. Besides the additional shield layer and its corresponding dielectric core layer, the thickness of both cores must be increased from 0.002” to 0.003”. For a design employing ½ OZ copper, the finished flex thickness escalates to over 0.011” following lamination. This restricts the bend capabilities to over 0.200”.

Flexible to Rigid Impedance Transitions

A distinctive feature of rigid-flex PCB designs with controlled impedance is the necessity to alter the configuration of the impedance lines as the circuits transition from the flexible regions to the rigid regions. In higher-layer-count designs, it is common for the impedance lines in the flexible areas to be configured as Microstrip structures. However, due to the requirement for power and ground planes in the rigid areas, the configuration often transitions to a Stripline structure. Ideally, the same line width and spacing should be maintained in both the flexible and rigid areas by increasing the core thickness in the rigid area adjacent to the plane. Nevertheless, higher layer counts may not accommodate the additional thickness. Consequently, the solution involves adjusting the line width and/or spacing as the circuits transition from the flexible to the rigid areas. This scenario does not apply to Stripline configurations, as the reference planes in the flexible areas extend continuously into the rigid areas.

Controlled Impedance & Handling Higher Currents

Both impedance control and higher current carrying requirements together can pose a design challenge. To meet the current requirements, we need thicker copper. But when combined with the impedance requirements, it results in an overlay thick flex construction. And this construction might not be able to meet the specified bend requirements reliably. Usually, the solution is specific to each design. Some of the following methods might be involved:

1. Isolate high-current traces by putting them on separate flex layers, using the air gaps in an Air-Gap Multilayer Flex design for better electrical separation.
2. Widen the flex circuit area to make room for thicker high-current traces, especially when using thinner ½ oz copper (which needs more width to carry the same current).
3. Combine (“gang”) multiple parallel traces on several flex layers to share high-current flow, reducing load on any single trace.
4. Only place reference planes directly above/below impedance-sensitive traces. Use leftover space elsewhere for grouped (“ganged”) high-current traces.

The goal is always to minimize the flex thickness as much as possible so that we can reliably meet the bend requirements.

Implementing controlled impedance in a flexible circuit or rigid-flex PCB design entails additional considerations owing to the mechanical bend requirements of the component. By necessity, controlled impedance contributes to an increase in flex thickness. When combined with higher current-carrying requirements, this can further augment the flex thickness to such an extent that the bend requirements may no longer be achievable. These challenges can typically be mitigated through a comprehensive design review and collaboration with an experienced supplier. 

The primary objective remains to minimize the flex thickness to the greatest extent possible, thereby ensuring reliable compliance with the bend requirements.