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Impedance is a measure of the limit imposed by a circuit on current. It is similar to a resistor, but it also considers the effects of inductance and capacitance. Impedance control in the flexible stack is essential to reduce signal reflections and achieve reliable signal integrity.
Controlled impedance (CI) is the characteristic impedance of the transmission line in the PCB conductor and its associated reference plane. It is especially needed when high-frequency signals propagate through circuit board traces.
Why do we need to control the impedance of the flexible PCB?
In modern times, flexible circuit boards have become smaller, faster, and more complex. Flex boards are commonly used in high-frequency applications, such as RF communications, telecommunications, calculations that use signal frequencies above 100MHz, high-speed signal processing, and high-quality analog video, such as DDR, HDMI, Gigabit Ethernet, etc.
The signal trace has impedance at every point in the signal path. If the impedance point is different from the point, there will be signal reflection, the amplitude of which depends on the difference between the two impedances. This reflection will propagate in the opposite direction to the signal, which means that the reflected signal will be superimposed on the original signal. In order to better understand controlled impedance readings, why is controlled impedance really important?
What is impedance matching in PCB?
When it comes to flexible PCB design, impedance matching becomes critical because they are often used in high-speed applications. It refers to matching the load impedance with the characteristic impedance of the transmission line. If the load impedance and the characteristic impedance are equal, the reflection in the transmission line will be eliminated. This ensures that the reception of the original signal is not attenuated.
Factors affecting the impedance of flexible circuit boards
Flexible impedance control can be achieved by changing the physical dimensions of the PCB traces and the characteristics of the dielectric materials used. The following are the factors that affect the impedance of the flexible PCB.
The physical size of the trace
Trace height
The width of the top surface of the trace
Width of trace bottom
The width difference between the top of the trace and the bottom of the trace
The height of the trace from the ground plane
The dielectric properties of the dielectric materials used
The dielectric constant of the added dielectric material
The dielectric height between the trace and the reference plane
The dielectric constant of the solder mask or cover layer
Controlled impedance configuration of flexible board
The most common configuration for impedance control of flexible boards is:
Single-ended microstrip
How to build a Flex Stack-Up with controllable impedance
Single-ended microstrip line for flexible PCB
H1: The dielectric height between the trace and the reference plane
W1: the width of the bottom of the trace
W2: the width of the top surface of the trace
T1: The thickness of the trace
Er1: The dielectric constant between the trace and the reference plane
This configuration has a transmission line made of uniform conductors (thickness and width) on the outer layer of the circuit board stack. The reference plane provides a current return path for the signal transmitted on the transmission line. Single-ended microstrip lines allow for thinner flexible structures, which also increases flexibility and reduces overall cost.
Edge-coated differential microstrip line
Edge-coupled differential microstrip line for flexible PCB
H1: The dielectric height between the trace and the reference plane
W1: the width of the bottom of the trace
W2: the width of the top surface of the trace
T1: The thickness of the trace
S1: The interval between the two traces of the differential pair
C1, C2, and C3: Cover layer thickness at different positions
CEr: Dielectric constant of the cover layer
When the signal and its complement are transmitted on two independent traces, it is called a differential signal. These traces are called differential pairs. The traces are routed at a constant pitch. One of the main advantages of having an edge-coupled differential pair is that the noise on the reference plane is common to both traces. This cancels out the noise at the receiving end.
Single-ended ribbon line
How to build a Flex Stack-Up with controllable impedance
Single-ended stripline for flexible PCB
H1: The height of the first dielectric
H2: The height of the second layer of dielectric
W1: the width of the bottom of the trace
W2: the width of the top surface of the trace
Er1: The dielectric constant of the first dielectric
Er2: Dielectric constant of the second dielectric
T1: The thickness of the trace
It implements signal routing between two ground planes in a multilayer PCB. The return path of the high frequency signal is located above and below the signal trace on the plane.
Edge-coupled differential stripline
How to build a Flex Stack-Up with controllable impedance
Edge-coupled differential stripline for flexible PCB
H1: The height of the first dielectric
H2: The height of the second layer of dielectric
W1: the width of the bottom of the trace
W2: the width of the top surface of the trace
Er1: The dielectric constant of the first dielectric
Er2: Dielectric constant of the second dielectric
T1: The thickness of the trace
S1: The interval between the two traces of the differential pair
This configuration has two controlled impedance traces sandwiched between two planes. It is similar to a single-ended ribbon line. The only difference is that it has a pair of conductors separated by a uniform distance.
Cross-hatched reference plane in flexible PCB
The ratio of the cross-hatched conductor width (HW) to the cross-hatched pitch (HP) plays an important role in characterizing the cross-hatched plane. If the ratio is about 0.293, 50% copper removal can be achieved. The smaller the ratio, the greater the percentage of copper to be removed. Compared with a rigid copper plane, the only disadvantage of flexible control impedance is the need to have a higher control impedance value.
The cross-hatched reference plane means that a large percentage of copper has been removed from the plane. It has a significant impact on the controlled impedance in the flexible PCB. The cross-hatched plane cannot provide 100% shielding for signal traces. The main purpose of the cross-hatched reference plane is to increase the flexibility of the circuit board.
The impedance control in the flexible design requires a thicker flexible core than the standard flexible core to achieve the required impedance value. A thicker flexible core increases the overall thickness and reduces bendability.
The surface microstrip configuration gives way to the thinnest flexible core possible, providing the highest degree of flexibility. The stripline configuration allows shielding on either side of the trace. However, this configuration significantly increases the deflection thickness, which in turn reduces the deflection capability.
The flexible board is usually made of a polyimide substrate. Compared to rigid materials, these substrates provide lower Dk values (3 to 3.5). The thickness of the flexible material is always uniform. This makes them ideal for flexible control impedance design.
There are two types of polyimide materials: adhesive-based materials and adhesive-free materials. Both adhesive-free and adhesive-based materials can be used for flexible CI designs. However, due to its consistent results, binder-free materials are more suitable for high-speed applications.
Advanced materials such as Teflon and Teflon/Polyimide hybrid materials are suitable for high-speed applications. These materials are more expensive than polyimide materials. Standard adhesive-free polyimide materials meet the design requirements of controlled impedance while reducing costs. Sierra Circuits uses DuPont materials for flexible PCBs.
Controlled impedance is one of the key factors to minimize signal reflections in the PCB.
