PCB Technical - Material issues that need to be considered when designing a wearable PCB

Due to the small size and size, there is almost no ready-made PCB printed circuit board standard for the growing wearable IoT market. Before these standards came out, we had to rely on the knowledge and manufacturing experience learned in board-level development and think about how to apply them to unique emerging challenges. There are three areas that require our special attention. They are: circuit board surface materials, RF/microwave design and RF transmission lines.

PCB material

PCBs are generally composed of laminates, which may be made of fiber-reinforced epoxy (FR4), polyimide, or Rogers materials or other laminate materials. The insulating material between the different layers is called a prepreg.

Wearable devices require high reliability, so when PCB designers are faced with the choice of using FR4 (the most cost-effective PCB manufacturing material) or more advanced and more expensive materials, this will become a problem.

If wearable PCB applications require high-speed, high-frequency materials, FR4 may not be the best choice. The dielectric constant (Dk) of FR4 is 4.5, the dielectric constant of the more advanced Rogers 4003 series material is 3.55, and the dielectric constant of the brother series Rogers 4350 is 3.66.

Figure 1: Stacking diagram of a multilayer circuit board, showing FR4 material and Rogers 4350 and the thickness of the core layer

The dielectric constant of a laminate refers to the ratio of the capacitance or energy between a pair of conductors near the laminate to the capacitance or energy between the pair of conductors in vacuum. At high frequencies, it is best to have a small loss. Therefore, Roger 4350 with a dielectric constant of 3.66 is more suitable for higher frequency applications than FR4 with a dielectric constant of 4.5.

Under normal circumstances, the number of PCB layers for wearable devices ranges from 4 to 8 layers. The principle of layer construction is that if it is an 8-layer PCB, it should be able to provide enough ground and power layers and sandwich the wiring layer. In this way, the ripple effect in crosstalk can be kept to a minimum and electromagnetic interference (EMI) can be significantly reduced.

In the circuit board layout design stage, the layout plan is generally to put a large ground layer close to the power distribution layer. This can form a very low ripple effect, and the system noise can also be reduced to almost zero. This is especially important for the radio frequency subsystem.

Compared with Rogers material, FR4 has a higher dissipation factor (Df), especially at high frequencies. For higher performance FR4 laminates, the Df value is around 0.002, which is an order of magnitude better than ordinary FR4. However, Rogers' stack is only 0.001 or less. When FR4 material is used for high frequency applications, there will be a significant difference in insertion loss. Insertion loss is defined as the power loss of the signal from point A to point B when using FR4, Rogers or other materials.

Manufacturing problem

Wearable PCB requires stricter impedance control, which is an important factor for wearable devices. Impedance matching can produce cleaner signal transmission. Earlier, the standard tolerance for signal carrying traces was ±10%. This indicator is obviously not good enough for today's high-frequency and high-speed circuits. The current requirement is ±7%, and in some cases even ±5% or less. This parameter and other variables will severely affect the manufacture of wearable PCBs with extremely strict impedance control, thereby limiting the number of businesses that can manufacture them.

The dielectric constant tolerance of the laminate made of Rogers UHF materials is generally maintained at ±2%, and some products can even reach ±1%. In contrast, the dielectric constant tolerance of the FR4 laminate is as high as 10%. Therefore, compare These two materials can be found that Rogers' insertion loss is particularly low. Compared with the traditional FR4 material, the transmission loss and insertion loss of the Rogers stack are half lower.

In most cases, cost is the most important. However, Rogers can provide relatively low-loss high-frequency laminate performance at an acceptable price point. For commercial applications, Rogers can be made into a hybrid PCB with epoxy-based FR4, some of which are made of Rogers material, and other layers are made of FR4.

When choosing a Rogers stack, frequency is the primary consideration. When the frequency exceeds 500MHz, PCB designers tend to choose Rogers materials, especially for RF/microwave circuits, because these materials can provide higher performance when the upper traces are strictly controlled by impedance.

Compared with FR4 material, Rogers material can also provide lower dielectric loss, and its dielectric constant is stable in a wide frequency range. In addition, Rogers material can provide the ideal low insertion loss performance required by high frequency operation.

The Coefficient of Thermal Expansion (CTE) of Rogers 4000 series materials has excellent dimensional stability. This means that compared with FR4, when the PCB undergoes cold, hot and very hot reflow cycles, the thermal expansion and contraction of the circuit board can be maintained at a stable limit under higher frequency and higher temperature cycles.

In the case of mixed stacking, it is easy to use common manufacturing process technology to mix Rogers and high-performance FR4 together, so it is relatively easy to achieve high manufacturing yield. The Rogers stack does not require a special via preparation process.

Ordinary FR4 cannot achieve very reliable electrical performance, but high-performance FR4 materials do have good reliability characteristics, such as higher Tg, still relatively low cost, and can be used in a wide range of applications, from simple audio design to Complex microwave applications.

RF/Microwave design considerations

Portable technology and Bluetooth have paved the way for RF/microwave applications in wearable devices. Today's frequency range is becoming more and more dynamic. A few years ago, very high frequency (VHF) was defined as 2GHz~3GHz. But now we can see ultra-high frequency (UHF) applications ranging from 10GHz to 25GHz.

Therefore, for wearable PCBs, the radio frequency part requires more attention to the wiring issues, and the signals should be separated separately, and the traces that generate high-frequency signals should be kept away from the ground. Other considerations include: providing a bypass filter, adequate decoupling capacitors, grounding, and designing the transmission line and return line to be almost equal.

The bypass filter can suppress the noise content and the ripple effect of crosstalk. Decoupling capacitors need to be placed closer to the device pins carrying power signals.

High-speed transmission lines and signal loops require a ground layer to be placed between the power layer signals to smooth the jitter generated by noise signals. At higher signal speeds, small impedance mismatches will cause unbalanced transmission and reception of signals, resulting in distortion. Therefore, special attention must be paid to the impedance matching problem related to the radio frequency signal, because the radio frequency signal has a high speed and a special tolerance.

Radio frequency transmission lines require controlled impedance in order to transmit radio frequency signals from a specific IC substrate to the PCB. These transmission lines can be implemented on the outer layer, the top layer, and the bottom layer, or they can be designed in the middle layer.

The methods used during the PCB RF design layout are microstrip lines, floating strip lines, coplanar waveguides, or grounding. The microstrip line consists of a fixed-length metal or trace, and the entire ground plane or part of the ground plane directly below it. The characteristic impedance in the general microstrip line structure ranges from 50Ω to 75Ω.

Figure 1: Stacking diagram of a multilayer circuit board, showing FR4 material and Rogers 4350 and the thickness of the core layer

Suspended stripline is another method of wiring and suppressing noise. This line consists of fixed-width wiring on the inner layer and a large ground plane above and below the center conductor. The ground plane is sandwiched between the power plane, so it can provide a very effective grounding effect. This is the preferred method for wearable PCB radio frequency signal wiring.

Coplanar waveguides can provide better isolation between RF lines and lines that need to be routed closer. This medium consists of a central conductor and ground planes on either side or below. The best way to transmit radio frequency signals is to suspend strip lines or coplanar waveguides. These two methods can provide better isolation between the signal and RF traces.

It is recommended to use so-called "via fences" on both sides of the coplanar waveguide. This method can provide a row of ground vias on each metal ground plane of the center conductor. The main trace running in the middle has fences on each side, thus providing a shortcut for the return current to the ground below. This method can reduce the noise level associated with the high ripple effect of the RF signal. The dielectric constant of 4.5 remains the same as the FR4 material of the prepreg, while the dielectric constant of the prepreg-from microstrip, stripline, or offset stripline-is about 3.8 to 3.9.

In some devices that use a ground plane, blind vias may be used to improve the decoupling performance of the power supply capacitor and provide a shunt path from the device to the ground. The shunt path to the ground can shorten the length of the via, which can achieve two purposes: you not only create a shunt or ground, but also reduce the transmission distance of devices with a small area, which is an important RF design factor.