Summary of contents: Due to their small size and size, there is almost no ready-made 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 resin (FR4), polyimide, or Rogers or other laminate materials. The insulating material between different layers is called a prepreg.

Wearable devices require high reliability, so when PCB designers are faced with the choice of using FR4 (PCB manufacturing materials with the highest cost performance) 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.

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

Normally, 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 provide enough ground and power layers and sandwich the wiring layer. In this way, ripple effects in crosstalk can be kept to a minimum and electromagnetic interference (EMI) can be significantly reduced.

In the layout design stage of the circuit board, the layout arrangement scheme is generally to place a large ground layer close to the power distribution layer. This can create a very low ripple effect, and the system noise can be reduced to almost zero. This is especially important for RF subsystems.

Compared with Rogers materials, FR4 has a higher dissipation factor (Df), especially at high frequencies. For higher-performance FR4 stacks, 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 smaller. When FR4 materials are used in high-frequency applications, there will be significant differences in insertion loss. Insertion loss is defined as the power loss of a signal from point A to point B when using FR4, Rogers, or other materials.

Manufacturing problem

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

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

In most cases, cost is the most important. However, Rogers can provide relatively low-loss high-frequency stack performance at an acceptable price. For commercial applications, Rogers can be combined with epoxy-based FR4 to make a hybrid PCB, some of which use Rogers material, and others use 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, its dielectric constant is very stable in a wide frequency range. In addition, Rogers materials can provide the ideal low insertion loss performance required for high frequency operation.

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

In the case of hybrid stacking, Rogers and high-performance FR4 can be easily mixed and used with common manufacturing process technology, so it is relatively easy to achieve high manufacturing yield. Rogers stacking does not require a special via preparation process.

Ordinary FR4 cannot achieve very reliable electrical performance, but high-performance FR4 materials do have good reliable 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 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 UHF applications ranging from 10GHz to 25GHz.

Therefore, for the wearable PCB, the RF part requires more close attention to wiring issues, and the signals should be separated separately to keep the high-frequency signal traces away from the ground. Other considerations include: providing bypass filters, sufficient decoupling capacitors, grounding, and designing transmission lines and return lines to be almost equal.

The bypass filter can suppress the ripple effect of noise content and crosstalk. The decoupling capacitor needs to be placed closer to the device pin carrying the power signal.

High-speed transmission lines and signal loops require a ground layer to be placed between the power layer signals to smooth out the jitter generated by the noise signal. At higher signal speeds, very small impedance mismatches will cause unbalanced transmission and reception signals, resulting in distortion. Therefore, special attention must be paid to the impedance matching problems associated with RF signals, because RF signals have high speeds and special tolerances.

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 in the outer layer, top layer and bottom layer, or they can be designed in the middle layer.

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

Floating striplines are another way to route and suppress noise. This line consists of fixed-width wiring on the inner layer and large ground planes above and below the center conductor. The ground plane is sandwiched between the power planes, so it can provide a very effective grounding effect. This is the preferred method for wearable PCB RF signal wiring.

Coplanar waveguides can provide better isolation between RF lines and lines that need to be routed close together. This medium consists of a central conductor and ground planes on both sides or below. The best way to transmit RF signals is to suspend the stripline or coplanar waveguide. These two methods can provide better isolation between signals 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 traces running in the middle are fenced on each side, thus providing a shortcut to the underlying formation for the return current. This method can reduce the noise level related to the high ripple effect of the RF signal. The dielectric constant of 4.5 remains the same as the prepreg FR4 material, 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 holes may be used to improve the decoupling performance of the power supply capacitor and provide a shunt path from the device to ground. The shunt path to ground can shorten the length of the via hole, which can achieve two purposes: you not only create a shunt or ground, but also reduce the transmission distance of devices with small ground, which is an important RF design factor.