PCB Blog - Wearable PCB Board Design Requires Focus on Basic Materials

Due to their small size and size, there are few ready-made printed circuit boards standards for the growing wearable IoT market. Before these standards became available, we had to rely on what we learned in board-level development and manufacturing experience and think about how to apply them to unique emerging challenges. There are three areas that require our special attention: circuit board surface materials, RF/microwave design, and RF transmission lines.

PCB board material

PCB boards typically consist of laminates, which may be fabricated from fiber-reinforced epoxy (FR4), polyimide or Rogers materials, or other laminates. The insulating material between the different layers is called a prepreg. Wearables require high levels of reliability, so this becomes a problem when PCB board designers are faced with the choice of using FR4 (a cost-effective PCB manufacturing material) or more advanced and more expensive materials. If a wearable PCB board application requires high-speed, high-frequency materials, FR4 may not be the choice. The dielectric constant (Dk) of FR4 is 4.5, the more advanced Rogers 4003 series material has a dielectric constant of 3.55, and the sibling Rogers 4350 has a dielectric constant of 3.66.

The dielectric constant of a stack refers to the ratio of the capacitance or energy between a pair of conductors in the vicinity of the stack to the capacitance or energy between the pair of conductors in vacuum. At high frequencies, there is very little loss, so 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 layer construction principle is that if it is an 8-layer PCB, it should provide enough ground and power planes and sandwich the routing layers. In this way, the ripple effect in crosstalk is preserved and electromagnetic interference (EMI) can be significantly reduced. In the circuit board layout design stage, the layout plan is generally to place a large stratum close to the power distribution layer. This results in a very low ripple effect and the system noise can be reduced to almost zero. This is especially important for RF subsystems. Compared to 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 regular FR4. But Rogers' stack is only 0.001 or less. When FR4 material is used for high frequency applications, there is a noticeable difference in insertion loss. Insertion loss is defined as the loss of power in signal transmission from point A to point B when using FR4, Rogers, or other materials.

Manufacturing problems

Wearable PCB board require tighter impedance control, an important factor for wearable devices, as impedance matching can result in 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 high-speed circuits. The current requirement is ±7%, and in some cases even ±5% or less. This parameter, along with other variables, can severely impact the manufacture of these wearable PCBs with particularly tight impedance control, thereby limiting the number of merchants who can manufacture them. The dielectric constant tolerance of laminates made of Rogers UHF materials is generally maintained at ±2%, and some products can even reach ±1%. Rogers can be found to have exceptionally low insertion loss with these two materials. Rogers stacks have half the transmission and insertion loss compared to conventional FR4 materials. In most cases, cost matters. However, Rogers can provide relatively low loss high frequency stack-up performance at an acceptable price point. For commercial applications, Rogers can be combined with epoxy-based FR4 to make hybrid PCBs, with some layers using Rogers material and others using FR4. When selecting a Rogers stack, frequency is the primary consideration. When frequencies exceed 500MHz, PCB board designers tend to choose Rogers materials, especially for RF/microwave circuits, because these materials can provide higher performance when the traces above are strictly controlled by impedance. Rogers materials also offer lower dielectric losses compared to FR4 materials, and their dielectric constants are stable over 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 the thermal expansion and contraction of the board can be maintained at a stable limit at higher frequency and higher temperature cycling when the board is subjected to cold, hot and very hot reflow cycles compared to FR4. In the case of a hybrid stack, Rogers and high-performance FR4 can be easily mixed together using common manufacturing process technology, so it is relatively easy to achieve high manufacturing yields. The Rogers stackup does not require a dedicated via preparation process. Regular FR4 cannot achieve very reliable electrical performance, but high-performance FR4 materials do have good reliability characteristics, such as higher Tg, are still relatively low cost, and can be used in a wide variety of applications, from simple audio designs to complex microwave applications.

RF/Microwave Design Considerations

Portable technology and Bluetooth paved the way for RF/microwave applications in wearables. Today's frequency range is becoming increasingly dynamic. A few years ago, very high frequency (VHF) was defined as 2GHz ~ 3GHz. But now we can see ultra-high frequency (UHF) applications in the range of 10GHz to 25GHz. Therefore, for the wearable PCB board, the RF part requires more close attention to the wiring issues, separate signals, and keep the traces that generate high-frequency signals away from the ground. Other considerations include: providing bypass filters, adequate decoupling capacitors, grounding, and designing transmission and return lines nearly equal. A bypass filter suppresses the ripple effects of noise content and crosstalk. Decoupling capacitors need to be placed closer to the device pins that carry the power signal. High-speed transmission lines and signal loops require a ground plane between power plane signals to smooth out jitter from noisy signals. At higher signal speeds, small impedance mismatches can cause unbalanced transmit and receive signals, resulting in distortion. Therefore, special attention must be paid to impedance matching problems associated with RF signals, which have high speeds and special tolerances. RF transmission lines require controlled impedance in order to transmit RF signals from a specific IC substrate to a PCB board. These transmission lines can be implemented in the outer, top and bottom layers, and can also be designed in the middle layer. The methods used during RF design layout of the PCB are microstrip, floating stripline, coplanar waveguide or grounding. A microstrip line consists of a fixed length of metal or trace and the entire ground plane or part of the ground plane directly beneath it. The characteristic impedance in a general microstrip line structure is from 50Ω to 75Ω.

Suspended striplines are another method of routing 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 planes and thus provides a very effective grounding effect. This is the preferred method for RF signal routing on wearable PCB boards. Coplanar waveguides can provide better isolation near RF lines and lines that need to be traced close together. This medium consists of a length of center conductor and ground planes on either side or below. The method of transmitting RF signals is suspended striplines or coplanar waveguides. These two methods provide better isolation between signal and RF traces. The use of so-called "via fences" is recommended on both sides of a coplanar waveguide. This approach provides a row of ground vias on each metal ground plane of the center conductor. The main trace running in the middle is fenced on each side, thus giving return current a shortcut to the formation below. This approach reduces noise levels associated with high ripple effects on RF signals. The dielectric constant of 4.5 remains the same as the prepreg FR4 material, while the prepreg-from microstrip, stripline, or offset stripline-has a dielectric constant of 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 capacitors and provide a shunt path from the device to ground. The shunt path to ground can shorten the length of the via, which serves two purposes: you not only create a shunt or ground, but you can reduce the transmission distance of devices with small ground, which is an important PCB board RF design factor .