Wearable devices demand high reliability, so this is a problem 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.

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

PCB material

The PCB typically consists of a laminate 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.

Wearable devices demand high reliability, so this is a problem 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.

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: Stacked view of a multilayer board showing FR4 material and Rogers 4350 and core layer thickness.

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 the vacuum. At high frequencies, it is preferable to have a small loss. Therefore, the Roger 4350 with a dielectric constant of 3.66 is more suitable for higher frequency applications than the 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 is built on the principle that if it is an 8-layer PCB, it should 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 board layout phase, the layout scheme generally follows the large formation close to the power distribution layer. This creates a very low ripple effect and the system noise can be reduced to almost zero. This is especially important for the RF subsystem.

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 normal FR4. However, Rogers' stack is only 0.001 or less. When FR4 materials are used in high frequency applications, there is a significant difference in insertion loss. Insertion loss is defined as the power loss of a signal transmitted from point A to point B when FR4, Rogers, or other materials are used.

Manufacturing problem

Wearable PCBs require tighter impedance control, which is an important factor for wearable devices, and impedance matching can result in cleaner signal transmission. Earlier, the standard tolerance for signal-bearing 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 and other variables can severely impact the manufacture of wearable PCBs with particularly tight impedance controls, which limits the number of merchants that can make them.

The dielectric constant tolerance of laminates made with Rogers UHF materials is typically maintained at ±2%, and some products can even reach ±1%, compared to a dielectric constant tolerance of up to 10% for FR4 laminates. Both materials can be found to have a particularly low insertion loss from Rogers. The Rogers stack has a transmission loss and insertion loss that is half that of conventional FR4 materials.

In most cases, cost is the most important. However, Rogers can provide relatively low loss high frequency stack performance at acceptable price points. For commercial applications, Rogers can be combined with epoxy-based FR4 as a hybrid PCB, with some layers using Rogers and the other layers using FR4.

Frequency is a primary consideration when choosing a Rogers stack. When the frequency exceeds 500MHz, PCB designers tend to choose Rogers materials, especially for RF/microwave circuits, which provide higher performance because the traces above are subject to strict impedance control.

Compared to FR4 materials, Rogers materials also offer lower dielectric loss and a constant dielectric constant over a wide frequency range. In addition, Rogers materials offer the ideal low insertion loss performance 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 is subjected to cold, hot and very hot reflow cycles, the thermal expansion and contraction of the board can be kept at a stable limit at higher frequencies and higher temperature cycles than FR4.

In the case of hybrid lamination, it is easy to mix Rogers and high-performance FR4 using common manufacturing process technology, so it is relatively easy to achieve high manufacturing yield. Rogers laminates do not require a dedicated via preparation process.

Ordinary FR4 does not 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 simple audio design 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 more and more dynamic. Still a few years ago, VHF was defined as 2 GHz to 3 GHz. But now we can see ultra high frequency (UHF) applications ranging from 10 GHz to 25 GHz.

Therefore, for the wearable PCB, the RF part requires closer attention to the wiring problem, and the signals should be separated separately so that the traces that generate high-frequency signals are far away from the ground. Other considerations include: providing a bypass filter, sufficient decoupling capacitors, grounding, and designing the transmission line and return line to be nearly equal.

The bypass filter suppresses the ripple effects of noise content and crosstalk. Decoupling capacitors need to be placed next to the device pins that carry the power supply signal.

High-speed transmission lines and signal loops require a ground plane between the power plane signals to smooth the jitter generated by the noise signal. At higher signal speeds, a small impedance mismatch can cause unbalanced transmission and reception of signals, resulting in distortion. Therefore, special attention must be paid to the impedance matching problem associated with RF signals because of the high speed and special tolerance of the RF signals.

The RF transmission line requires control of the impedance to transfer RF signals from a particular IC substrate to the PCB. These transmission lines can be implemented in the outer layer, the top layer and the bottom layer, or in the middle layer.

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

Figure 2: Coplanar waveguides provide better isolation near RF lines and lines that require close traces.

The floating stripline is another way to route and suppress noise. This line consists of a fixed width of the inner layer and a large ground plane above and below the center conductor. The ground plane is sandwiched between the power planes and therefore provides a very effective grounding effect. This is the preferred method for wearable PCB RF signal routing.

Coplanar waveguides provide better isolation near RF lines and lines that require traces to be close. This medium consists of a central conductor and a ground plane on either or both sides. The best way to transmit RF signals is to suspend stripline or coplanar waveguides. These two methods provide better isolation between the signal and the RF traces.

The so-called "via fence" is recommended on both sides of the coplanar waveguide. This method provides a row of ground vias on each of the metal ground planes of the center conductor. The main traces running in the middle have fences on each side, thus providing a shortcut to the return current to the underlying formation. 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 that of the prepreg FR4, while the prepreg - from the microstrip line, the strip line or the offset strip line - has a dielectric constant of about 3.8 to 3.9.

Figure 3: A via fence is recommended on both sides of the coplanar waveguide.

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

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