Several key points to focus on in wearable PCB design
Date：August 20, 2025 Views：24
Due to their small volume and size, there are almost no ready-made printed circuit board standards for the growing wearable Internet of Things market. Before these standards were introduced, 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 layers, which may be made of fiber-reinforced epoxy resin (FR4), polyimide or Rogers materials or other laminated materials. The insulating material between different layers is called a semi-cured sheet.
Wearable devices require very high reliability. Therefore, when PCB designers are faced with the choice between using FR4 (the PCB manufacturing material with the highest cost performance) or more advanced and expensive materials, this will become a problem.
If wearable PCB applications require high-speed and 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 materials is 3.55, while that of the sibling series Rogers 4350 is 3.66.
Figure 1: Layering diagram of the multilayer circuit board, in which the FR4 material, Rogers 4350 and the thickness of the core layer are shown.
The dielectric constant of a stack refers to the ratio of the capacitance or energy between a pair of conductors near the stack to that between the two conductors in a vacuum. At high frequencies, it is best to have very little loss. Therefore, the dielectric constant is 3. The Roger 4350 of 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 used in wearable devices ranges from 4 to 8. The construction principle of the layers is that if it is an 8-layer PCB, it should be able to provide sufficient ground layers and power layers and sandwich the routing layers in the middle. In this way, the ripple effect in crosstalk can be kept to a minimum, and electromagnetic interference (EMI) can be significantly reduced.
During the circuit board layout design stage, the layout arrangement scheme is generally to place large ground layers 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 particularly important for the radio frequency subsystem.
Compared with Rogers materials, FR4 has a higher dissipation factor (Df), especially at high frequencies. For higher-performance FR4 laminations, the Df value is around 0.002, which is one order of magnitude better than that of ordinary FR4. However, Rogers' layering was only 0.001 or smaller. When FR4 materials are used in high-frequency applications, significant differences will occur in terms of insertion loss. Insertion loss is defined as the power loss of the signal transmitted from point A to point B when using FR4, Rogers or other materials.
Manufacturing problems
Wearable PCBS require more stringent impedance control, which is an important factor for wearable devices. Impedance matching can generate 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 it even reaches ± 5% or less. This parameter and other variables will seriously affect the manufacturing of these wearable PCBS with particularly strict impedance control, thereby limiting the number of manufacturers capable of manufacturing them.
The dielectric constant tolerance of the laminations made of Rogers UHF material is generally maintained within ± 2%, and some products can even reach ± 1%. In contrast, the dielectric constant tolerance of FR4 laminations is as high as 10%. Therefore, by comparing these two materials, it can be found that the insertion loss of Rogers is particularly low. Compared with the traditional FR4 material, the transmission loss and insertion loss of Rogers stacking are half as low.
In most cases, cost is the most important. However, Rogers can offer relatively low-loss high-frequency lamination performance at an acceptable price. For commercial applications, Rogers can be made into a hybrid PCB together with epoxy-based FR4, with some layers using Rogers material and others using FR4.
When choosing Rogers stacking, frequency is the primary consideration. When the frequency exceeds 500MHz, PCB designers tend to choose Rogers materials, especially for RF/microwave circuits, as these materials can offer higher performance when the traces above are subject to strict impedance control.
Compared with FR4 materials, Rogers materials can also provide lower dielectric loss, and their dielectric constant remains 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 compared with FR4, when the PCB undergoes cold, hot and very hot reflow soldering cycles, the thermal expansion and contraction of the circuit board can be maintained at a stable limit at a higher frequency and higher temperature cycle.
In the case of hybrid stacking, Rogers and high-performance FR4 can be easily mixed and used together using general manufacturing process technology, and thus it is relatively easy to achieve a high manufacturing yield. Rogers lamination does not require a dedicated 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.
Considerations for RF/microwave design
Portable technology and Bluetooth have paved the way for radio frequency/microwave applications in wearable devices. Today's frequency range is becoming increasingly dynamic. Just a few years ago, very high frequency (VHF) was defined as 2GHz to 3GHz. But now we can see ultra-high frequency (UHF) applications ranging from 10GHz to 25GHz.
Therefore, for wearable PCBS, the RF part requires closer attention to the wiring issues. Signals should be separated separately to keep the traces that generate high-frequency signals away from the ground. Other considerations include: providing bypass filters, sufficient decoupling capacitors, grounding, and designing the transmission line and return line almost equally.
Bypass filters can suppress the ripple effect of noise content and crosstalk. Decoupling capacitors need to be placed closer to the pins of the devices that carry power signals.
High-speed transmission lines and signal loops require the arrangement of a ground layer between the power layer signals to smooth out the jitter caused by noise signals. At higher signal speeds, even a very small impedance mismatch can cause unbalanced transmission and reception of signals, thereby resulting in distortion. Therefore, special attention must be paid to the impedance matching problem related to radio frequency signals, because radio frequency signals have a very high speed and a special tolerance.
Rf transmission lines require control of impedance in order to transmit RF signals from a specific IC substrate to a 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 include microstrip lines, suspended striplines, coplanar waveguides or grounding. The microstrip wire consists of a fixed-length metal or trace wire and the entire or part of the ground plane located directly beneath. The characteristic impedance in the general microstrip line structure ranges from 50Ω to 75Ω.
Figure 2: Coplanar waveguides can provide better isolation near RF lines and lines that require very close traces.
Floating ribbon is another method of wiring and noise suppression. This kind of wire consists of fixed-width wiring on the inner layer and large ground planes above and below the central conductor. The ground plane is sandwiched in the middle of the power layer, thus providing a very effective grounding effect. This is a preferred method for the RF signal routing of wearable PCBS.Coplanar waveguides can provide better isolation near RF lines and lines where traces need to be close. This medium consists of a central conductor and the ground planes on either side or beneath it. The best way to transmit radio frequency signals is a floating ribbon or a coplanar waveguide. These two methods can provide better isolation between the signal and the RF trace.
It is recommended to use the so-called "through-hole fences" on both sides of the coplanar waveguide. This method can provide a row of grounding vias on each metal ground plane of the central conductor. The main routing running in the middle is fenced on each side, thus providing a shortcut for the return current to the lower strata. This method can reduce the noise level related to the high ripple effect of radio frequency signals. The dielectric constant of 4.5 remains the same as that of the semi-cured sheet FR4 material, while the dielectric constant of the semi-cured sheet - from microstrip lines, stripline or offset stripline - is approximately 3.8 to 3.9.
Figure 3: It is recommended to use through-hole fences on both sides of the coplanar waveguide.
In some devices that use the ground plane, blind holes may be employed to enhance the decoupling performance of power capacitors and provide a shunt path from the device to the ground. The shunt path to ground can shorten the length of the vias, thus achieving two purposes: you not only create shunt or ground, but also reduce the transmission distance of devices with small ground blocks, which is an important RF design factor.免责声明： 本文章转自其它平台，并不代表本站观点及立场。若有侵权或异议，请联系我们删除。谢谢！ Disclaimer: This article is reproduced from other platforms and does not represent the views or positions of this website. If there is any infringement or objection, please contact us to delete it. thank you! |
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