Wearable PCB design should be cautious, considering these three major issues
Time:2023-09-27
Views:660
Due to their small size 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 consider 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 material
PCB is generally composed of layers, which may be made of fiber reinforced epoxy resin (FR4), polyimide or Rogers materials, or other laminated materials. The insulation material between different layers is called a semi cured sheet.
Wearable devices require high reliability, so when PCB designers face the choice of using FR4 (the most cost-effective PCB manufacturing material) 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, while the dielectric constant of the more advanced Rogers 4003 series material is 3.55, while the dielectric constant of the brother series Rogers 4350 is 3.66.
Figure 1: Layered diagram of a multi-layer circuit board, showing FR4 material, 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 near the stack to the capacitance or energy between these conductors in a vacuum. At high frequencies, it is best to have a very small loss, so the dielectric coefficient 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 for wearable devices ranges from 4 to 8. The construction principle of the layer is that if it is an 8-layer PCB, it should be able to provide sufficient layer and power layer, and clamp the wiring layer 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.
In the circuit board layout design stage, the layout arrangement scheme generally involves placing large layers of ground next to the power distribution layer. This can form a very low ripple effect and reduce system noise to almost zero. This is particularly important for the RF 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 an order of magnitude better than regular FR4. However, Rogers‘ stacking is only 0.001 or smaller. When FR4 material is used for high-frequency applications, there will be significant differences in insertion loss. Insertion loss is defined as the power loss of signal transmission from point A to point B when using FR4, Rogers, or other materials.
Manufacturing issues
Wearable PCBs require stricter impedance control, which is an important factor for wearable devices. Impedance matching can generate cleaner signal transmission. Earlier, the standard tolerance for signal carrier routing 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 up to ± 5% or less. This parameter and other variables will seriously affect the manufacturing of wearable PCBs with particularly strict impedance control, thereby limiting the number of merchants able to manufacture them.
The dielectric constant tolerance of layers made of Rogers ultra-high frequency materials is generally maintained at ± 2%, and some products can even reach ± 1%. In contrast, the dielectric constant tolerance of FR4 layers is as high as 10%. Therefore, comparing these two materials, it can be found that Rogers‘ insertion loss is particularly low. Compared with traditional FR4 materials, the transmission loss and insertion loss of Rogers layers are half lower.
In most cases, cost is the most important. However, Rogers can provide relatively low loss high-frequency stacking performance at an acceptable price point. For commercial applications, Rogers can be combined with epoxy resin based FR4 to form a hybrid PCB, with some layers made of Rogers material and others made of 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 provide higher performance when the wiring above is strictly controlled by impedance.
Compared with FR4 material, Rogers material can also provide lower dielectric loss, and its dielectric constant is stable over a wide frequency range. In addition, Rogers materials can provide the ideal low insertion loss performance required for high-frequency operation.
The Thermal Expansion Coefficient (CTE) of Rogers 4000 series materials exhibits excellent dimensional stability. This means that compared to 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 higher frequencies and higher temperature cycles.
In the case of mixed stacking, Rogers and high-performance FR4 can be easily mixed together using universal manufacturing process technology, making it relatively easy to achieve high manufacturing yield. Rogers lamination does not require a specialized through-hole preparation process.
Ordinary FR4 cannot achieve very reliable electrical performance, but high-performance FR4 materials do have good reliability characteristics, such as higher Tg, relatively low cost, and can be used for 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 increasingly dynamic. 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 wiring issues, separating signals separately and keeping the wiring that generates high-frequency signals away from the ground. Other considerations include providing bypass filters, sufficient decoupling capacitors, grounding, and designing transmission and return lines almost equally.
Bypass filters can suppress the ripple effects of noise content and crosstalk. The decoupling capacitor needs to be placed closer to the pin of the device carrying the power signal.
The high-speed transmission line and signal circuit require a layer to be arranged between the power layer signals to smooth out the jitter generated by noise signals. At high signal speeds, small impedance mismatches can cause imbalanced transmission and reception of signals, resulting in distortion. Therefore, special attention must be paid to impedance matching issues related to RF signals, as RF signals have high speeds and special tolerances.
RF transmission lines require impedance control to transmit RF signals from specific IC substrates to PCBs. These transmission lines can be implemented in the outer layer, top layer, and bottom layer, and can also be designed in the middle layer.
The methods used during PCB RF design layout include microstrip lines, suspended striplines, coplanar waveguides, or grounding. A microstrip line consists of a fixed length of metal or wiring, as well as the entire or part of the ground plane located directly below it. The characteristic impedance in general microstrip line structures ranges from 50 Ω to 75 Ω.
Figure 2: Coplanar waveguides can provide better isolation near RF lines and lines that require close routing.
Suspended stripline is another method of wiring and noise suppression. This type 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 the preferred method for wearable PCB RF signal wiring.
Coplanar waveguides can provide better isolation near RF lines and lines that need to be routed close together. This medium consists of a central conductor and a ground plane on either side or below. The best method for transmitting RF signals is through floating striplines or coplanar waveguides. These two methods can provide better isolation between signal and RF routing.
It is recommended to use the so-called "through hole fence" 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 wiring running in the middle has fences on each side, providing a shortcut for the return current to the underlying strata. This method can reduce the noise level related to the high ripple effect of RF signals. The dielectric constant of 4.5 remains the same as that of the FR4 material of the semi cured sheet, while the dielectric constant of the semi cured sheet - from microstrip line, strip line, or offset strip line - is about 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 using ground planes, blind holes may be used to improve the decoupling performance of power capacitors and provide a shunt path from the device to ground. The shunt path to the ground can shorten the length of the via, which can achieve two goals: you not only create a shunt or ground, but also reduce the transmission distance of devices with small plots of ground, which is an important RF design factor.
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