Wearable device PCB design
Due to its small size and size, there are few printed circuit board standards for the growing wearable networking market. Before these standards came out, we had to rely on the knowledge and manufacturing experience we had in the development of the board level, and we thought about how to apply them to unique, emerging challenges. There are three areas that require our special attention: they are Radio frequency PCB, surface materials, RF / microwave design, and RF transmission lines.
1. PCB material for Wearable device PCB design
PCB is usually made up of laminated sheets, which may be made of fiber reinforced epoxy resins (FR4), polyimide or Rodgers (Rogers) materials or other laminates. The insulating material between the layers is called a prepreg.
Wearable devices require very 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 the choice of more advanced and expensive materials.
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, and the dielectric constant of the more advanced Rogers 4003 series is 3.55, while the dielectric constant of brother series Rogers 4350 is 3.66.
A laminated dielectric constant refers to the capacitance or energy between a pair of conductors near the stack and the capacitance or energy between the conductors in the vacuum. At high frequencies, it is best to have very small losses, therefore, the dielectric constant of 3.66 Roger 4350 is better than the dielectric constant of 4.5 FR4 for higher frequency applications.
Normally, the number of PCB layers for wearable devices ranges from 4 to 8 layers. Layer construction principle is that, if it is 8 PCB, it should be able to provide sufficient formation and power layer, and the wiring layer in the middle. Thus, ripple effects in crosstalk can be minimized, and electromagnetic interference (EMI) can be significantly reduced.
In the layout design phase of the circuit board, the layout plan usually relies on the large stratum to be close to the power distribution layer. This can result in very low ripple effects, and system noise can be reduced to almost zero. This is especially important for radio frequency subsystems.
Compared with Rogers material, FR4 has higher dissipation factor (Df), especially at high frequency. For higher performance FR4 stacks, the Df value is around 0.002, an order of magnitude better than the regular FR4. However, the Rogers stack is only 0.001 or less. When FR4 materials are used in high frequency applications, significant differences are observed in insertion loss. Insertion loss is defined as the loss of power transmitted from the A point to the B point when using FR4, Rogers or other materials.
2. Manufacturing problems in Wearable device PCB design
Wearable PCB requires more stringent impedance control, wearable devices, this is an important factor, impedance matching can produce more clean signal transmission. Earlier, the standard tolerance for signal carrying lines was + 10%. This indicator is clearly not good enough for today’s high-frequency high speed circuits. The requirement is now + 7%, and in some cases even + 5% or less. This parameter, as well as other variables, can seriously affect these impedance control, especially the manufacture of wearable PCB, which limits the number of businesses that can produce them.
Using Rogers UHF made of laminated dielectric constant tolerance is generally maintained at + 2%, even some products can reach + 1%, compared with the dielectric constant of FR4 tolerance stack up to 10%, therefore, comparison of the two kinds of materials can be found in the special low insertion loss Rogers. Compared with the conventional FR4 material, the transmission loss and insertion loss of the Rogers stack are half lower.
In most cases, the cost is most important. However, Rogers can provide relatively high frequency stack performance with relatively low loss at acceptable prices. For commercial applications, Rogers can be made into hybrid PCB with epoxy based FR4, some of which are made of Rogers material and others using FR4.
Frequency is the primary consideration in selecting the Rogers stack. When the frequency is more than 500MHz, PCB designers tend to choose the Rogers materials, especially for RF / microwave circuits, because the line above is strictly impedance control, these materials can provide higher performance.
Compared with FR4 material, Rogers material can provide lower dielectric loss, and its dielectric constant is stable in a wide frequency range. In addition, the Rogers material provides ideal low insertion loss performance for high-frequency applications.
The thermal expansion coefficient (CTE) of Rogers 4000 series materials has excellent dimensional stability. This means that, compared with the FR4, when the PCB undergoes a cold, hot and very hot reflow cycle, the thermal expansion and contraction of the circuit board can be kept at a steady limit at higher frequencies and higher temperature cycles.
In the case of a hybrid stack, it is easy to use a common manufacturing process to mix Rogers with high-performance FR4, so it is relatively easy to achieve high manufacturing yield. The Rogers stack does not require special through-hole preparation procedures.
FR4 can not achieve the electrical performance is very reliable, but high performance FR4 material does have good properties such as reliability, higher Tg, still relatively low cost, and can be used for a wide variety of applications, the application of complex microwave from simple audio design.
3. RF / microwave design considerations Wearable device PCB design
Portable technology and Bluetooth pave the way for RF / microwave applications in wearable devices. Today’s frequency range is becoming more dynamic. A few years ago, very high frequency (VHF) was defined as 2GHz~3GHz. But now we can see ultra high frequency (UHF) applications ranging from 10GHz to 25GHz.
Therefore, for wearable PCB, the RF part requires closer attention to wiring problems, the signal should be separated separately, so that the high frequency signal traces away from the ground. Other considerations include providing bypass filters, sufficient decoupling capacitors, and grounding to nearly equal the transmission line to the back line design.
The bypass filter can suppress the ripple effects of noise content and crosstalk. The decoupling capacitor needs to be placed next to the device pin that is closer to the bearer power signal.
High speed transmission lines and signal circuits require a layer to be arranged between the power layer signals to smooth the jitter generated by the noise signal.
At higher signal speeds, very small impedance mismatches result in unbalanced transmission and reception of signals, resulting in distortion. Therefore, particular attention must be paid to the problem of impedance matching associated with radio frequency signals, since radio frequency signals have very high speed and special tolerances.
The radio frequency transmission line requires control impedance to transmit the radio frequency signal from a particular IC substrate to the PCB. The transmission lines can be implemented at the outer, top and bottom levels and also at the intermediate level.
Microstrip lines, suspended stripline, coplanar waveguides, or ground are used during the design of the PCB radio frequency domain. A microstrip line consisting of a fixed length of metal or wire and the entire ground plane or part of the plane below it. The characteristic impedance in general microstrip lines is from 50 to 75.
Suspended stripline is another method of wiring and noise suppression. The line is made up of fixed width wiring on the inner layer and large ground planes up and down the center conductor. The ground plane is sandwiched between the power layers and thus provides very effective grounding effects. This is a preferred approach for wearable PCB radio frequency signaling cabling.
Coplanar waveguides can provide better isolation at radio frequency lines and near lines that require routing. The medium consists of a central conductor and a plane of earth on either side or below. The best way to transmit radio frequency signals is suspended stripline or coplanar waveguide. These two methods provide better isolation between signal and radio frequency routing.
The so-called “through the hole” fence is recommended on both sides of the coplanar waveguide”. The method can provide a row of ground vias in each metal ground plane of the central conductor. The main line running in the middle has a fence at each side, so the return current provides a short cut to the ground below. 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 that of the prepreg FR4 material, while the prepreg – from the microstrip line, ribbon line or offset band – has a dielectric constant of about 3.8 to 3.9.
In some devices that use ground planes, blind vias ,via in pad may be used to increase the decoupling performance of the power capacitor and provide a shunt path from device to ground.
The path to shunt can shorten the length of the hole, it can reach two goals: you not only created a diversion or, but also can reduce the transmission distance of the device is small, this is one of the important factors of RF design.