Due to its small size and size, there is almost no ready-made printed circuit board standard for the growing wearable Internet of things market. Before these standards came out, we had to rely on our knowledge and manufacturing experience in board level development and think about how to apply them to unique emerging challenges. There are three areas that need our special attention: circuit board surface materials, RF / microwave design and RF transmission lines.
PCB generally consists of laminates, which may be made of fiber reinforced epoxy resin (FR4), polyimide or Rogers materials or other laminates. The insulating material between different layers is called prepreg.
Wearable devices require high reliability, so when PCB designers are faced with the choice of FR4 (the most cost-effective PCB manufacturing material) or more advanced and expensive materials, it will become a problem.
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, that of the more advanced series of Rogers 4003 is 3.55, and that of the brother series of Rogers 4350 is 3.66.
Figure 1: stack diagram of multilayer circuit board, showing FR4 material and Rogers 4350 as well as core layer thickness.
The dielectric constant of a stack is the ratio of the capacitance or energy between a pair of conductors near the stack to that between the two conductors in vacuum. At high frequency, it is better to have a small loss. Therefore, Roger 4350 with a dielectric coefficient 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. The principle of layer construction is that if it is an 8-layer PCB, it should be able to provide enough layers and power layers, and clamp the wiring layer in the middle. In this way, the ripple effect in crosstalk can be kept to a minimum and the electromagnetic interference (EMI) can be significantly reduced.
In the stage of circuit board layout design, the layout plan is to place the large layer close to the power distribution layer. In this way, the ripple effect is very low and the system noise can be reduced to almost zero. This is particularly important for the RF subsystem.
Compared with Rogers material, FR4 has higher dissipation factor (DF), especially at high frequency. For higher performance FR4 stack, DF value is about 0.002, which is one order of magnitude better than ordinary FR4. But Rogers’ stack is only 0.001 or less. When FR4 material is used in high frequency applications, there will be obvious differences in insertion loss. Insertion loss is defined as the power loss of signal transmission from point a to point B when FR4, Rogers or other materials are used.
Wearable PCB requires more strict impedance control, which is an important factor for wearable devices. Impedance matching can produce cleaner signal transmission. Earlier, the standard tolerance for signal carrier routing was ± 10%. This index is obviously not good enough for today’s high frequency and high speed circuits. The current requirement is ± 7% and in some cases ± 5% or less. This parameter and other variables will seriously affect the manufacture of these wearable PCBs with strict impedance control, thus limiting the number of manufacturers that can manufacture them.
The permittivity tolerance of the laminates made of Rogers UHF materials is generally kept at ± 2%, and some products can even reach ± 1%. Compared with FR4 laminates, the permittivity tolerance is as high as 10%. Therefore, comparing the 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 stack are half lower.
In most cases, cost is the most important. However, Rogers can provide relatively low loss high frequency stack performance at an acceptable price. For commercial applications, Rogers can be combined with FR4 based on epoxy resin to make hybrid PCB. Some layers are made of Rogers material, while others are made of FR4.
When choosing Rogers stack, frequency is the first consideration. When the frequency exceeds 500MHz, PCB designers tend to choose Rogers materials, especially for RF / microwave circuits, because these materials can provide higher performance when the above wiring is strictly impedance controlled.
Compared with FR4 material, Rogers material can also provide lower dielectric loss, and its dielectric constant is stable in a wide frequency range. In addition, Rogers material can provide 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, compared with FR4, when PCB undergoes cold, hot and very hot reflow cycles, the thermal expansion and contraction of PCB can be maintained at a stable limit at higher frequency and higher temperature cycles.
In the case of hybrid lamination, it is easy to mix Rogers and high-performance FR4 using general manufacturing process technology, so it is relatively easy to achieve high yield. Rogers stack does not need special via preparation process.
Ordinary FR4 can not achieve very reliable electrical performance, but high-performance FR4 materials do have good reliable 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.
RF / microwave design considerations
Portable technology and Bluetooth pave the way for RF / microwave applications in wearable devices. Today’s frequency range is becoming more and 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 in the range of 10 GHz to 25 GHz.
Therefore, for the wearable PCB, the RF part needs to pay more attention to the wiring problems, separate the signals separately, so that the high-frequency signal lines are far away from the ground. Other considerations include: providing bypass filter, sufficient decoupling capacitance, grounding, and almost equal design of transmission line and return line.
The bypass filter can suppress the ripple effect of noise content and crosstalk. The decoupling capacitor needs to be placed closer to the device pin carrying the power signal.
High speed transmission lines and signal circuits require a layer between the signals in the power layer to smooth the jitter caused by noise signals. At high signal speed, a small impedance mismatch will cause unbalanced transmission and reception of signals, resulting in distortion. Therefore, it is necessary to pay special attention to the impedance matching problems related to RF signals, because RF signals have high speed and special tolerance.
RF transmission lines require impedance control to transmit RF signals from a specific IC substrate to a PCB. These transmission lines can be implemented in the outer layer, top layer and bottom layer, and can also be designed in the middle layer.
Methods used in PCB RF layout design include microstrip line, suspended stripline, coplanar waveguide or grounding. The microstrip line is composed of fixed length metal or wire and the whole or part of the ground plane directly below. In general, the characteristic impedance of microstrip line is from 50 Ω to 75 Ω.
Figure 2: coplanar waveguide can provide better isolation between RF lines and lines that need to be routed close to each other.
Suspended stripline is another way of wiring and noise suppression. This line consists of a fixed width of wiring on the inner layer and a large ground plane above and below the central conductor. The ground plane is sandwiched in the middle of the power layer, so it can provide very effective grounding effect. This is the preferred method for RF signal wiring of wearable PCB.
Coplanar waveguide can provide better isolation between RF lines and lines that need to be close to them. The medium consists of a central conductor and ground planes on both sides or below. The best way to transmit RF signal is to suspend stripline or coplanar waveguide. These two methods can provide better isolation between signal and RF routing.
It is recommended to use the so-called “via fence” on both sides of coplanar waveguide. This method can provide a row of ground vias on each metal ground plane of the central conductor. The main route running in the middle is fenced on each side, thus 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 signal. The dielectric constant of fr4.5 remains the same as that of FR4, while the dielectric constant of FR4 from microstrip line, stripline or offset stripline is about 3.8 to 3.9.
Figure 3: through hole fence is recommended on both sides of coplanar waveguide.
In some devices using ground plane, blind holes may be used to improve the decoupling performance of power supply capacitor and provide shunt path from device to ground. The shunt path to ground shortens the length of the via, which achieves two goals: you not only create shunt or ground, but also reduce the transmission distance of devices with small patches, which is an important RF design factor.
Source: Electronic Engineering