Because of the size and size are very small, the growing market for wearable Internet products almost no ready-made PCB printed circuit board standards. Before these standards are available, we have to rely on the level of knowledge and manufacturing experience in board level development, and think about how to apply them to unique emerging challenges. There are three areas we need to pay special attention to, they are: circuit board surface material, RF / microwave design and RF transmission line.
PCB material
PCB is generally made up of layers, which may be made of fiber reinforced epoxy resin (FR4), polyimide or Rodgers (Rogers) or other laminated materials. The insulating material between the different layers is called a prepreg.
Wearable devices require high reliability, so when PCB designers are faced with the use of FR4 (with the highest cost of PCB manufacturing materials) or more advanced and more expensive material choice, this will become a problem.
If wearable PCB applications require high-speed, high-frequency materials, FR4 may not be the best choice. The dielectric constant of FR4 (Dk) is 4.5, and the dielectric constant of the more advanced Rogers series material is 3.55, while the dielectric constant of the brother series Rogers 4350 is about 3.66.
The design of wearable PCB material issues need to be considered
Figure 1: stacked layer of multilayer circuit board, shown in the FR4 material and Rogers 4350 and core layer thickness
A laminated dielectric constant is the ratio of the capacitance or energy of a pair of conductors near the stack to the capacitance or energy of the pair of conductors in the vacuum. At high frequencies, it is best to have a very small loss, and therefore, the dielectric coefficient of Roger 4350 is higher than the dielectric constant of 4.5 FR4 and is more suitable for higher frequency applications.
Under normal circumstances, wearable devices with PCB layer from 4 to the 8 layer. The construction principle of the layer is that if it is a 8 layer PCB, it should be able to provide sufficient layer and power layer and the wiring layer sandwiched in the middle. In this way, the ripple effect in crosstalk can be minimized, and the electromagnetic interference (EMI) can be significantly reduced.
In the layout design phase of the circuit board, the layout scheme is usually close to the power distribution layer. This results in a very low ripple effect, and the system noise can be reduced to almost zero. This is especially important for RF subsystems.
Compared with Rogers, FR4 has a higher dissipation factor (Df), especially at high frequencies. For the higher performance of the FR4 stack, the Df value is about 0.002, an order of magnitude higher than the average FR4. But the Rogers stack is only 0.001 or less. When the FR4 material is used for high frequency applications, there will be a significant difference in the insertion loss. The insertion loss is defined as the power loss when the signal is transmitted from A to B by using FR4, Rogers or other materials.
Manufacturing problems
Wearable PCB requires more stringent impedance control, wearable devices, which is an important factor, impedance matching can produce a cleaner signal transmission. Earlier, the standard deviation of the signal carrying line was + 10%. This indicator is not good enough for today's high frequency circuits. The current requirement is + 7%, in some cases even up to + 5% or less. This parameter as well as other variables will seriously affect the production of these impedance control particularly stringent wearable PCB, thereby limiting the number of businesses that can make 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 traditional FR4, the transmission loss and insertion loss of the Rogers stack is less than half.
In most cases, cost is the most important. However, Rogers can provide relatively low loss of high frequency stack performance at acceptable prices. For commercial applications, Rogers can be mixed with epoxy based FR4 to make a hybrid PCB, some of which are made of Rogers material, the other layer using FR4.
Frequency is the primary consideration when 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 materials, Rogers materials can also provide lower dielectric loss, and the dielectric constant is stable in a wide frequency range. In addition, the Rogers material can provide ideal low insertion loss performance for high frequency operation requirements.
The thermal expansion coefficient (CTE) of Rogers 4000 series has excellent dimensional stability. This means that the thermal expansion and contraction of the circuit board can be maintained at a stable limit at higher frequencies and higher temperatures when compared to FR4, when the PCB undergoes a cold, hot and very hot reflow cycle.
In the case of a mixed stack, it is easy to use the common manufacturing process technology to mix Rogers with the high performance FR4, so it is relatively easy to achieve high manufacturing yield. Rogers stack does not require special hole preparation process.
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.
RF / microwave design considerations
Portable technology and Bluetooth pave the way for wearable devices in RF / microwave applications. 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 the range between 10GHz and 25GHz
