The importance of basic materials for wearable PCB design

- Jul 20, 2017-

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 circuit boards, surface materials, RF / microwave design, and RF transmission lines.

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.

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.

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.

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.

For wearable PCB, the RF part requires closer attention to wiring issues, to separate signals, 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.

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.

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.

In some devices that use ground planes, blind vias 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.

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