Wearable PCB Designers Need To Pay Attention To Several Aspects

Due to their small size and volume, there is almost no ready-made printed circuit board standard for the growing wearable IoT market. Before these standards came out, we had to rely on the knowledge and manufacturing experience learned in board-level development and think about how to apply them to unique emerging challenges. There are three areas that require our special attention. They are: circuit board surface materials, RF/microwave design, and RF transmission lines.

PCB material

PCBs are generally composed of laminates, which may be made of fiber-reinforced epoxy resin (FR4), polyimide, or Rogers (Rogers) materials or other laminate materials. The insulating material between different layers is called a prepreg.

Wearable devices require high reliability, so when PCB designers are faced with the choice of using FR4 (PCB manufacturing materials with the highest cost performance) or more advanced and more expensive materials, 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 (Dk) of FR4 is 4.5, the dielectric constant of the more advanced Rogers 4003 series materials is 3.55, and the dielectric constant of the brother series Rogers 4350 is 3.66.

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 the pair of conductors in a vacuum. At high frequencies, it is best to have very small losses, so the dielectric coefficient is 3. 66 Roger 4350 is more suitable for higher frequency applications than FR4 with a dielectric constant of 4.5.

Normally, the number of PCB layers for wearable devices ranges from 4 to 8 layers. The principle of layer construction is that if it is an 8-layer PCB, it should provide enough ground and power layers and sandwich the wiring layer. In this way, ripple effects in crosstalk can be kept to a minimum and electromagnetic interference (EMI) can be significantly reduced.

In the layout design stage of the circuit board, the layout arrangement scheme is generally to place a large ground layer close to the power distribution layer. This can create 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 materials, FR4 has a higher dissipation factor (Df), especially at high frequencies. For higher-performance FR4 stacks, the Df value is around 0.002, which is an order of magnitude better than ordinary FR4. However, Rogers' stack is only 0.001 or less. When FR4 materials are used in high-frequency applications, there will be significant differences in insertion loss. Insertion loss is defined as the power loss of a signal from point A to point B when using FR4, Rogers, or other materials.

Manufacturing problem

Wearable PCBs require stricter impedance control, which is an important factor for wearable devices. Impedance matching can produce cleaner signal transmission. Earlier, the standard tolerance for signal carrying traces 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 ±5% or less. This parameter and other variables will seriously affect the manufacture of wearable PCBs with particularly stringent impedance controls, thereby limiting the number of businesses that can manufacture them.

The dielectric constant tolerance of the stack made of Rogers UHF material is generally maintained at ±2%, and some products can even reach ±1%, compared with the dielectric constant tolerance of FR4 stack up to 10%. These two materials can find Rogers' insertion loss to be particularly low. Compared with traditional FR4 materials, the transmission loss and insertion loss of the Rogers stack are lower by half.

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 epoxy-based FR4 to make a hybrid PCB, some of which use Rogers material, and others use FR4.

When choosing a Rogers stack, frequency is the primary 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 upper traces are strictly controlled by impedance.

Compared with FR4 material, Rogers material can also provide lower dielectric loss, its dielectric constant is very stable in a wide frequency range. In addition, Rogers materials can provide the ideal low insertion loss performance required for high frequency operation.

The coefficient of thermal expansion (CTE) of Rogers 4000 series materials has excellent dimensional stability. This means that when the PCB undergoes cold, hot and very hot reflow cycles compared to FR4, 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 hybrid stacking, Rogers and high-performance FR4 can be easily mixed and used with common manufacturing process technology, so it is relatively easy to achieve high manufacturing yield. Rogers stacking does not require a special via preparation process.

Ordinary FR4 cannot 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.