Some experiences on wearable PCB design

Some experiences on wearable PCB design

Because of their small size and dimensions, there are few y printed circuit board standards for the growing wearable IoT market. Until they are available, we must rely on what we know about board-level development and manufacturing experience and carefully consider how they can be applied to the unique challenges that arise there. Three areas in particular that we should focus on are: board surface materials, RF/microwave design, and RF transmission lines.

PCB material The PCB layer is composed of a laminate, which can be made of FR4 (epoxy resin reinforced with fiber), polyimide or other materials or layered materials. The isolation between different layers is called prepreg. Wearable devices require a high degree of reliability, which becomes a problem when PCB developers are faced with a choice between using FR4 (an economical material for manufacturing printed circuit boards) or more advanced and expensive materials.

If wearable PCB applications require high-speed, high-frequency materials, FR4 may not be the answer. The dielectric constant (Dk) of FR4 is 4.5, while the dielectric constant of the more advanced Rogers 4003 series materials is 3.55, while the dielectric constant of its supporting series Rogers 4350 is 3.66.

The Dk of the laminate refers to the capacitance or energy between a pair of conductors near the laminate compared to the pair of conductors in the vacuum. At high frequencies, it is desirable that the loss is very small, so the Dk of Rogers 4350 is 3.66, which is more ideal for high-frequency circuits, while the Dk of FR4 is 4.5.

Usually, the number of layers of wearable devices ranges from four to eight layers.The layer structure is like this. If it is an eight-layer PCB, it will provide enough ground plane and power layer to sandwich the wiring layer in the middle.Therefore, the ripple effect in crosstalk is kept to a limit, and electromagnetic interference or EMI is significantly reduced.

In the circuit board layout stage, the layout schedule is that the ground plane is adjacent to the power distribution layer.This will produce a low ripple effect and the system noise will be almost reduced to zero.This is particularly important for the RF subsystem.

Compared with conventional materials, FR4 has a higher dissipation factor (Df), especially at high frequencies.The Df value of higher-performance FR4 laminates is in the range of 0.002, which is an order of magnitude better than ordinary FR4.However, Rogers' laminate is 0.001 or less.Therefore, when the FR4 material is subjected to high frequencies, a meaningful difference in insertion loss will occur.Insertion loss is defined as the signal power loss caused by the use of laminates such as FR4, Rogers, or other materials during transmission from point A to point B.

Manufacturing problem

The PCB of wearable devices requires stricter impedance control, which is an essential element of wearable devices and can achieve cleaner signal propagation.Earlier, the standard tolerance for signal carrying traces was +/-10%.This is not good enough for today's high-frequency and high-speed circuits.The current requirement is +/-7%, and in some cases +/-5% or even lower.This and other variables have a negative impact on the manufacture of wearable device PCBs with extremely strict impedance control, thereby limiting the number of manufacturing workshops that can manufacture them.

The laminate of Rogers very high frequency materials is maintained at a Dk tolerance of +/-2%.Some can even maintain a DK tolerance of +/-1%, while the Dk tolerance of FR4 laminate is 10%, so when comparing the two materials, the insertion loss is extremely low.Compared with traditional FR4 materials, this will limit the transmission and insertion loss of the material to less than half.

In most cases, cost is important.However, Rogers provides a relatively low-loss laminate with high-frequency performance at an acceptable cost. For commercial applications, it can be used with epoxy resin-based FR4 for hybrid PCBs, where some layers are the same material and other layers are FR4.

When choosing the right laminate, frequency is the primary consideration. As the frequency increases above 500 megahertz (MHz), PCB designers tend to use Rogers materials instead of FR4, especially for RF/microwave circuits, because these materials perform better when the traces are strictly impedance controlled.

Compared with FR4, Rogers material also has a lower dielectric loss and provides a stable dielectric constant over a wide frequency range. In addition, they have low insertion loss and are ideal for high-frequency operation.

The coefficient of thermal expansion (CTE) of the Rogers 4000 series has excellent dimensional stability.This means that when the PCB undergoes cold, hot, and very hot reflow soldering cycles, compared with FR4, the expansion and contraction of the circuit board remain at a stable limit at higher frequencies and higher temperature cycles.

In the case of mixed laminated stacking, Rogers can easily mix with high-performance FR4 using common manufacturing process techniques, thereby obtaining a good manufacturing yield relatively easily. Rogers' laminate does not require special through-hole preparation.

In terms of reliable electrical performance, FR4 is usually not well done, but high-performance FR4 materials do have good reliability characteristics, such as higher Tg, still relatively low cost, and it can be used in a variety of applications from simple to extensive audio design to complex microwave applications.