Fundamentals guide wearable PCB designs

Because of their small size and dimensions, few printed circuit board standards exist for the growing wearable Internet of Things market. Until they emerge, we’ll have to depend on what we have learned about board level development and manufacturing experience and carefully consider how they apply to the unique challenges emerging there. Three areas we should be paying particularly close attention to are: board surface materials, RF/microwave design, and RF transmission lines.

PCB materials
PCB layers are composed of laminates, which can be made of FR4 (fiber reinforced epoxy), polyimide, or Rogers materials or laminates. Insulation between different layers is called pre-preg.

Wearables demand a high degree of reliability, which becomes an issue when the PCB designer is confronted with the choice of using FR4, which is the most cost-effective PCB fabrication material, or a more advanced, more expensive material.

If the wearable PCB application requires high-speed, high frequency materials, FR4 may not be the best answer. FR4 has a dielectric constant (Dk) of 4.5, whereas the more advanced Rogers 4003 Series materials have a Dk of 3.55, while its companion series Rogers 4350 has a Dk of 3.66.

A stack up of a multilayer board showing both FR4 material and Rogers 4350 along with the thickness of the cores.

A stack up of a multilayer board showing both FR4 material and Rogers 4350 along with the thickness of the cores.

The Dk of a laminate refers to the capacitance or energy between a pair of conductors in a vicinity of the laminate compared to that pair of conductors in a vacuum. At high frequencies, it’s desirable to have a very small loss, thus a Dk of 3.66 in Rogers 4350 would be more desirable for higher frequency circuits versus FR4, which has a Dk of 4.5

Normally, layer count varies from four to eight layers for wearable devices. Layer structuring is such that if it’s an eight layer PCB, it provides enough ground and power plane to sandwich the routing layers. Thus, the ripple effect in crosstalk is kept to a minimum and electromagnetic interference or EMI is significantly reduced.

At board layout stage, the layout schedule is such that the ground plane is solid next to the power distribution layer. This creates a low ripple effect and system noise is reduced to virtually zero. This is especially important for RF subsystems.

FR4 has a high dissipation factor (Df) compared to Rogers’ material, especially at high frequencies. Df values for higher performance FR4 laminates are in the range of 0.002, an order of magnitude better than regular FR4. However, Rogers’ laminates are 0.001 or less. A meaningful difference in insertion loss is thus created when FR4 material is subjected to high frequencies. Insertion loss is defined as a loss of signal power in transmission from point A to B resulting from using a laminate such as FR4, Rogers, or other materials.

Fabrication issues
Wearables PCBs require much tighter impedance control, which is an essential element for a wearable device, resulting in cleaner signal propagation. Earlier, the standard tolerance was +/-10% for signal carrying traces. That’s not good enough for today’s high frequency, high-speed circuitry. The requirement now is for +/-7% and in some cases +/-5% or even lower. This and other variables negatively impact fabrication of those wearables PCBs that have extremely tight impedance control, thereby limiting the number of fabrication shops able to build them.

Laminates from a Rogers’ extremely high-frequency material are held to +/-2% of Dk tolerance. Some can even hold +/-1% of DK tolerance, compared to 10% Dk tolerance in FR4 laminates, hence insertion loss is extremely low when the two materials are compared. This would limit the transmission and insertion losses in Rogers to less than half, compared to a traditional FR4 material.

In most instances, cost is paramount. However, Rogers offers a relative low loss laminate with high-frequency performance at an acceptable cost point. For commercial applications, it can be used in conjunction with epoxy-based FR4 for a hybrid PCB with some layers being Rogers’ material and others FR4.

When selecting Rogers’ laminates, frequency is the top consideration. As frequency increases beyond 500 megahertz (MHz), PCB designers tend to favor Rogers materials over FR4, especially for RF/microwave circuits because these materials perform better when traces are tightly impedance controlled.

Rogers materials also offer low dielectric loss compared to FR4 and provide a Dk that is stable for a wide range of frequencies. Plus, they offer a low insertion loss ideal for high frequencies operation.

Coefficient of thermal expansion (CTE) for Rogers 4000 Series has exceptional dimension stabilities. This means that when the PCB goes through a reflow cycle of cold, hot, and very hot, the circuit board’s expansion and contraction are kept to a stable limit at higher frequencies and higher temperature cycles compared to FR4.

In a hybrid laminate stackup situation, Rogers can easily be mixed with a high performance FR4 using common fabrication processing techniques, making it relatively easy to get good fabrication yields. Rogers’ laminates don’t require specialized via preparation.

FR4 normally doesn’t do well as far as reliable electrical performance, but high performance FR4 material, does have good reliability characteristics, such as higher Tg, still relatively lower cost, and its ability to be used in a wide variety of applications from simple audio designs to complex microwave applications.

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