Wearable PCB design requires attention to basic materials

- Sep 22, 2018-

Manufacturing problems

Wearable PCB requires more stringent impedance control, which is an important factor for wearables, and impedance matching can produce cleaner signal transmission.In earlier times, the standard tolerance of the signal load line was + / - 10%.This indicator is clearly not good enough for today's high-frequency high-speed circuits.Now the answer is plus or minus 7%, and in some cases it's plus or minus 5% or less.This parameter, along with other variables, can seriously affect the production of these impedance controls for particularly stringent wearable PCB, thereby limiting the number of merchants able to manufacture them.

The permittivity tolerance of the laminated layers made by Rogers uhf materials is generally kept at + / -2%, and some products can even reach + / -1%, while the permittivity tolerance of FR4 laminated layers is as high as 10%. Therefore, when comparing these two materials, Rogers's interpolation loss can be found to be particularly low.Compared with the traditional FR4 materials, the transmission loss and insertion loss of the Rogers stack are half lower.

In most cases, cost is most important.However, Rogers can provide relatively low loss of high frequency lamination performance at an acceptable price.For commercial applications, Rogers can mix PCB with epoxy based FR4, some layers are made of Rogers materials, others are made of FR4.

Frequency is the primary consideration when choosing a Rogers stack.When frequencies over 500MHz, PCB designers tend to choose Rogers materials, especially for rf/microwave circuits, which can provide higher performance when the walkway above is subject to strict impedance control.

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

The thermal expansion coefficient (CTE) of Rogers 4000 series materials has excellent dimensional stability.This means that when the PCB undergoes a cold, hot and very hot reflow cycle compared to FR4, the circuit board's thermal expansion and contraction can be maintained at a stable limit at a higher frequency and higher temperature cycle.

In the case of mixed lamination, it is easy to use the general manufacturing technology to mix Rogers and high-performance FR4, so it is relatively easy to achieve high manufacturing yield.The Rogers stack does not require a special drill preparation procedure.

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 for a wide range of applications, from simple audio designs to complex microwave applications.