Selection of magnetic beads in PCB circuit design

- Oct 17, 2018-

PCB material

PCB is typically composed of laminated layers that may be made from fiber reinforced epoxy (FR4), polyimide or Rogers materials or other laminated materials.The insulating material between the layers is called a semi-solidified sheet.

Wearables require high reliability, so this becomes a problem when PCB designers face the choice of using FR4(the most cost-effective PCB manufacturing material) or more advanced and expensive material.

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 permittivity of the more advanced Rogers 4003 series materials is 3.55, and that of the sibling Rogers 4350 is 3.66.

The dielectric constant of a laminated layer refers to the ratio of the capacitance or energy of a pair of conductors near the laminated layer to that of a conductor in a vacuum.At high frequencies, it is better to have a small loss, so Roger 4350 with a dielectric coefficient of 3.66 is more suitable for higher frequency applications than a FR4 with a dielectric constant of 4.5.

Normally, wearables use four to eight layers of PCB.The construction principle of the layer is that if it is 8 layers of PCB, it should be able to provide enough layers and power supply layer and place the wiring layer in the middle.In this way, ripple effects in crosstalk can be kept to a minimum and EMI can be significantly reduced.

In the circuit board layout design stage, the layout arrangement plan is to place the large stratum close to the power distribution layer.This creates a very low ripple effect, and the system noise can be reduced to almost zero.This is especially important for the rf subsystem.

Compared with Rogers materials, FR4 has a higher dissipation factor (Df), especially at high frequency.For higher performance FR4 overlays, Df values are around 0.002, an order of magnitude better than normal FR4.But the Rogers stack is only 0.001 or smaller.When FR4 materials are used for high frequency applications, there will be significant differences in insertion loss.Insertion loss is defined as the loss of power from point A to point B in the use of FR4, Rogers or other materials.

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.

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.

Rf/microwave design considerations

Portable technology and bluetooth pave the way for radio frequency/microwave applications in wearables.Today's frequency range is becoming more dynamic.A few years ago, VHF was defined as 2GHz~3GHz.But now we can see UHF applications that range from 10GHz to 25GHz.

Therefore, for wearable PCB, the rf part needs to pay more attention to wiring problems, and separate the signals so that the high frequency signal is routed away from the ground.Other considerations include the provision of bypass filters, adequate decoupling capacitors, grounding, and the design of transmission lines and return routes almost equal.