PCBMASTER shares with you how to prevent signal integrity issues in high - frequency PCB design.

Author: Jack Wang

In 5G communication and millimeter - wave radar applications, out - of - control signal integrity (SI) of high - frequency PCBs can directly lead to system failures. Engineers often face three major challenges: signal reflection, crosstalk, and loss, and the root causes usually lie in basic details during the design phase.

 

 

1. Material Selection: The Trade - off between Dielectric Constant and Loss Factor

The dielectric constant (Dk) stability of high - frequency materials (such as Rogers 4350B) is more than 3 times higher than that of FR - 4, but the cost is 5 - 8 times higher. It is recommended to preferentially use medium - loss PTFE materials (such as Isola IS620) below 6GHz. Its Dk stability of 3.0±0.04 can meet the requirements of most scenarios. Be alert to the roughness of the copper foil - the additional loss of 1μm - roughness copper foil at 28GHz can reach 0.15dB/inch.

 

 

 

2.Hidden Traps in Impedance Control

Even if the Polar SI9000 is used to calculate the line width, there may still be a ±10% deviation in actual production. A case of an automotive radar project shows that when the actual impedance of the differential lines drifts from 100Ω to 92Ω, the eye - diagram opening decreases by 40%. The solution is to require the PCB manufacturer to provide the etching compensation coefficient and reserve a 3% line - width redundancy during the design.

 

 

2. Resonance Disasters Caused by Vias

In a case of a WIFI6E module on a 12 - layer board, the unoptimized via stubs caused resonance at 5.8GHz, resulting in a 6dB deterioration of the S21 parameter. Using the back - drilling technique to control the stub within 8mil and using a T - shaped grounded via array can reduce the resonance amplitude to an acceptable range.

 

 

 

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Author: Jack Wang