High speed Designs - Part 4

One of the challenging aspects in the high speed board design is the selection of the PCB material. At low frequencies, the characteristics of the PCB material are of less significance as the signal characteristics doesn't change with type of material. But when it comes to high frequencies, dielectric material properties always a crucial role in the electrical performance. As most of us worked on low frequencies, the most available, cheap and widely used laminate material is FR4. The characteristics of FR4 does not allow it to be used for high frequencies. To decide on which material works best for a given frequency, we have to check out the specifications of the material datasheet as we do for the component selection. One more approach by the design engineers and fab houses is to use some simulation tool to decide the material characteristics to be used for a given scenario.

           The circuit designs range from low frequency digital and analog type to a high frequency digital and RF designs. As designers, we understand that characteristics of high speed digital and RF/microwave circuits differ a lot. So, in dielectric selection, considering the signal type is of primary importance and we understand that even in the high frequency domain, not all signal types will use the same laminate material. For a high frequency digital signal, the tolerance is more than the RF/Microwave. So, component selection for  RF/Microwave is more critical.

               As designers, we have to understand that quality should be the prime motto and the timeline for the project delivery must follow it. As a quality check, thorough analysis of application vs dielectric type to be used is a must and some significant amount of time must be spent on this. 

What are the material characteristics that need to be considered for selecting a laminate material?

1. Dielectric constant
2. Dielectric thickness
3. Loss tangent
4. Impedance characteristic
5. Temperature curve
6. Cost

Effects of  improper dielectric material selection:

1.  Impedance mismatch and hence reflection
2. Dielectric loss (signal loss in the material) - Skin effect

Impedance of the material seen by the signal varies as per the signal frequency. Also, with temperature the impedance varies. Another major factor is the process variation by different fab vendors resulting in minute variations which can create signal losses post fabrication.

The below are the dielectric materials preferred for high frequency applications:

1. Nelco 4000 
2. Rogers 4350

The below figure describes the core and pre-peg terminologies in a stack up.