3.2.3 Extra Tips

Important: Almost from the start of the project, or when the PCB layout is advanced enough so you can define the number of layers and the PCB technology that will be required, anticipate discussing with the PCB manufacturer to make sure that they can manufacture that specific stack-up.

The PCB manufacturer may not have the required materials in stock and may have to order it specifically, which can increase the overall production cost.

Also, the manufacturer can recommend a different layer stack-up that they can produce more cheaply with the materials in stock.

In such cases, you can easily adapt your layout to the proposed stack-up by changing the width of the traces so that the required impedances are preserved. With the help of an impedance calculator tool, make sure that the recommendations previously given can still be respected (for example, check that enlarging the traces will satisfy the impedance while not infringing on the minimal spacing).

Designing proper transmission lines is easier nowadays with the help of impedance calculators available on the market.

The following example shows the Altium Designer impedance calculator.

Here, after defining the PCB stack-up, you can either use the software to compute the ideal trace width that will yield a specific impedance or input the trace width so that the tool calculates the resulting impedance.

Figure 3-5. Impedance Calculation

The SAMA7D65-Curiosity was designed on a four-layer PCB so that the final thickness of the board would be 1.6 mm. Designs that require a different board thickness can be obtained only by modifying the central dielectric height. This does not impact the previously calculated trace impedances.