3.2.3 Extra Tips

Important: Make sure that your PCB manufacturer can produce that specific stack-up.

The PCB manufacturer may not have the required materials in stock, and 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 cheaper 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 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 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 3-6. Impedance Calculation

The SAMA7G54-EK was designed on a four-layer PCB so that the final thickness of the board should be 1.6 mm. Designs which require a different board thickness can be obtained only by modifying the central dielectric height. This does not impact the previously calculated trace impedances.