NXP Semiconductors
JN-RM-2079
QN9090 module development reference manual
JN-RM-2079
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© NXP Semiconductors N.V. 2020. All rights reserved.
Reference manual
Rev. 1.0
— 17 Jan 2020
8 of 31
The QN9090-001-M10 and QN9090-001-M13 (OM15069) modules are built on a
standard 4
–layer printed circuit board (PCB) with the individual layers organized as
shown in Fig 4.
Fig 4. PCB stack-up
Note
: The NXP PCB layouts assume use of the layers defined above. If a different PCB
stack-up is used, then NXP does not guarantee performance.
NXP strongly recommends the use of the above stack-up.
As shown in Fig 4, regarding transmission lines, it is important to copy not just the
physical layout ofthe circuit, but also the PCB stack-up. Any small change in the
thickness of the dielectric substrate under the transmission line will have a significant
change in impedance; all this information can be found on the fabrication notes for
each board design. As an illustration, consider a 50-ohm microstrip trace that is 18-mils
wide over 10 mils of FR4. If that thickness of FR4 is changed from 10 to 6 mils, the
impedance will only be about 36 ohms.
In any case the width of the RF lines must be re-calculated according to the PCB
characteristics in order to ensure a 50-ohm characteristic impedance.
When the top layer dielectric becomes too thin, the layers will not act as a true
transmission line, even though all the dimensions are correct. There is not universal
industry agreement on which thickness at which this occurs, but NXP prefers to use a
top layer dielectric thickness of no less than 8-10 mils.
There is also a limit to the ability of PCB fabricators to control the minimum width of a
PCB trace and the minimum thickness of a dielectric layer. +/- 1 mil will have less
impact on an 18-mils wide trace and a 10-mil thick dielectric layer, than it will on a much
narrower trace and thinner top layer.
This can be an especially insidious problem. The design will appear to be optimized
with the limited quantity of prototype and initial production boards, in which the bare
PCB's were all fabricated in the same lot. However, when the product goes into mass
