nRF52832-CIAA HDI PCB layout tips

[Sumpli7]

Hello,

We have plan to use nRF52832-CIAA with external antenna connector (MHF4).  I noticed that in 4-layer reference design layout there is quite big square keepouts on inner layers 2(GND) and 3 (VDD) under C3/L1 tuning components.

Our HDI PCB is under design. We have short "RF" trace from L1 to antenna connector pin and now its time to calculate trace width to match impedance to 50ohms.  Because of those keepout in inner layers, there can't be solid reference plane on layer2 (for RF trace).

1. Can we "shrink" keepout from layer2  to get more solid copper for reference?

2. Or is it better to have keepouts  and use layer 4 as reference layer  ->RF trace width increases a lot. How wide keepout (on layers 2 &3) must be under RF trace?

Do you have tips how to go forward?

 BR

Markku

[AmbystomaLabs]

The big reason for the keepout is that HDI boards normally have about a 3 mil outer layer dielectric with a big core.  The dielectric needs to be thin so the laser can burn through. With 3 mil and a healthy trace to ground pour spacing even a 10 mil trace will give a 33ohm characteristic impedance.

Plus since the tolerance is normally +/- some fixed mil amount. This means board to board impedance changes will be large even it you spec a characterized board.

So, much easier and repeatable to do a ground keepout on inner layers and then use bottom as ground reference for top and traditional coplanar with ground configuration for the strip.

[Sumpli7]

Thank you. This helps a lot to go forward.