BALF-SPI2-02D3 datasheet stackup: what does “35 µm + 31 µm” mean?

[cehmke]

I’m using BALF-SPI2-02D3 with the S2-LP at 433 MHz and have a question about Figure 13 in the datasheet.

The recommended PCB stackup shows:

• Top plane: 35 µm + 31 µm
• Mid layer: 31 µm
• Bottom plane: 35 µm + 31 µm

What exactly does “35 µm + 31 µm” mean for the outer layers?

Is this:

• base copper + plated copper,
• finished copper thickness in ST’s stackup notation,
• or something else?

I’m trying to choose a 1.0 mm, 4-layer PCB stackup from a commercial manufacturer and would like to match the recommendation as closely as possible.

Thanks for any clarification.