Hi,
We are currently at the process of manufacturing prototype boards, the manufacture cannot do pitch size under 0.25mm. They kept telling us that the chip's BGAs exceed that limit, we are using the nrf52840 eagle library from nordic's github. On document, the chip satisfies that requirement, so we don't know what the problem is.
Thank you,
IPC7351 defines the landing pad sizes for bga balls, for a ball diameter of 0.25mm it gives a pad diameter of 0.2mm (Table 14-3, unfortunately I cannot link to the standard). But I see that our official altium library has a pad diameter of 0.275 so you can definetively increase the pad size up to that (I will update the eagle library to use that diameter as well).
But be aware that then the clearance between the pads is 0.225mm witch is below 0.25mm, and the solder mask sliver becomes 0.125mm witch is also below 0.25mm. See attached picture.
Any pad sizes between 0.2mm and 0.275mm should work fine, so you could try a pad size of 0.25mm, then the clearance between the pads also becomes 0.25mm, and only the solder mask sliver is outside of spec.
If the manufacturer does not accept this either then it seems you will have to use a manufacturer witch can handle smaller sizes (the most common smallest primitive pcb manufacturers can do is 0.125mm). Or as suggested by jdts use a pre assembled module for prototyping.
Best regards,
Øystein
P.S.
The package for nRF52832 is not technically a BGA package thus the table on BGA landing pad sizes from IPC7351 might not give the absolute answers. We know that a pad size of 0.275mm works as it is used in our altium library.
