I would like to do the routing of the stm32wle5jbi6 chip that comes in a tiny UFBGA73 package. Please see the datasheet here: https://www.mouser.de/datasheet/2/389/dm00648230-1799409.pdf The dimensions are provided on p128 and p129.

I'd like to send the PCB for manufacturing to pcbway.com and they have a minimum trace width and spacing of 4mil: https://www.pcbway.com/capabilities.html But they recommend 6mil for both trace width and spacing.

Looking at my board file: STM32WLE routed chip

I see that the size of the solder mask (tstop layer) might be the problem. I cannot do any routing in the chip. What do you think about just reducing the size of the soldermask, what is the minimum distance it should have to the cream layer? Can I eventually not use it at all?

EDIT: Please see more details on the dimensions (in mm) here: enter image description here

Thank you!


1 Answer 1


Not being able to use a package due to manufacturing constraints is a real and frequent thing.

Solder mask expansion has a real role to play - it gives tolerance against offsets in manufacturing: enter image description here

So obviously you don't wan to shrink the opening too much... This is really something that's driven both by the device datasheet (less) AND manufacturer requirements (more). PcbWay has a blog post which seems relevant and they tell you that you can go down to 50 microns of expansion. Also note that ST says 330 microns typical, depends on solder mask registration tolerance, by which they mean to stick with what your manufacturer can deal with:

enter image description here

  • \$\begingroup\$ thank you for the helpful comment, I've added more details on the dimensions to my question. The pad has a size of around 230µm, the Cream layer 280µm and the soldermask 430µm. I'll reduce the soldermask to around 350µm and see if that helps me for the routing. \$\endgroup\$ Mar 6, 2021 at 11:12
  • \$\begingroup\$ @MarcoBobinger Sure, btw what did you mean by "Can I eventually not use it at all?" \$\endgroup\$
    – Dzarda
    Mar 8, 2021 at 21:51
  • \$\begingroup\$ All good, reducing the soldermask will do (I think), I didn't do the routing yet but looking at the specs of the datasheet and the manufacturer (pcbway) it should work now if I reduce the soldermask from 430µm to 350 or 330µm. \$\endgroup\$ Mar 9, 2021 at 7:10

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.