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We are struggling to source STM32L4Q5CG chips due to global semiconductor shortage. Normally I am using LQFP48 package which allows me to easily rework PCB if necessary. But sometimes UFQFPN chips become available, and those can be installed by our fabrication house.

While looking at datasheet I noticed that outside of UFQFPN48 footprint is 7.3 mm, which is exactly the same as inside of LQFP48 footprint. Since the pinout of the chips is identical, it is theoretically possible to combine both footprints and have an option to install either chip.

My question is - could this affect the chip alignment during reflow?

I suspect the majority of alignment force comes from surface tension on the exposed ground pad of UFQFPN. And I can mask the ground pad on the stencil for LQFP to avoid blob of solder under the chip.

My second question is about that ground pad stencil. A lot of packages with exposed pad recommend splitting the hole into smaller parts to reduce amount of paste by about 10-20%. The ST datasheet does not mention this. Would it be better than one big opening?

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My question is - could this affect the chip alignment during reflow?

Probably not, most designs have traces connected to every pin and the pads of the UFQFPN48 are not unlike traces (the pads of the UFQFPN48 won't have soldermask on them, and that is probably the only difference and it's thermal effects would be minimal, infact having no soldermask may improve your thermal situation during solder heating because of better conduction to air with no soldermask).

Designs with lots of traces don't normally have issues (sometimes if you have a via array to ground or Vcc it can be a thermal sink, in that case the assembly may require more 'soak time' to allow the PCB to equilateral its inner planes to higher temperatures to allow solder to flow on the outer layers.)

However, if the pads are touching directly you might have an issue (it might be best if there were a bit of soldermask between footprints). Also you may want to consider different stencils\soldermask for either loading pattern, I would consult your assembler for suggestions.

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  • \$\begingroup\$ "However, if the pads are touching directly you might have an issue" I was thinking about simply extending UFQFPN pads by 1.2mm outwards. Our fabrication house has 0.25mm minimal solder mask requirement, so at 0.5 pitch it will be one big opening in the solder mask anyway. I was planning on having stencil for UFQFPN and soldering LQFP by hand. Having different solder masks, however, defeats the purpose of combined footprint. Might as well design two different boards. Hmm... maybe this was not such a good idea after all \$\endgroup\$
    – Maple
    Aug 12, 2021 at 18:07
  • \$\begingroup\$ I'm not saying it couldn't work, usually the surface tension in the pads will center the parts, if you extend the pins and solder there is no certainty to what would happen. The pads size is designed to give adequate amounts of solder and surface tension to the pins extending them would get away from specs like JEDEC OR IPC that have lots of thought put into them to maximize yields. At this point if you really wanted to continue down this path, I would prototype it and see how it works out, odds are it would work fine. But just looking at this on paper there could be issues. \$\endgroup\$
    – Voltage Spike ♦
    Aug 12, 2021 at 18:14
  • \$\begingroup\$ I would also think that the UFQFPN48 would have little or no problems, the larger part might have more trouble centering itself. \$\endgroup\$
    – Voltage Spike ♦
    Aug 12, 2021 at 18:15

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