I’m embarking on my first design with a BGA, to be hand-assembled (prob hot air). Do any of you have some bookmarks for tutorials on how to lay out the board (I’m using kicad) and also how to reflow with hot air? The chip in question is a stm32wle5jc, which is a 0.5mm pitch 9x9 BGA. Thanks!
My suggestion is to look for and read up on whitepapers from various manufacturers for “escape routing” of their various BGA offerings. Most likely, you’ll want to just use “dog bones” to escape from the inner pads to get out.
For layout, it’s best to study any existing reference designs first. In your case, I’d use the STM Nucleo board with a similar part here: https://www.st.com/content/st_com/en/products/evaluation-tools/product-evaluation-tools/mcu-mpu-eval-tools/stm32-mcu-mpu-eval-tools/stm32-nucleo-boards/nucleo-wl55jc.html
For general BGA layout recommendations, Lattice Semi has a decent app note here: https://www.latticesemi.com/~/media/LatticeSemi/Documents/ApplicationNotes/PT/PCBLayoutRecommendationsforBGAPackages.pdf?document_id=671
With BGA packages, it’s important to choose a compatible board stackup and design constraints from the start. You’ll need to confirm that your chosen PCB vendor can handle the trace/space/via dimensions that you need to make the 0.5mm pitch BGA work. Given the RF functions, you also need to consider the impedance controlled traces. Settle on a PCB vendor and stackup before you start routing.
As for hand placement: YouTube videos are a good place to start. You’ll have better results if you pre-heat the board from the bottom before going to full reflow temperatures from the top. Plan on buying extra parts and boards if this is your first time.
Yup, “escape routing” brings up many hits. https://macrofab.com/blog/escaping-bgas-methods-routing-traces-bga-footprints/ is both basic and specific.
Dumb question: what is the difficulty with via-in-pad? Is it a PCB manufacturing issue or is it a BGA soldering issue?
Tom, thanks for suggesting the nucleo board design. I could have thought of that! Duh! (I guess now I earn the “asked dumb question” badge… )
Something I had to look up is SMD pad vs. NSMD pad, terms that the lattice app note uses but doesn’t explain (or I missed it). These stand for Solder Mask Defined pad and Non Solder Mask Defined pad. I found https://macrofab.com/blog/escaping-bgas-methods-routing-traces-bga-footprints/ to be informative.
Normal vias wick solder away from joints. If you put a via under a BGA pin, the via could wick enough solder away to break the connection.
The solution is to backfill via-in-pad holes with epoxy. This stops them from wicking away solder and allows them to be used under BGA pads.
The downside, of course, is extra cost. You won’t find via-in-pad as an option at the cheaper board houses.