I’m making a board with a [LIS331DLF accelerometer and a [LPR550AL gyroscope and I think I need someone more experienced so tell me if my understanding of the datasheets and ST land patterns are correct.
I couldn’t find the landpatterns in PCB Matrix LP Calculator V2009 for either of them. The footprint for LIS331DLF is called LGA16 and for LPR550AL is called LGA16L. The land patterns are not shown in either of the datasheets, just says that they can be found on the ST website. I found a pdf with [ST land patterns and there it says the width of the PCB land width should be the LGA solder pad width + 0.1mm. Since LIS331DLF has a solder pad width of 0.25mm with a pitch of 0.5mm, this would make the PCB land width 0.35mm on 0.5 mm pitch, giving an pad-pad of just under 6mil. Making it impossible to have the board made by BatchPCB. Is my reasoning correct, or is it possible to use a PCB land of 0.25mm, same as the solder pad?
For the LPR550AL, the solder pad width are 0.5mm on 0.8mm pitch, giving a PCB land width of 0.6mm on 0.8mm pitch and a clearance of 0.2mm (just under 8mil). Here I guess it’s possible to reduce the PCB land slightly to get it to 8mil. Or what would the experts say?
If you are soldering them by hand, it won’t matter much. Just make the pads longer so that you have some room for the soldering iron tip. If you are having them assembled professionally, I’d get them made by someone who can meet the ST footprint spec.
I believe Batch PCB can go down to 7mil and still pass DRC (Can someone confirm that? It’s been a while, and I don’t have any of my designs that I’ve run through it handy), so that would help.
The Manufacturer’s recommended footprints are more useful when doing mass production runs, where a few percent fallout due to bad reflow caused by a bad footprint can add up quickly in re-working costs and time – And often the assembly house will recommend changes to what the manufacturer’s recommended footprint themselves based on their results with their combination of paste, ovens, profiles, etc.
So long story short, you’re fine to change the footprint around a bit to make it work for your project.
I would probably hot-air reflow them instead of trying to use a soldering iron. I don’t have any of them handy so I can’t check, but I don’t think any parts of the solder pads are accessible with a soldering iron. This is just a board to play around with these two ICs, so no mass-production…
It said minimum 8mil for 2-layer and 6mil for 4-layer for BatchPCB in their FAQ.
I think 8 mil is the official party line, but unofficially I think it’ll pass 7 mil boards… I’m pretty sure I’ve put 7 mil through before, and seen reference to that on here.
Their vendor (Gold Phoenix) does 7mil standard, I think BatchPCB just bumped it up a mil to reduce problems.
