Bulk Capacitance for iMX6 Designs

[markwilliams]

The iMX6 design guide recommends one 22uF capacitor for each of the _CAP power supplies, under the processor. Due to the tight space under the iMX6 processor ideally these caps would be 0402, however finding 22uF 0402 is proving difficult - they exist but seem to be rare, expensive and not available in the current capacitor market.

0603 are abundant in this value but I can't fit them under the processor in my design.

I looked at the Sabre AI design and the footprint follows the IPC standard for 0402 but the BOM lists these as 0603 caps? Are you able to place 0603 successfully on 0402 footprint? From quick glance it will fit, but not sure of the manufacturability?

While our board assembly house looks into the smallest 0603 footprint they could get away with, would it be possible to use 15uF for these positions as this is more widely available on 0402? Or are these _CAP pins exactly 22uF requirement?

[gusarambula]

Hello Mark Williams,

There is some variation on the footprint sizes, it’s not rare to see slight variations in different libraries so it’s good that you are careful with this. While similar in proportions, the sizes of 0603 and 0402 footprints are not the same. The SABRE AI BOM do contain both 0603 and 0402 capacitors and it’s important that the footprint corresponds to the component. Due to the size difference you should not place a 0603 capacitor in a 0402 footprint.

The recommended bulk capacitors should be 22uF for i.MX6 processors (except i.MX6QP/DP which require larger capacitors). We do not have information on behavior with smaller capacitors but it could be that a smaller capacitor wouldn’t be enough to provide stability on the power supply.

I hope this helps!
Regards,

[markwilliams]

Thank you for your reply. I managed to create a "minimal" 0603 footprint and fit that between the vias. Our manufacturer can tolerate the smaller footprint. This means we can stick with 22uF and buy these in 0603.