FB236 PCB Routing

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alwalker
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Joined: Sun Apr 08, 2018 11:19 pm

FB236 PCB Routing

Post by alwalker »

Hello,

I'm looking at the best way of routing out the connections from an FB236 package which according to IPC specifies a nominal 0.25 mm diameter pad for the 0.3 mm diameter solder ball. The minimum advised track width and clearance I'm seeing advised by PCB manufacturers is 0.1 mm, which means it's not possible to route tracks between pads given the 0.5 mm pitch as I would need a minimum distance between the pads of 0.3 mm. In theory, I could reduce the pad diameter to 0.2 mm to create this desired 0.3mm gap and stay within IPC guidelines, but this doesn't necessarily feel like the right way to go.

So I'm looking at options for via-in-pad and micro-vias and am looking for recommendations please. I think that the 0.25 mm pad size is likely too small for via-in-pad, even to only go down to the next layer, but would welcome any guidance and experience.

For example, would it be ok to increase the pad diameter to allow via-in-pad, and if so by how much?

Thanks!

Al


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CousinItt
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Post by CousinItt »

Xilinx has a very good document for this. Search for UG1099.
alwalker
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Post by alwalker »

Hi @CousinItt

Very helpful, thanks very much! Having done some more research it makes sense to do a small sub-board using HDI technology for the XMOS device plus any associated components that can be accommodated such as the crystal and 1V0 regulator, rather than to go to the expense of using HDI technology for the whole of the (much larger) main PCB.

As I need to do this work anyway, would there be any interest in general-purpose development boards that utilise the FB236 and FB374 packages? The current XMOS offerings are limited to the TQ128 package with a significantly smaller I/O count. I'd welcome input from the community about what features should be included. Such boards might also be of interest for production as they would eliminate the need for the added expense of HDI technology on the associated PCB.
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CousinItt
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Post by CousinItt »

I already have a general purpose board for my own (company) use, based on the XL216 in TQ128 package - see pic. I am planning to upgrade to FB374 for XL232, but I think my uses may vary from what might be considered general purpose. I just need pin compatibility with the XL216 module, local 3V3 and 1V0 supply converters with sequencing and reset control (extensible to arrays of boards), debug signals, local clock, an extra clock generator (e.g. Si5351a) for external use, serial flash, power led, maybe an eeprom, and a bunch of series terminations for the external I/O. That's about it. Also I don't need particularly high density for external connections, so I'm using a PCI-E edge connector.

I think you're going the right way in using a PCB that can properly address your routing requirements.

HTH
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alwalker
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Post by alwalker »

Thanks very much, apologies I thought I'd replied already. Yes, having had some discussions an HDI sub-board is definitely the way to go.

Nice PCB layout!