BGA/LGA chips

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Chris Hamilton

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Feb 18, 2014, 9:42:40 PM2/18/14
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I was curious if anyone had experience laying out BGA chips.  I know I don't and they appear to require multilayer boards just to be fully connected.  I have been quite unsure of exact chip selection vs. limiting the number of boards.  Motorola just announced some new K20 based 802.15.4 capable MCUs and I already am evaluating a low end K20 as our standard USB driver. - http://cache.freescale.com/files/rf_if/doc/data_sheet/MKW22D512V.pdf
The top end version combines the USB OTG, but it uses LGA (and is only available in tray, but that is a minor issue).  I know there is only about half a dozen people here, but I thought I would ask.

Chris Hamilton

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Feb 18, 2014, 10:06:49 PM2/18/14
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OK, well I was looking at the LGA for this chip, and it appears that the 6 pins under it are all factory test, so I don't think it will be hard to lay this out with just a special ground plain that doesn't touch those pins.   Still, I am curious if anyone has worked with real BGA :).  There are quite a lot of tiny charger and fuel gauge chips that utilize it and BGA would open us up to more small MCU designs.  I know that BGA+heat+lead free solder usually ends up with the balls failing.

Also, I just noticed I had to turn on post by email.  Any other group features that are missing, let me know.

On Feb 18, 2014, at 6:42 PM, Chris Hamilton <ccha...@gmail.com> wrote:

I was curious if anyone had experience laying out BGA chips.  I know I don't and they appear to require multilayer boards just to be fully connected.  I have been quite unsure of exact chip selection vs. limiting the number of boards.  Motorola just announced some new K20 based 802.15.4 capable MCUs and I already am evaluating a low end K20 as our standard USB driver. - http://cache.freescale.com/files/rf_if/doc/data_sheet/MKW22D512V.pdf
The top end version combines the USB OTG, but it uses LGA (and is only available in tray, but that is a minor issue).  I know there is only about half a dozen people here, but I thought I would ask.

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Chris Hamilton

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Feb 25, 2014, 6:21:08 PM2/25/14
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Just some more info about this.  I spent some time researching ‘proper’ footprints for chips.  I am not sure there is a solid standard, but diptrace ‘oval’ (actually rounded rectangle) pads appear to be what the PCB Expert/IPC people recommend.  I have gone through most of the footprints I already did and tried to make them more correct.

I think it is debatable whether the length of the pad should be based on the maximum or nominal published in the datasheet.  It appears that many manufacturers recommend laying out a pad twice the length centered on the chip edge for similar QFP D shaped pads (which the LGA in this case really is).  So the rounded rectangle is a pad shaped like an elongated D back to back.  I have seen a couple designs that stop at exactly the edge though and so only have a single D side.  The width standard appears to be nominal.  For now I think we should go with rounded rectangle maximum length, nominal width.

I plan to use the reference nRF51822 DCDC reference design with 0402 with a stencil and see how that works.  I met some other hardware guys at a meetup last week and was recommended a cheap place to get laser cut polyimide stencils.  The hardware community here is definitely building.
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