BGA routing problems?

[ryan.kaveh]

Hi,

Does anyone have any suggestions for BGA escape routing? Between autorouting/hand-routing I haven't been able to make much progress. 

My main issue seems to be that I can't wire between the BGA pins on the top layer. This suggests a package/footprint error but I couldn't find anything.

Here's a picture as reference - right now, it's impossible for me to get the +3v3 pin at c9 anywhere I could make a via. Intuitively I should be able to route down to the center of the package and put a via there but Eagle isn't letting me. My min trace width is .005" while the pin pitch is .008" (so it should be able to route between pins without a problem).

Any suggestions would be much appreciated! I can provide the parts/files if need be!

Kindest regards,

Ryan

I'm using Eagle 8.1.0 premium to make a custom PCB with a uControllor (specifically an LPC4088).

[Frazer2.BarnesAXF26]

I think the problem is your DRC setup.

You really want your vias fanning out in a diagonal fashion from the pads. 

However your pin pitch looks very fine and your will likely need to use micro vias to fan out that BGA, although that will come with additional cost during fabrication. 

I typically try to use BGAs with pitches no smaller than 0.6mm as that works well with many fab houses. 

Other people may have butter suggestions but I think microvias might be the best option for a chip with a pitch that fine.

[ryan.kaveh]

Sorry, I mistyped the pin pitch. The BGA has a pitch of 8mm not 8 mils (I don't know what I was thinking).

Hmm it might be my DRC file - I'll double check my pin/wire dimensions. But if I set my wire width to something really really small (say .01mm) Eagle still won't let me route it through the pins.

Regardless, thank you so much for the advice on wiring diagonally and for the screenshot, once I get this working I'll follow your example. Thank you!

Kindest regards,

Ryan

[don.pitchford]

Hi,

I wonder if you could guide me on how to size my via.  I have a BGA with a pitch of 0.4mm.   I am trying to resize the via to fit on the ball (I will have the vias filled), but I can't seem to size the via pad small enough because of my DRC.   I am wondering which rule is forcing the pad size.   I been playing with it and can't seem to get the pad size of the via small enough.  

Any input would be helpful.

Don

Eagle 7.6

[rachaelATWH4]

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@don.pitchford wrote:

Hi,

I wonder if you could guide me on how to size my via.  I have a BGA with a pitch of 0.4mm.   I am trying to resize the via to fit on the ball (I will have the vias filled), but I can't seem to size the via pad small enough because of my DRC.   I am wondering which rule is forcing the pad size.   I been playing with it and can't seem to get the pad size of the via small enough.  

Any input would be helpful.

Don

Eagle 7.6

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Hi Don,

I suspect you might be falling foul of the minimum size settings in the "Restring" tab of the DRC. You'll likely need to drop these values down to do via in pad on your very fine pitch BGA.

Best Regards,

Rachael

[C.Nicks]

Hi Ryan,

Be sure to check the net class rules you have setup. The new routing features use the net classes to set clearances along with the DRC.

Regards,

- Cameron

[don.pitchford]

Thank you for the quick response.   

1.  Experimenting - I set my netclass to all 0 mil.

2.  Restring - Set everything to min of 4mil with 1%.

When I create a via of 5 mil and diameter of 8 mil, the system still draws a via with 5 mil drill and 13 mil inner / outer layer diam.    I am not sure what I am missing.  

Don

[rachaelATWH4]

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@don.pitchford wrote:

Thank you for the quick response.   

1.  Experimenting - I set my netclass to all 0 mil.

2.  Restring - Set everything to min of 4mil with 1%.

When I create a via of 5 mil and diameter of 8 mil, the system still draws a via with 5 mil drill and 13 mil inner / outer layer diam.    I am not sure what I am missing.  

Don

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Its because the restring is defining the minimum width if copper remaining after the hole is drilled so in your case 5mil + 4mil each side. If you want an 8mil overall size then you need to set the restring to 1.5mil.

Best Regards,

Rachael

[jorge_garcia]

Hi @don.pitchford,

Thanks for visiting our forums the diameter is still being dominated by the DRC. A 4mil minimum restring will yield a diameter of 5mils + 2×4 = 13mils. If you need a smaller via diameter you will have to adjust the restring even further.

What diameter are you going for? Additionally, if you want vias in pad you will have to use microvias. These are setup in the DRC sizes tab make sure that min micro via is smaller than minimum drill to enable microvias.

Please let me know if there's anything else I can do for you.

Best Regards,

[don.pitchford]

Georgio,

Good to hear from you.  I was actually looking to do a via with a drill of 5 mil (minimum my board house can do) with pad of 10 mil (total diameter).   I was hoping NOT to do micro vias because my board is not that complex and I can escape the bga on a 4 layer through hole via with fill.  When I place 5mil with a 13 mil ring, the rings are just too close for comfort. 

I guess I don't fully understand the DRC / restring.  I have reset the DRC via out to 5 mil with 25%.  The percentage doesn't seem to matter (changed to 1%).  I think something else is overriding my restring.  Right now I am getting a drill of 5 mil with outer / inner diameter of 15 mils and I am stuck.   Do you have a DRC that I can test that would allow me to create this via? 

Don

[rachaelATWH4]

As I said in my reply, the restring is the minimum width of copper around the hole so with a 5mil hole and 5mil restring (gives 5mil all around the hole) that is 15mil total. Reduce the restring to 2.5mil or less then it will work.

Best Regards,

Rachael

[don.pitchford]

Rachael,

Sorry I didn't listen.. mostly because I didn't fully understand it.   I changed the restring to 2.5mil and worked like a champ.   I still have much more learning, but it solves me issue.   

Super Thanks and I apologize I didn't listen the first time. 

Don

[rachaelATWH4]

It's okay, not apology needed, sometimes it's not easy to follow everything people say when your new and still have lots to learn. You got it sorted now so that's all that matters 🙂

[jorge_garcia]

Hi Don,

Just for your reference, this blog post covers how the Restring features works in detail and I think it will help.

https://www.autodesk.com/products/eagle/blog/what-you-didnt-know-about-eagle-restring/

Please make sure to give Kudos on the posts that helped you, since they may also help others. Any post that solves an issue should be Accepted as a solution.

Thanks for contributing.

Best Regards,