autorouting BGAs

[tgcoombs]

I am working on a board with several 256-pin BGA parts with 1mm x 1mm pitch.  The footprints came from the Ultiboard library.  The autorouter works fine on all parts except the BGA parts.  I have the trace widths and isolation, as well as pad and via isolations set to 4 mils.  The autorouter seems to only route to the outer perimeter of the BGA parts.  Manually routing to the inner pads does not cause any DRC errors.  I have the 'fanout BGA option set, yet I don't see any evidence that this does anything.  Am I doing something wrong, or is there some reason I'm going to have to manually route the parts (about 500 connections).

[Tien_P]

Hi,

Please go to http://www.ni.com/support/ and create a service request,  send us your file and hopefully, we can find a solution for you.

[kittmaster]

Go to view > clearances.  Look at the BGA pads, are the clearance "shells" outside the pad diameter?  If so, you need to set the SMT pad clearnaces to ZERO.  Most times, the clearances are compounding each other i.e. trace clearnace =  10 mils, SMT clearnance = 10, so the autorouter is directed to keep a total of 20 mils total between objects > which is completely wrong.  The only thing that should be set IMO, is the trace clearance, since when the board is manufactured, will be the only concern.  Most fab houses can deal with 5 mil clearances between any two objects.  This will also allow you to most likely get your BGA's routed.  Make sure you verify your PCB house's miniumum clearance limits or you will pay dearly to have the pcb fabbed for out of spec needs.........Regards, Chris