Sam, It depends on how many multi gig traces/via are needed in BGA design. You could try eliminating stub altogether by transition from top to last layer; and design a transparent via to account for capacitance using a 3D field solver. Other options - use coreless substrates or build-up substrates with blind/buried vias. -Vijaya -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Aubrey Sparkman Sent: Wednesday, July 22, 2009 9:41 AM To: 'Bill Stube'; cygnul@xxxxxxxxx; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Backdrill under BGA We looked at backdrilling an 82 mil thick pcb under a 1mm (40 mils) pitch BGA once. Almost all of the low cost PCB suppliers said no. Aubrey Sparkman Aubrey.K.Sparkman@xxxxxxxx -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Bill Stube Sent: Wednesday, July 22, 2009 11:36 AM To: cygnul@xxxxxxxxx; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Backdrill under BGA Sam, I do not have experience with backdrilling under BGAs specifically. My suggestion would be to avoid putting vias on the signal at BGA in the first place, or if you must have vias to transition from layer 1 to layer N and avoid the stub. I realize this doesn't exactly address your question, but if you avoid having a via/stub altogether, you will avoid the added complexity and cost of having to backdrill and account for the items David mentions below. Just thought I would throw it out there. Bill Stube Hardware Engineer Plexus - http://www.plexus.com The Product Realization Company -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of David Mullenex Sent: Wednesday, July 22, 2009 4:13 AM To: cygnul@xxxxxxxxx; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Backdrill under BGA Sam, The critical factors missing here are your BGA pitch and location of the proposed back-drills relative to the BGA grid and each other. This as Back-drilled vias require larger anti-pads to accomodate the larger diameter of the back-drill head that assures removal of the matal stub, as well as the minor mis-alignment tollerace for the back-drill. Note, that you will also lose routing channels on the back drilled layers due to the larger antipad and back-drill diameter. Looking at the drill size, anti-pad, routing channels, board thickness, and ground/power plane coverage under the BGA, may indicate that back-drilling is not the preffered solution for fine pitch BGAs. Also, selection of the right routing layers and pad stack may allow you to get by with negligable stubs... assuming you do not have a boat load of multi-gig signals on a poorly pinned-out BGA. Porvide more details here or work with your fab house to arrive at the manufacturable solution. cheers, - David. ----- Original Message ---- From: "cygnul@xxxxxxxxx" <cygnul@xxxxxxxxx> To: si-list@xxxxxxxxxxxxx Sent: Tuesday, July 21, 2009 5:46:21 PM Subject: [SI-LIST] Backdrill under BGA SI Gurus, Has anybody done back drilling on BGA vias for multi gig traces? is that achievable? If so, what was the via structure? -Sam ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu