Getting started with PCB Layout for BGA Packages

Question

Are there any good resources for learning the intricacies of PCB layout when dealing with BGA packages?

I'm very familiar with layout for almost every SMT part that has leads on the edge (QFP, TSSOP, QFN, etc...) However, I have not ever had a chance to work with BGA parts because of the difficulties involved in their assembly, as the shop where I work does not have the facilities to do so.

Anyways, I've been looking into farming assembly out, and am hoping for some reference material for dealing with BGA devices.

I'm interested in both general and specifics. Escape routing, blind vias, vias in pad, SMD pads vs NSMD pads, filled and open vias, etc...

I've done a lot of sporadic reading (blogs, mainly), but am missing the bigger picture, namely how different techniques interact and a lot of basic common sense knowledge that likely comes through actual experience, even if only by proxy.

So far, I have spent some time studying any open-source project that uses BGA I can find (the BeagleBoard, Mainly) but most open source projects are not at the level of complexity that requires a BGA device, and the ones that do are rather rare.

Answer 1

If you want affordable boards, forget about blind vias, via in pad, and filled vias. This is a good presentation on BGA routing, albeit for very high-density boards, but the basic principles will be the same for less demanding layouts.

SMD pads vs NSMD pads is something you need to ask the company doing your BGA assembly about. The latter seems to be preferred. Some chip manufacturers have recommendations, as well.

If you have questions, this forum is very useful. You can also learn a lot by reading the various posts.

Answer 2

They're a nightmare to use, most manufacturers use an x ray to check the connections properly! - not sure if I've got one lying around in the tool shed :)

I found this BGA design tips PDF helpful, it has a pretty no nonsense look at BGA design and should give you a few pointers on PCB layout.

As a side point - there's issues with heat and mechanical stress affecting the BGA's connections, although they dissipate heat well, they hate movement and flex in the PCB. The Xbox 360 technical problems are a good example of this - much of the problems are based around insufficient heat dissipation warping the PCB and effecting delicate BGA connections such as the one's on the graphics chip, a flat pack SMD has a certain amount of flexibility in the leads and they stand up better to expansion and movement caused by heat even if they don't dissipate the heat into the PCB as efficiently.

Answer 3

Here's a PDF from Altera. I've usually seen the diagonal via location because this gives the most space between the via and ball. You can also Google "BGA fan out" to get more help.