We often get asked about the best way to route very fine pitch BGA and CSP packages. Often times designers think they need to route their traces between the BGA pads. But if you try that, then you will violate our 3-mil trace width and gap. So what’s to be done?

Here’s a typical scenario. A layout engineer is trying to use a part like MAX20353. This part has a 0.4mm pitch from pad to pad and each pad is 0.25mm in diameter. This leaves only 0.15mm of clearance between the pads. When it comes time to route the inner pads to the rest of the circuit board, how are you supposed to fit a trace between the outer pads? Well, the simple answer is, you don’t. That’s far too small of a gap to fit a trace. On a one ounce copper board we recommend a trace width no smaller than 0.075mm and a gap from copper to copper of no less than 0.075mm. So if you’re doing the math, that means you need a gap between the pads of at least 0.225mm. And last I checked 0.15 is less than 0.225.

If you find yourself in this scenario, then you must use multiple layers and route the traces using internal layers. This is sometimes referred to as “via-in-pad”. This is accomplished by laser drilling a via down the middle of the pad and then copper plating the walls of this hole. This will carry the signal to the internal layers for routing purposes. Then these vias must be filled and plated for proper assembly.

One of the best guides we’ve ever found is written by MicroSemi which you can download here. And just in case MicroSemi ever decide to change their website and that link dies, we’ve saved it locally to our website so you can download it here.

As always, please let us know if you have any questions about this article or any other PCB layout and assembly questions.