During the early days of BGA design there was some debate over whether pads should be Solder Mask Defined (SMD) or Non-Soldermask Defined (NSMD). As you can see from the diagram SMD pads are defined only by an aperture in the soldermask. NSMD pads are your regular type where the soldermask is pulled back a little to expose a narrow gap where you can see bare board. The preference these days is for NSMD for all footprints except in some extreme exceptions where an ultra-fine pitch might lead to bridging. That’s not the case for me so it’ll be NSMD for the BGA footprint on my board.
All the major components are on the top layer and hence that’s where the majority of the traces are routed. Let’s have a closer look at the BGA escape routing.
The pads are routed either to a via or to the edge of the footprint with 6 mil traces where they then expand to a wider trace for further routing across the board. The vias are all 0.3mm hole size with an overall diameter of 0.4mm. Teardrops were enabled across the entire design to ensure a stronger bond between pad and trace.
All the differential pairs are length as well as impedance matched. This meant adding these funky looking snake traces to keep the lengths within a few mills of each other. An additional design rule keeps the differential pairs at least 20 mils away from the ground fill on the top layer as well.
The internal signal layer has a 3.3V flood fill which takes care of many of the power connections. The 1.8V supply is routed using traces. A small number of signal traces are also routed on this layer
The bottom layer has the remaining traces, mainly those going out to the GPIO header and some others going to the differential pin headers. The traces on the bottom layer are not 50Ω controlled due to the lack of a ground plane directly below them.
The design is now ready to be manufactured so all that remains is to export it as a set of Gerbers and upload it to my choice of the Chinese prototype houses. I’ve been using PCBWay lately for all four layer designs because the results have been consistently good and with this board I don’t really want to be venturing into the unknown by taking a punt on a slightly cheaper manufacturer.
The board thickness will be the full 1.6mm because I don’t want any accidental flexing of this board that could crack the BGA joints. Navigating their site was easy enough and the final price with slow shipping was around 65 American bucks for ten copies.
