Today’s smaller packaging can create confusion within the PCB designer ranks. When designing with BGAs with pitch sizes of 0.5mm and above, there are certain rules of thumb related to pad sizes and solder mask opening. One is to maintain pad size to about 85 % of a BGA’s ball size. Another is to use a non-solder mask defined (NSMD) pad. In this case, the solder mask is larger than the BGA pad.
The solder mask is usually opened to a diameter of about the ball size of the BGA. The recessed solder mask provides stress relief to the NSMD pad during reflow. In short, it creates a protective barrier and lets solder go around it.
Then, there are solder mask defined (SMD) pads, which are used for 0.4mm and below pitch BGA PCB designs. NSMD and SMD pad comparison is shown in Fig. 1 above. In the SMD pad case, the solder mask opening is smaller than the pad size. Better yields and results are achieved for these particular PCB designs.
There are a number of design issues and problematic areas that crop up if the wrong pad is used with 0.5mm pitch and above and 0.4mm pitch and below BGA-based designs. Check out our article to get more details!
In the meantime, here are some tips and hints go help avoid associated design mishaps in your projects.
- Using NSMD pad sizes for 0.4mm pitch BGA PCB design may provide favorable results at low volumes, but chances are low yields result at high-volume production runs.
- Keep in mind that ball size of 0.4mm pitch BGAs are smaller than those of 0.5mm pitch BGAs. Reducing pad size further may cause an insufficient solder-able area on the BGA pads. This can lead to PCBs being rejected at assembly inspection or latent failures in the field.
- Bridging can be created between pads when using NSMD pads for 0.4- and 0.3mm pitch BGAs. Bridging is created because there’s not enough solder mask webbing between pads.
- Since pad sizes are so small, and there is no solder mask webbing to provide adhesive strength, the pad may peel off, during reflow, or in the field.
- It’s best to have no trace between two BGA pads. Having them introduces shorts at pitches of 0.4mm and smaller on external layers.
- On internal layers, there should be just one trace between pads. Also, PCB fabrication problems surface when running multiple and thin 2 to 3 mil traces between two BGA pads.
It’s a good idea for the seasoned PCB designer to perform preliminary experimental layouts and prototype rounds to get the lay of the design before launching into a full-fledged layout. This means the PCB designer must be on top of his/her game, and certainly not be complacent after successfully completing many 0.5mm pitch and above BGA-based designs.