Two types of land patterns used for surface mount (SM) pads are non-solder mask defined or NSMD pads and solder mask defined or SMD pads. Fig. 1 shows the difference between the two.
NSMD pads have the solder mask opening larger than the pads. On the other hands, SMD pads have the solder mask opening smaller than the copper pad.
Most EMS providers suggest that NSMD be used because it provides tighter control of copper artwork registration compared to the positional tolerance of the solder masking process.
Moreover, SMD pad definition may introduce stress concentration points that may result in solder joint cracking under extreme fatigue conditions.
For NSMD, the pad size is typically reduced by 15 percent over the BGA diameter to achieve balanced stress on the solder joints. This method can also help to increase the routing channel on the primary layer.
Here are more details to help you decide on whether to go NSMD or SMD in your designs.
- 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.