We’ve been getting more questions about laying out the 0.4mm pitch Ti OMAP BGA, as is used in the Beagleboard. As I’ve written before, some of the rules change at these tiny geometries. The Beagleboard folks discovered that non soldermask defined pads (NSMD) can lead to bridging and poor yields and therefor they recommend soldermask defined (SMD) pads. Check out page 10 in their design guide. If in doubt, or if you’re concerned that your set up might be different enough to warrant NSMD pads, I’d suggest you give a Ti Applications engineer a call.
And speaking of the Beagleboard, they just recently reduced the price on their pre-built Beagleboards. Like $125 for the original and $149 for the new xM version. Very nice.
If you’ve got a 0.4mm BGA part from a different manufacturer, check with that particular part manufacturer for the final say. Some 0.4mm pitch parts have a staggered arrangement and in that case, there is enough room center to center to successfully use NSMD pads.
Joe Cool here.
This comment is in addition to the solder bridging issue. Most cell phone designs have solder mask defined BGA’s because it helps secure the land (pad) to the PCB. During drop tests, most of the failure starts with the land (pad) ripping away from the prepreg. i.e.: there are less drop test failures with solder ball detachment to the land (pad). Since the PCB material is an epoxy and solder mask is an epoxy, the solder mask aids in holding the land (pad) to the PCB.
While we’re on the subject of solder mask defined surface mount lands, it is recommended for all SMT lands on flexible circuit PCB’s to be Solder Mask Defined for the same reason – to aid in the land adhesion to the board.