I wrote not long ago and not far, far away about the type of pad recommended for 0.4mm pitch BGAs, as in non-soldermask defined vs. soldermask defined (NSMD vs SMD). The wisdom being that with the pads being so close on a 0.4 mm pitch land, the BGA balls may be too close together and may bridge if the pads are NSMD.
I got a comment from an engineer stating that not all 0.4mm pitch BGAs are created equal, that some have staggered lands which would still require NSMD pads. I searched and couldn’t find a picture of such a part or corresponding land — too many millions to go through — so I simulated what that might look like in this image.
IPC calls this an “Interspersed array” (IPC-7095B, March 2008, Figure 6-13) or a “Staggered Matrix” (IPC-7351A, February 2007, Figure 14-8). The thought being that, when staggered like this, the center to center distance between the pads is 0.57mm even though the column center to center distance is 0.4mm and that would put it back in the realm of getting better results with NSMD pads because 0.5’s and above generally want NSMD pads.
Hmmm. Makes sense. Thanks Mr. Pythagoras for the extra 0.1656854mm of space.
Use the reflow, Luke. Use the reflow
IPC talks about non-solder mask defined and solder mask defined BGA lands in their publications to explain the differences between the technology, but they never encourage solder mask defined BGA lands. But there are exceptions to every rule.
We all know that most SMT lands start out as 9um thick (1/4 OZ) copper foil over prepreg.
The IPC-7351B land (pad) size for a 0.4 mm pitch with a 0.25mm collapsing ball diameter is 0.2 mm (8 mils). If the BGA is used in a cell phone, it has to pass drop tests. I have learned that there normally is not a problem with the solder joint during the drop tests, but there are reported problems with the land detaching from the prepreg. Cell phone manufacturers have learned that adding a slight solder masked defined land improves the yeild of the drop tests for these fine pitch BGA’s. The solder mask in this case acts as additional glue to prevent the land from breaking away from the prepreg.
The land breaking away from the prepreg during drop tests is primarily due to the small land size.
Interesting you should mention that (pads detaching from the pre-preg). Were these ball pads that were not connected in any fashion to the PWB (stand alone, unconnected net pads)? If so, those balls should be removed from the chip.
Reason? On all our devices, all pads are connected somehow (with nets). Hence, each ball pad is connected mechanically to the PWB (e.g. via in pad at the ball pad; trace connecting to the ball pad; dog-bone via to the ball pad). This would make it much more structurally reliable than a simple ‘stand-alone’ ball pad.
Sounds like the drop-test might need to be investigated to make sure it’s not unfairly trying to create a bad situation (pad detach).
Make sense? The added copper (trace running to the pad) along with the soldermask holding it down should be providing adequate structural support.