For fine-pitch BGAs, at what point does it become practical to opt for via-in-pad?
I don’t know that it would be accurate to say that BGAs have ever been easy, but with 0.4mm pitch commonplace and 0.3mm pitch starting to appear, some of the older sizes – like a whole millimeter pitch larger – seem positively spacious. With 1mm and larger ball pitch, putting a via between the pads (not in the pads) is a no-brainer.
IPC-7095B classifies 0.8mm and smaller pitch as fine-pitch.1 It really starts to get complicated at around that point. For example, take a 0.5mm pitch BGA. When looking to put a via between the pads, the diagonal pitch is the critical measurement. In this case, it’s 0.71mm (17 mils). It might immediately seem like that’s plenty of room for a 6 mil via, but upon closer examination, not so much (Figure 1).
IPC states that a 0.5mm pitch BGA will have a nominal pad diameter of 0.3mm. It should be a non-solder mask defined pad, which will add about 0.07mm to the pad diameter. That gives 0.44mm total pad diameter. The radius is 0.22mm (8 mils). Take that out of the 0.35mm (14 mils) there is to work with and it doesn’t leave a lot of room.
If the fabricator can build 3 mil lines and spaces, the result will be enough room for a 0.06mm (5 mil) via, including annular ring. That’s not much space. Most designers would at that point seriously consider putting the via in the land pad and having it filled and plated over (Figure 2). You can’t leave the via open or unplated. It will cost more, but at these geometries, it’s your only viable option.
1. IPC-7095B, “Design and Assembly Process Implementation for BGAs,” March 2008.