Will a Via Fit Between?

I don’t know that it would be accurate to say that BGAs have ever been easy, but with 0.4mm pitch being common and 0.3mm pitch showing up, some of the older size, like a WHOLE millimeter pitch seem 0.5mm pitch padspositively 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. It really starts to get complicated at around that point. For example, take a 0.5mm pitch BGA. Since we’re looking to put a via between the pads, the diagonal pitch is the critical measurement. In this case, it’s 0.71mm (17 mil). It might immediately seem like that’s plenty of room for a 6 mil via, but upon closer examination, not so much.

0.5mm pitch pads viasIPC states that a 0.5mm pitch BGA will have a nominal pad diameter of 0.3 mm. It should be a non-soldermask defined pad, which will add about 0.07 mm to the pad diameter. That gives 0.44 mm total pad diameter. The radius is 0.22 mm (8 mil). Take that out of the 0.35 mm (14 mil) you have to work with and you’re not left with much space.

If your fab house can do 3 mil trace and space, you will end up with enough room for a 0.06mm (5 mil) via, including annular ring. That’s not much space. Most designers, at that point, will seriously consider putting the via in the land pad and having it filled and plated over. You can’t leave the via open or un plated.

Duane Benson
All was in chaos, ’till Euclid arose and made order


Reference Designators

A while back, I wrote about reference designators relative to family panels. Family panels can cause problems because often times, each individual circuit layout will have reference designators that start at the same place.

For example, circuit A, down in the lower right corner of the panel, will have resistors R1, R2, R3… Looking at the other three circuits on this hypothetical family panel, all of them also start their reference designators with R1, R2, R3… That’s bad. It can lead to confusion and wrong parts on the board in the wrong spots. If we see this here at Screaming Circuits, we may spend some extra time and sort through it manually or we may ask you to fix it first. Fixing it here is a labor intensive and risky process. It’s bad news.

Anyway, to the point of this post: In the original post, I listed one wrong way and three right ways. There are two other wrong ways not in the original post, which I’ll list here.

Wrong way number one: R1-1, R1-2, R1-3. Bad. Most assembly software will interpret a dash as meaning a range. It will see “R1-3” as equalling “R1, R2, R3”. That can be bad.

Wrong way number two: Leading zeros. Don’t do “R1, R01, R001”. The leading zeros are stripped and that can cause all of those the be seen as “R1”. Just don’t put leading zeros in your reference designators.

Duane Benson
Corrigan says Long Beach is actually in Ireland