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Comment by tirant | Hacker News Reader

In BGA soldering, the concept of partial alignment doesn’t really happen. Here’s why: In a 2D plane (which is the PCB), only two points are needed to define alignment. If two solder balls are aligned with their pads, surface tension in the reflow process ensures the entire BGA package aligns with the rest of the pads automatically.

Surface tension acts on all the solder balls, pulling the package into place along both the X and Y axes. So once the first few balls are in place, the whole thing “snaps” into alignment. Misaligning only some pads while others are perfectly aligned isn’t possible unless something’s seriously wrong, like a warped board or damaged package.

It’s an all-or-nothing situation—either everything aligns, or nothing does.

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> Misaligning only some pads while others are perfectly aligned isn’t possible

It's possible when your pcb-making process is on the odge of what designer wanted. Some pads on some boards may be not perfectly aligned in such cases. Or the board was heated not enough or in too short time and not all balls melted properly. Or it cracked under stresses because designer put too much vias in one place near the chip.

> It’s an all-or-nothing situation—either everything aligns, or nothing does.

In theory, practice and theory agrees. In practice, it sometimes does not.

I believe you forgot to quote "unless something’s seriously wrong". Your counter examples seem to describe exactly that: something going seriously wrong.

I've heard of head-in-pillow defects where the solder may touch but fails to wet the pad on the chip or board. BGA probably makes it almost impossible to nudge a leg to check for mechanically solid connections. I don't know if bad joints from the factory are a problem in practice, or they usually only crack in use.

The failure mode that I worry about and would need X-ray to detect is that temporary misalignment will merge/bridge some of the solder balls.

I like to "wiggle" the chip with tweezers (under hot air) to see surface tension in action and judge whether the balls have all melted, but a tiny bit too much wiggle causes that problem.

Indeed, the only common issue we have on BGA rework is bridged balls. If it's on the edge and it's a larger BGA, we can often wick it out with desoldering braid, cut up some fine solder wire, push it under, and "reball" it in-place. If it's on an inner row you're removing the BGA.

No wonder computers are binary.

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