Solder ball

In integrated circuit packaging, a solder ball, also a solder bump (ofter referred to simply as "ball" or "bumps") is a ball of solder that provides the contact between the chip package and the printed circuit board, as well as between stacked packages in multichip modules[1]; in the latter case, they may be referred to as microbumps (μbumps, ubumps), since they are usually significantly smaller than the former. The solder balls can be placed manually or by automated equipment, and are held in place with a tacky flux.[2]

For particles in solder paste, see Solder paste § Classification.
A grid array of solder balls under an integrated circuit chip, with the chip removed; the balls were left attached to the printed circuit board.
A MCM schematic for a stacked DRAM dice showing solder balls

A coined solder ball is a solder ball subject to coining, i.e., flattening to a shape resembling that of a coin, to increase contact reliability.[3]

The ball grid array, chip-scale package, and flip chip packages generally use solder balls.

Underfill

After the solder balls are used to attach an integrated circuit chip to a PCB, often the remaining air gap between them is underfilled with epoxy.[4][5][6]

In some cases, there may be multiple layers of solder balls -- for example, one layer of solder balls attaching a flip chip to an interposer to form a BGA package, and a second layer of solder balls attaching that interposer to the PCB. Often both layers are underfilled.[7][8]

Usage in flip chip method
  • Pads
  • Balls
  • Alignment
  • Contact
  • Solder reflow
  • Adhesive underfill
  • Final result
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gollark: For me, at least, it would be *worse* than a mediocre Android phone, since it would be more annoying to use ssh/mosh to access my server when I need to fix things, and generally worse to test things on portably.
gollark: I have learned by now that the bare minimum is not very good, having bought somewhat less RAM for my laptop than I should even though the price difference wasn't big and generated untold annoyance over time, but iPhones are quite far into diminishing returns territory.
gollark: I guess it depends on what you're comparing against pricewise.
gollark: iPhones are quite expensive, so if you value your time at $50/hour (this might be low, I'm not really sure), it would probably take a few years for the iPhone to pay off, but it could actually come out in favour if it does in fact save that much time.

See also

References
  1. Definition of: solder ball, PC Magazine glossary
  2. Soldering 101 - A Basic Overview
  3. Patent US 7622325 B2, prior art description
  4. "Underfill revisited: How a decades-old technique enables smaller, more durable PCBs". 2011.
  5. "Underfill". ScienceDirect.
  6. "Underfill".
  7. "BGA Underfill for COTS Ruggedization". NASA. 2019.
  8. "Underfill Applications, Materials & Methods". 2019.
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