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]

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]

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.[6][7]

Usage in flip chip method

• 
  Pads
• 
  Balls
• 
  Alignment
• 
  Contact
• 
  Solder reflow
• 
  Adhesive underfill
• 
  Final result

See also

• Head-in-pillow defect, a solder ball failure

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".
6. "BGA Underfill for COTS Ruggedization". NASA. 2019.
7. "Underfill Applications, Materials & Methods". 2019.