What Is Solder Bump?
In Integrated circuit (IC) packaging, a Solder Bump, also a Solder Ball, or referred simply as “ball” or “bumps”, is a ball of solder that provides the contact between chip package and printed circuit board, as well as between stacked packages in multichip modules. 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 pace with a tacky flux.
Solder Bumps are the small spheres of solder balls that are bonded to contact areas or pads of semiconductor devices or circuit boards. Subsequently, the solder bumps are used for face-down bonding. The length of the electrical connections between the chip and substrate can be minimized by:
- Placing solder bumps on the die
- Flipping the die cover
- Aligning the solder bumps with contact pads on the substrate
- Reflowing the solder ball in a furnace to establish the bonding between the die and the substrate
Ball Bumping Materials
A few key points on ball bumping materials. The primary bump material still used is solder. Solder bumps are fabricated by evaporation, plating and solder screening. Depending on your application, you may choose an alternative solder bump material. These include gold stud bump, conductive epoxy, copper balls or columns.
Flip-chip Devices
Flip-chip devices have solder bumps, other metal bumps, or even conductive adhesive bumps on the face of the device for I/O connections. During assembly, the devices are flipped face down, then mated and bonded to corresponding solder or metal pads on the package or interconnect substrate. In the quest to eliminate tin–lead solders, electrically conductive epoxy adhesives are beginning to be used for the bumps.
Ball-grid Array (BGA)
Ball-grid arrays are analogous to flip-chip devices except that the solder balls are formed or attached at the next level: the package or the chip carrier. Thus, one can have flip-chip or wire-bonded devices or combinations in a BGA package. BGAs were developed because other packaging approaches such as the QFP had reached their limit in the number of I/Os that were practical and producible. Reducing the lead pitch for QFPs had increased the number of leads but, since the leads were at the periphery, such packages became quite large and risked damage to the closely spaced leads. With bumps spread throughout the area of a BGA package, even though the I/O pitch is greater, the overall size is smaller and up to 1,000 I/O bumps are possible.