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Flip-Chip

A type of interconnect using solder balls or microbumps.
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Description

Flip-chip is an interconnect scheme, providing connections from one die to another die or a die to a board. It was initially developed in the 1960s. It is also known as controlled collapse chip connection, or C4.

In flip-chip interconnects, many tiny copper bumps are formed on top of a chip. The device is then flipped and mounted on a separate die or board. The bumps land on copper pads, forming an electrical connection. Originally, flip-chip interconnects were created using solder balls, but those became too large for many purposes. As an alternative, copper microbumps were introduced. Copper microbumps enable more I/Os with smaller pitches and better thermal conductivity than solder bumps, which is why they have become the mainstream interconnect technology for many midrange and high-end packages.

Side view of flip-chip mounting. Source: Wikipedia

Copper bumps consist of a copper pillar with a solder cap, based on a tin/silver alloy. To make copper bumps, a surface is deposited with an under-bump metallurgy (UBM). Then, a photoresist is applied on the UBM. The desired bump size is patterned and etched, forming a small gap in the resist. A copper layer is plated over the surface, forming a pillar in the gap. In some cases, this material is reflowed or heated, forming the bump.

To stack and connect bumped dies in packages, the industry uses thermal compression bonding (TCB). In operation, a TCB bonder picks up a die and aligns the bumps to those from another die, then bonds the bumps using force and heat. TCB, however, is a slow process.

Flip-chip is used to develop many packages, such as ball-grid array (BGA), double-sided, molded ball grid array (DSMBGA), and others. In DSMBGA, the components are situated on top and bottom of the substrate. This reduces the package size and also shortens the signal path of the devices.

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