What is Flip Chip

What is Flip Chip?

Flip Chip (FC) is not a specific package (like SOIC), or even a package type (like BGA). Flip Chip describes the method of electrically connecting the die to the package carrier. The package carrier, either substrate or leadframe, then provides the connection from the die to the exterior of the package. In "standard" packaging, the interconnection between the die and the carrier is made using wire. The die is attached to the carrier face up, then a wire is bonded first to the die, then looped and bonded to the carrier. Wires are typically 1-5 mm in length, and 25-35 µm in diameter. In contrast, the interconnection between the die and carrier in flip chip packaging is made through a conductive "bump" that is placed directly on the die surface. The bumped die is then "flipped over" and placed face down, with the bumps connecting to the carrier directly. A bump is typically 70-100 µm high, and 100-125 µm in diameter.

The flip chip connection is generally formed one of two ways: using solder or using conductive adhesive. By far, the most common packaging interconnect is solder Current solder types are: eutectic (63%Sn, 37%Pb) or high lead (95%Pb, 5%Sn) or lead-free (97.5%Sn, 2.5%Ag) compositions. The solder bumped die is attached to a substrate by a solder reflow process, very similar to the process used to attach BGA balls to the package exterior. After the die is soldered, underfill is added between the die and the substrate. Underfill is a specially engineered epoxy that fills the area between the die and the carrier, surrounding the solder bumps. It is designed to control the stress in the solder joints caused by the difference in thermal expansion between the silicon die and the carrier. Once cured, the underfill absorbs the stress, reducing the strain on the solder bumps, greatly increasing the life of the finished package. The chip attach and underfill steps are the basics of flip chip interconnect. Beyond this, the remainder of package construction surrounding the die can take many forms and can generally utilize existing manufacturing processes and package formats.

Benefits of Flip Chip:

Using flip chip interconnect offers a number of possible advantages to the user:

Reduced signal inductance - because the interconnect is MUCH shorter in length (0.1 mm vs 1-5 mm), the inductance of the signal path is greatly reduced. This is a key factor in high speed communication and switching devices.

Reduced power/ground inductance - by using flip chip interconnect, power can be brought directly into the core of the die, rather than having to be routed to the edges. This greatly decreases the noise of the core power, improving performance of the silicon.

Higher signal density - the entire surface of the die can be used for interconnect, rather than just the edges. This is similar to the com parison between QFP and BGA packages. Because flip chip can connect over the surface of the die, it can support vastly larger numbers of interconnects on the same die size.

Die shrink - for pad limited die (die where size is determined by the edge space required for bond pads), the size of the die can be reduced, saving silicon cost.

Reduced package footprint - in some cases, the total package size can be reduced using flip chip. This can be achieved by either reducing the die to package edge requirements, since you no longer have to leave space for wires, or in utilizing higher density sub strate technology, which allows for reduced package pitch