Flip chip
Flip chip, also known as Controlled Collapse Chip Connection or its acronym, C4, is a method for interconnecting semiconductor devices, such as IC chips and MEMS, to external circuitry with solder bumps that have been deposited onto the chip pads. The solder bumps are deposited on the chip pads on the top side of the wafer during the final wafer processing step. In order to mount the chip to external circuitry (e.g., a circuit board or another chip or wafer), it is flipped over so that its top side faces down, and aligned so that its pads align with matching pads on the external circuit, and then the solder is flowed to complete the interconnect. This is in contrast to wire bonding, in which the chip is mounted upright and wires are used to interconnect the chip pads to external circuitry.
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Process steps
- Integrated circuits are created on the wafer
- Pads are metalized on the surface of the chips
- Solder dots are deposited on each of the pads
- Chips are cut
- Chips are flipped and positioned so that the solder balls are facing the connectors on the external circuitry
- Solder balls are then remelted (typically using ultrasound)
- Mounted chip is “underfilled” using an electrically-insulating adhesive
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Comparison of mounting technologies
Wire bonding
In typical semiconductor fabrication systems chips are built up in large numbers on a single large "wafer" of semiconductor material, typically silicon. The individual chips are patterned with small pads of metal near their edges that serve as the connections to an eventual mechanical carrier. The chips are then cut out of the wafer and attached to their carriers, typically with small wires (see wire bonding). These wires eventually lead to pins on the outside of the carriers, which are attached to the rest of the circuitry making up the electronic system.
Flip chip
The processing of a flip chip is similar to conventional IC fabrication with the addition of a few additional steps.[1] Near the end of the process the attachment pads are "metalized" to make them more suitable for being soldered onto. This typically consists of several treatments. A small dot of solder is then deposited on each of the pads. The chips are then cut out of the wafer as normal. No additional processing is required, and there is no mechanical carrier at all.
To attach the flip chip into a circuit, it is inverted to bring the solder dots down onto connectors on the underlying electronics or circuit board. The solder is then re-melted to produce an electrical connection, typically using an ultrasonic process. This also leaves a small space between the chip's circuitry and the underlying mounting. In most cases an electrically-insulating adhesive is then "underfilled" to provide a stronger mechanical connection, provide a heat bridge, and to ensure the solder joints are not stressed due to differential heating of the chip and the rest of the system.
Advantages
The resulting completed flip chip assembly is much smaller than a traditional carrier-based system; the chip sits directly on the circuit board, and is much smaller than the carrier both in area and height.
Disadvantages
Flip chips have several disadvantages as well. The lack of a carrier means they are not suitable for easy replacement, or manual installation. They also require very flat surfaces to mount to, which is not always easy to arrange, or sometimes difficult to maintain as the boards heat and cool.
History
The process was originally introduced commercially by IBM in the 1960s for ICs being used in the mainframe systems.[2] DEC followed IBM's lead but was unable to achieve the quality they demanded, and eventually gave up on the concept. In the 1970s it was taken up by Delco Electronics, and has since become very common in automotive applications.
Alternatives
Since then a number of alternatives to the solder bumps have been introduced, including gold balls or molded studs, electrically conductive plastics, and the "plated bump" process that removes an insulating plating by chemical means. Flip chips have recently gained popularity among manufacturers of cell phones, pagers and other small electronics where the size savings are valuable.
References
See also
External links
- Flip chip assembly videos
- Flip chip tutorials
- Flip Chip Assembly
- Flip Chip (C4) Benefits
- Kyocera America, Inc. - White Paper: Flip Chip Challenges
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