Flip Chip for High Speed MMIC and RF Devices

Flip Chip Design and Process Considerations for High Speed MMIC and RF Devices

When a MMIC or RF device is mounted with the backside attached to a metal base, it is necessary to join the RF pads of the MMIC and motherboard track with a wire or ribbon bond. However, these are inductive and present a significant RF loss, especially at high frequencies (>50GHz). Furthermore, it is difficult to ensure the same bond length because of assembly and MMIC/motherboard tolerances. At low frequencies, the loss is small, as it is at high frequencies, if the bond length can be made small. However, at high frequencies, the loss is intolerable if the bond length is large. Such loss is not expected from such an intimate bond achieved by flip-chip mounting.While flip chip clearly offers a number of significant advanatges for high speed operation a number of important design and assembly considerations must be taken into account.

Flip Chip Background

Flip chip offers a number of benefits over traditional wire bond electrical interconnection techniques. The shorter electrical connection enabled by a small bump on the die in a flip chip processes, as opposed to a longer wire bond connection, improves the electrical performance in terms of reducing the inductance and improving the high speed electrical operation of a device.

For MMIC devices in particular there are a number of applications that require operation at 60GHz and above and flip chip interconnection techniques are seen as a way to enable to higher frequency wideband operation.

Flip Chip (FC) is a term used in the world of microelectronic assembly to describe an electrical interconnection process whereby electrical conducting bumps face down are mounted directly onto packages, substrates, carriers and printed circuit boards. In wire bonding, the chip is situated with the bond pads facing up and a wire connection to each pad.

Semiconductor die must be connected to external circuitry, somehow. The process steps for flip chip attaching a device to external circuitry is shown below;

•  IC’s are created on the wafer
• Bond pads are metalized on the surface of the IC
• Bonds pads have solder bumps added (top of wafer). Note: alternative bumps to solder bumps can be formed.
• The semiconductor die are then flipped upside down with the bumps facing down.
• The external circuitry (on the PCB, package, substrate) has its pads on top (facing upward).
• The bumps on the chip and the bond pads on the circuitry are aligned and made in contact.
• The solder bumps are reflowed or welded to the substrate (using one of several methods) and the connection is completed
• Mounted chips are typically underfilled with an adhesive

A number of advantages exist for a flip chip assembly compared with a traditional wire bonded assembly.

One reason is cost.  Interconnections can be made faster and cheaper than many other interconnection options and enjoy other advantages, discussed below, as well.

A second advantage is electrical performance. By eliminating the bond wires, the capacitance and inductance of the electrical connections are greatly reduced, thus improving speed by an order of magnitude and creating high speed electrical connections.

Another advantage of flip chip is form factor reduction. By flipping the device directly to the external circuitry, bond wires can be removed from the equation completely.  This can reduce package sizes significantly and commensurately reduce weight at the same time.  Reduced size and weight transfers to reduced production costs and can also be a big advantage for Space flight applications.

Flip chip also provides more flexibility. Wire bonding is a peripheral technology, limiting the number of external connections to the available surface area along the external periphery of the die.  More connections means larger die sizes. Flip chip, on the other hand, enables the entire die surface to be used for connections, therefore giving the maximum number of connections per unit of area.

The final main advantage is durability. Because flip chips are bonded directly onto other circuitry, and because they can be filled with an adhesive which hardens to a rock-like finish, flip chips can be used  in harsh environments – from industrial to military to Space.

It’s relatively obvious that the electrical conductivity is the primary purpose of the bump.  What’s not obvious is the fact that the bump also serves:

• as a thermal conductive path from chip to substrate. The thermal conductivity of the bump helps to reduce the device junction temperature, which is something that will be monitored in this project.
• as a rigid physical / mechanical mount to secure the chip. This will have an important impact on the reliability of the flip chip assembly.
• as a spacer to separate the chip and substrate. The stand-off distance between the chip and the substrate can have an important impact on the high frequency electrical operation and will be evaluated in this proposal.

 In the design and development of any flip chip assembly then consideration must be given to the die design, bump technology, substrate, flip chip methods and underfill options. The following sections review the detailed design, process and material considerations of each of these factors, mostly from a mechanical and assembly reliability perspective.