Andy - Flip Chip Bonding - Here's a "lengthy response"
CONVENTIONAL WIRE BONDING
Conventional Integrated circuits use wire bonding to connect the electrical signals from the silicon chip to the leads, or terminals, on the physical package.
Along the edges of an integrated circuit, rectangular Bond Pads are placed. These are typically square "lands' of aluminum film, roughly 100 microns x 100 microns in size.
During assembly operations, the IC is attached to a package with adhesive conductive epoxy or eutectic (Gold-Si) which bonds the BOTTOM SIDE of the silicon chip to the package. The actual active circuits are on the TOP side.
The conventional Wire bonding operation connects one bond pad on the top surface to a package lead using a thin wire. Aluminum wire, about 1 mil (0.001 inches) diameter is wedge bonded using ultrasonic vibration and compression to "weld" one end of a wire using a "wedge" type tool to a bond pad and the other end to a metal trace on a package or to a package lead. This is also known as "wedge" bonding. The aluminum wire is metallurgically bonded to the aluminum bond pad on the die at one end and to the package lead or metal trace on the other end.
Another technique is gold wire bonding in which a 1.25 mil (0.00125) diameter gold (with small amounts of beryllium) wire is thermosonically bonded to the die. An electric arc melts the tip of the gold wire and a small sphere is formed, about 3 or 4 mils in diameter. The gold ball is on the end of wire which is threaded up through a ceramic capillary tube.
The capillary comes down on top of the center of an aluminum bond pad and "smashes" the gold ball onto the pad while ultrasonic vibration is applied to the capillary, making the gold ball form a metallurgical bond with the aluminum pad. At the other end, the gold wire is wedge bonded to the package lead or trace. During this operation, the package and die are simultaneously heated to 200+ degrees centigrade.
In both aluminum ultrasonic wire bonding and Gold Thermosonic wire bonding, each pad on the die (IC) is connected to each lead on the package ONE AT A TIME. For modern CPUs, this can involve 300 or 400 or more wires! Automatic wire bonders with "automatic" alignment are used for this purpose and typically about 10 wires can be bonded PER SECOND!
FLIP CHIP BONDING
This technology does away with wires.
Instead of flat, square aluminum pads on the die, near-spherical balls of low melting temperature alloys are used to connect electrical signals from the die to the package. These "solder balls" are created though either thin film deposition/etching/reflow operations or thorough electroplating (and sometimes electroless plating) techniques.
The IC die will have these solder bumps distributed across the surface of the die and they do not have to be along the edges - they can even be in the middle of the die.
In assembly, each die is "flipped" over - that it is, it is held by a collet from the backside with the electrical surface FACING DOWN towards the package. The solder bumps re of course facing down as well.
The collet lowers the die onto the substrate/package and a series of reflective optics helps align the die and solder balls to individual traces or pads on the substrate/package surface.
The collet/die is heated to just below the melting point of the solder bumps. The package is heated ABOVE the melting point of the solder bumps and the area is purged with a heated, inert gas to prevent oxidation.
When the die is placed on the substrate, the solder bumps melts and begin to pull the die slightly downwards due to capillary/surface tension. This same surface tension will actually rotate the die slightly to align it exactly with the bonding traces on the substrate.
When removed from the flip chip assembly station, the solder bumps cool and solidify and the die is attached MECHANICALLY and ELECTRICALLY to the substrate through these solder bumps.
IBM, the inventor of this technology, calls this " C4" bonding for Controlled Collapse Chip/Circuit Connection. This refers to the slight collapse of the solder bumps, pulling the die down towards the substrate during the attachment operation.
The solder bumps themselves have various metallurgical compositions. Indium-Lead or Indium-Tin-Lead alloys are such combinations but others are used as well.
Note that the C4 Flip Chip bonding operation connects ALL terminations and the silicon die itself in ONE OPERATION. Thus, it is much faster than a wire bonding operation. This is a classic example of parallel processing versus serial processing.
Advantages of C4 Flip Chip Connection
As noted above, speed and efficiency are two main advantages. All leads and the die are connected simultaneously instead of a die-attach operation followed by a slow, one wire at a time wire bonding operation.
From an electrical standpoint, the solder bumps have much much less inductance than a tiny wire. This reduction in inductance is of great significance in reducing current transients (dI/dt), called switching noise, when many I/O signals switch states (low to high or high to low) at once.
Also, the thermal conduction of heat from the top side of the die - where it is generated - through MULTIPLE METAL alloy solder bumps - is more efficient than dissipating heat via conduction through the backside of the silicon die.
Paul |
