1. Technical Field
The present invention relates to integrated circuits. More particularly, the present invention relates to integrated circuit carriers.
2. Description of the Prior Art
Advances in integrated circuit technology continue to produce smaller and denser devices. As a result of such increased device density, the number of connections that may be made to an integrated circuit ("pin-out") has increased substantially, while the size of the integrated circuit has decreased substantially. Traditional approaches to integrated circuit packaging cannot accommodate these modern device pin-out densities. Accordingly, considerable effort is currently being expended to develop and improve integrated circuit packaging technologies.
One emerging packaging technology that is particularly promising is that of microcarriers. FIG. 1 is a partial plan view of a typical microcarrier 10, in which a carrier base 12 includes rows of terminals 14 to which an integrated circuit (not shown) is connected. An array of bump-shaped pads 16 provides a microcarrier pin-out. The microcarrier shown in FIG. 1 provides a microcarrier having a size of 2 cm (0.8 inches) per side, and a pin-out of 408 pads having a 0.6 mm (24 mil) pitch. Microcarriers presently available are capable of providing a pin-out array of up to 1000 pads having a 0.25 mm (10 mil) pitch.
In early microcarrier applications, an integrated circuit was wire bonded to the microcarrier. The microcarrier was then mounted onto a substrate, such as a printed circuit board, using area array solder joints. In more recent microcarrier applications, the microcarrier may be designed to accept integrated circuits having either high density TAB or area array flip chip solder bump connections, in addition to wire bonds.
Initially, microcarriers were made of alumina ceramics. Such microcarriers provided about 400 input/output terminals "I/O") in a package having dimensions of about 2.03 cm (0.8 inches) per side. This compares quite favorably with other packaging technologies, such as pin grid arrays, which have dimensions of about 5.08 cm (2 inches) per side.
When first introduced, ceramic microcarriers were mounted onto multiplayer ceramic substrates having matching pads by solder screen printing and IR reflow of the microcarrier/substrate assembly. Currently, microcarriers may be made of many different materials. For example, printed circuit board materials may be used for low cost applications having low to medium I/O count, whereas copper polyimide Cu/PI on ceramic (such as alumina, aluminum nitride, etc.) may be used for very high density, high performance applications.
A problem arises in microcarrier applications when dissimilar materials are used for the carrier and the substrate to which the carrier is attached because of the thermal coefficient of expansion ("TCE") mismatch between the dissimilar materials. Temperature cycling normally encountered under circuit operating conditions often results in some movement of the carrier and the substrate relative to one another. The use of standard area array soldering to join the carrier to the substrate produces a mechanical bond of insufficient strength to mediate a significant TCE mismatch. Thus, under conditions of thermal cycling, the solder joints between the carrier and the substrate may be broken, disrupting circuit operation and necessitating assembly repair.
When contact area is increased to strengthen the carrier-to-substrate bond enough to maintain the bond during thermal cycling, pin out density is decreased dramatically. For example, current practice permits solder joints between similar materials in which a contact area of 0.25 mm (10 mils) with a gap between contacts of 0.25 mm (10 mils) provides a contact array having a 0.50 mm (20 mil) pitch. When dissimilar materials are used for the carrier and substrate, a 1.0 mm (40 mil) contact must be provided, resulting in a contact array having a 1.25 mm (50 mil) pitch.
It is known that the height of the solder joint between the carrier and the substrate is critical in mediating severe TCE mismatch encountered when dissimilar materials are used for the substrate and the carrier. That is, a longer joint somewhat reduces the likelihood of failure at a solder joint under conditions of TCE mismatch. It is also known that the height of the solder joint is critical for effective cleaning of the solder joint after the joint is formed by heating. If effective cleaning is not performed at the solder joint, the residual corrosive materials remaining at the joint after soldering will attack the joint and eventually cause the joint to fail.
The height of the solder joint, i.e. the separation between the carrier and the substrate, may be increased by any of various known techniques. These techniques include making a post print of tungsten metal onto a bond surface of either the substrate or the carrier to raise the height of the interconnecting pads and thereby add height to a resulting solder joint; and the use of a stand-off or spacer, such as a metal pin or ball (e.g. made of tungsten or copper) at each interconnecting pad of the substrate or carrier.
Although post printing raises the solder pads a small amount, such that it is possible to compensate somewhat for a lack of coplanarity between the substrate and the carrier, it does not provide adequate height to mediate the TCE mismatch when dissimilar materials are used for the substrate and the carrier.
Metal pins provide a partial solution to the problem of adding height to a solder joint, but they are prohibitively expensive (currently about 15 times standard processing costs). Tiny metal balls that are useful as spacers between a substrate and a carrier are commercially available, for example from such well known vendors as Kester and Alpha Metals. Although such metal balls have been found to be of limited use to raise the height of the solder pads, the tendency of a substrate and a carrier to lack coplanarity requires that metal balls of various sizes be provided at different locations in a solder pad array to mate the interconnect surface of the carrier to that of the substrate. The metal balls must also be of sufficient strength to provide adequate support for heavier carriers such as those including multiple circuits, etc., when the metal balls are subjected to temperatures that are sufficient to produce a solder bond.
Presently, when using a microcarrier, a circuit designer is constrained to using a substrate and carrier formed of a similar material. This lack of choice limits the designer's ability to exploit the properties of various materials as an application may require. The full value of microcarriers will be realized when the limitations and problems their use poses are solved. The ability to attach a microcarrier to a substrate formed of a dissimilar material in a commercially and technically reasonable fashion would be a significant advance in the integrated circuit packaging art.