This invention relates generally to electronic component packaging techniques and more particularly to mounting of electronic devices within a package.
As is known in the art, electronic devices in particular integrated circuits such as monolithic microwave integrated circuits include a semiconductor crystalline substrate such as gallium arsenide, silicon, or other semiconductor materials having disposed over a first surface thereof an electronic circuit manufactured by conventional integrated circuit techniques. Generally, disposed over the opposite surface of the substrate is a back plane conductor which serves as a heat sink and ground plane for the electrical circuit.
In particular, with monolithic microwave integrated circuits (MMIC's) the circuit is bonded to a machined carrier typically comprised of a metal or a dielectric having a metal coating over a surface thereof. The metal carriers are subsequently bonded or packaged into a circuit package. Two techniques are commonly employed to bond MMIC devices to a package. One technique is pretinning in which the package or carrier is provided with a quantity solder. The carrier is heated to remelt the solder as the electronic circuit is placed upon the carrier.
With the second technique, the integrated circuit is soldered to the carrier using a solder pre-form. A solder pre-form is a flat slab of solderable material which is placed between the ground plane conductor of the integrated circuit and the carrier. The carrier is heated to an elevated temperature and slight pressure is brought to bear on the circuit to cause the solder pre-form slab to melt and form a solder bond between the package and the circuit.
Problems with pretinning and the solder pre-form technique are similar. For example, a solder pre-form is often relatively thick and thus the solder pre-form provides a relatively thick solder bond. The solder pre-form also provides a relatively high thermal impedance bond, since typically the material of the solder has a thermal conductivity which is lower than the thermal conductivity of the material of the machined carrier, and the material of ground plane conductor on the substrate of the MMIC. Generally, gold is the material used as the ground plane conductor whereas a gold layer is often provided on the package or carrier.
Furthermore, since the solder pre-form tends to be thick, there is a possibility that excess solder will build up around the outside edges of the chip. This situation provides the potential for a short circuit between the back contact of the integrated circuit and the circuit fabricated on the front side or top portion of the substrate. Moreover, the requirement for applying pressure to the circuit may result in damage to the circuit.
Also, solder pre-form bonds often have inconsistent and unreproducible mechanical characteristics. Such occurrences necessitate addition rework operations which increase the cost of packaging, and have the potential to cause damage to the circuit.
The use of solder pre-forms or pre-tinning also increase the cost of packaging of such electronic circuits, since such solder bonding techniques are labor intensive.