1. Field of the Invention
The present invention relates to the field of packaging of high current semiconductor devices and integrated circuits.
2. Prior Art
As integrated circuits become smaller and more complex, packaging becomes a substantial problem, both in terms of size and cost. Classically an integrated circuit wafer is diced, and each individual die is mounted on a respective lead frame having a plurality of leads integrally connected at their outer ends. Then each contact pad on the integrated circuit is wire bonded to a respective lead on the lead frame, and the integrated circuit and wire bonds are encapsulated in a suitable resin. Finally the leads on the lead frame are trimmed to electrically separate the leads, and the leads are bent to their final shape.
An increasing number of end products cannot tolerate the size of integrated circuits packaged in this manner, because of the size of the package, because of the number of leads required, or both. As a result, smaller packages have been developed, such as surface mount packages. While various such packages are currently in use, in general such packages have solder contacts on their mounting surface, typically with a solder ball on each contact, so that on placement of the surface mount package on a circuit board and heating, the solder will flow and wet the circuit board and contact, thereby physically and electrically connecting the integrated circuit to the board.
More recently, the individual die themselves have been solder ball mounted as described above. In such mounting, contact pads on the integrated circuit are exposed through circular openings in the insulative layer or layers on the integrated circuit, to each of which a solder ball is attached. The mounting surface of such die is shown schematically in
The present invention is related to a method of fabricating an integrated circuit using wafer level packaging (WLP) for high electrical current capabilities in a way that is less expensive than other methods. In particular, the invention utilizes soldering alloys on the chip to create an electrical contact of a sufficiently large area to maximize the allowable electrical current per pin. The invention further accomplishes high current capabilities using a wafer-level chip-size packaging, which results in a low cost and small form-factor.
Another feature of the present invention is the use of multiple contact pads 22 for connections requiring high current capacity. Use of multiple contact pads 22 on the integrated circuit 24 rather than a single large pad is preferred, as multiple pads, all of the same area, can conveniently fall within an array of multiple pads, more easily facilitating placement of the solder balls, and better assuring the right amount of solder is available per contact pad, as opposed to too much or too little, either of which could lead to difficulties during soldering to a circuit board. It also reduces stress on the substrate. Such a connection is also illustrated in
In the foregoing embodiment, an opening is created in the passivation layer over an underlying thick conductive layer 26 to create an electrical connection from a printed circuit board (PCB) of an electronic device. This opening should be of a sufficiently large area to be capable of carrying a targeted electrical current. This passivation opening is covered with a solder alloy material that is re-flown post deposition, in order to provide a low-resistance, high-mechanical-robustness electrical connection to the PCB. The solder may be screen-printed to the contact pads by distributing a solder paste over the wafer. Alternatively, solder may be electroplated. Alternatively, pre-formed solid solder objects, typically spheres (balls), can be dropped onto the wafer in the loci of the passivation openings.
In accordance with another aspect of the present invention,
Schematic cross sections taken along lines 4-4, 5-5 and 6-6 of
For the source contact in this example (
Of course the devices illustrated are exemplary only. In other examples, there may only be one high current device on the integrated circuit, or multiple high current devices connected differently. By way of example, in many switching converters and motor drives having on-chip drive transistors, high current high side and low side transistor switches may be connected in series and alternately operated. In this case, there will be three high current contacts needed, each of which in accordance with the present invention may preferably use one (or more) of three adjacent contact pads. If more than one contact pad is used for each contact, preferably the three contacts are brought out on three adjacent contact pads aligned in a first direction (say in the x direction), with the additional contact pads for each contact being aligned in a second direction (the y direction). Centering the multiple contacts as a group with respect to each high current device to the extent possible further reduces the resistance of the current path from the contact to the high current device through the thick interconnect layer. In that regard, all aspects of the present invention are directed to minimize the i2R loses between a circuit board on which the integrated circuit may be mounted and the device in the integrated circuit so as to minimize the heat generated beyond that generated by the device itself, thereby allowing the device to operate at the highest current possible for the cooling capacity of the chip scale package and its mounting. In the case of high side and low side transistor switches alternately operated, the center contact will have a higher duty cycle than the other two contacts, and thus may be connected to more contact pads than the other two contacts.
Thus the present invention has a number of aspects, which aspects may be practiced alone or in various combinations or sub-combinations, as desired. While a preferred embodiment of the present invention has been disclosed and described herein for purposes of illustration and not for purposes of limitation, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the full breadth of the following claims.