Patent Application
20090324906
Embodiments of the present invention relate generally to electronic component packaging. More particularly, these embodiments relate to forming a compliant wrap-around contact on the top side of a semiconductor.
Electronic components are packaged in order to interconnect them with other devices. The packaging of the electronic device usually includes contacts for transmitting signals and providing power and ground connections between the internal circuitry of the device and external circuitry. Some examples of prior art contacts include wire bonds protruding from the ends of a discrete diode or resistor, or metal caps located on the ends of a fuse. Sophisticated electronic devices such as microprocessors may require several hundred contacts. Those devices are usually produced in a package having multiple pins for mounting to a printed circuit board. The electronic component is typically placed in a package, and each required contact area on the electronic component is wire bonded to the corresponding pin on the package. Because each wire bond is individually added to the circuit, large numbers of contacts make wire bonding expensive. Additionally, because of the precision required for wire bonding, wire bonding may result in short circuits and similar problems. Furthermore, wire bonds can degrade chip performance because of the length of the wires.
One prior art method of solving the problems of wire bonds is the flip chip. Solder balls act as attachment material that allows the flip chip to be attached to a substrate or circuit board. The silicon of the flip chip usually has a different coefficient of thermal expansion (CTE) than that of the substrate or circuit board. The CTE for the silicon flip chip can be as low as 3 parts per million (PPM) while the circuit board (as well as the solder balls) can be in the range of 18-22 PPM. As a result of the CTE mismatch, the chip and circuit board expand and contract at different rates due to thermal cycling. A lack of compliance in the contact area can cause failures. Attempts to solve the CTE mismatch have included introducing large volumes of solder. This is contrary, however, to the desires of the market for electronic packaging for smaller, cheaper, and faster components. For example, in U.S. Pat. No. 6,441,487, the inventors specify a minimum sized solder ball that is 229 um (0.009″) as a means of improving reliability. The market is seeking the ability to use solder balls smaller than 100 um (0.004″).
U.S. Pat. No. 6,051,489 presented a circuit package which addressed CTE mismatch by providing lead connections that are not directly on the surface of the die, but instead are formed on posts composed of an encapsulant material. This allows for some compliancy and smaller bond pads or junction areas on the die and thus minimizes the stress on the die surface. Nevertheless, smaller silicon area, greater flexibility, and greater compliancy are still desired.
A method and apparatus are described for an electronic component package. A standoff is formed on an active side of a substrate. This substrate has an electronic circuit. A conductive layer is deposited over at least a portion of the standoff and a portion of the active side of the substrate. The conductive layer electrically couples a contact area on the active side of the substrate. The standoff is removed to create a flexible conductor.
Other features and advantages of embodiments of the present invention will be apparent from the accompanying drawings and from the detailed description that follows.
Embodiments of the present invention are illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements, and in which:
A method and apparatus are described for an electronic component package using wafer level processing and flexible contacts on the top side of the substrate over an active or passive circuit.
A temporary standoff is formed from the front of the substrate. The electronic device contact areas are exposed. The exposed contacts and standoffs are covered with conductive material which is photoformed into individual contact to standoff conductors. After photoforming the conductors, a portion of the standoff layer is removed, releasing the formed conductors, resulting in a flexible contact. Once the floating flexible conductor is formed and the device is soldered to a circuit board, the conductor will flex to absorb the size changes resulting from the mismatch of coefficients of thermal expansion.
For one embodiment, a barrier metal such as titanium tungsten (Ti/W) or titanium tungsten/gold (TiW/Au) is first sputter deposited over the entire circuit prior to depositing the metal beams 510. For one embodiment, prior to depositing the metal beams 510, a barrier metal such as titanium tungsten (Ti/W) is first sputter deposited over the entire circuit and followed by a plating of nickel (Ni) and a flash of gold (Au). Alternatively, copper (Cu) plating is used. The barrier metal provides a barrier layer between metals and enhances adhesion of the metal beams 510. Additionally, the underside of the resulting conductive layer—i.e., metal beams 510—does not accept solder while the top plating layer does accept solder. After the deposition of conductive layer 510, the barrier metal layer is etched away from the remaining areas of the electronic component.
For one embodiment, the metal beams 510 comprise a first gold layer, a nickel layer, and a flash gold layer. For one embodiment, the nickel layer is deposited using electroless deposition—i.e., by chemical reduction. For one embodiment, the first gold layer is 4-8 microns and the nickel layer is 4-6 microns in thickness. The nickel layer is used because the gold layer should not be in contact with solder because it might affect solder joint reliability. The nickel layer, however, is susceptible to oxidization. To avoid oxidization, for one embodiment, a flash gold layer is deposited over the nickel layer.
Coating layer 710 functions as a solder resist. For one embodiment, coating layer 710 is approximately 25 um thick while the standoffs are approximately 25 to 200 um in height. The coating layer 710 surrounds the base of the metal beams 510 and remains under them and over the entire die surface. Any of coating layer 710 that remains on the solder surface of metal beams 510 is so thin that the coating layer is easily penetrated and removed during the reflow solder process joining the package to the circuit board. Nevertheless, the bulk of the coating layer 710 prevents solder form wicking completely over the beam rendering it rigid.
For one embodiment, the back side of the semiconductor substrate is thin. The back side of the semiconductor substrate is thinned to 3-10 mils. Generally, a semiconductor substrate is relatively thick but only has active components on or near the surface. The thickness of the substrate simplifies processing. The semiconductor substrate is thinned by sandblasting, grinding, etching, or other known techniques. The etched back side of semiconductor is a relatively flat semiconductor surface.
Once the floating flexible conductor 510 is formed and the device is coupled with a circuit board, the flexible conductor 510 will flex to absorb the expansion and contraction resulting from the mismatch of CTE. For one embodiment, the flexible conductor 510 is patterned to have a suitable surface for coupling with a circuit board via one or more of the following techniques: soldering, ultrasonic bonding, conductive epoxy, or the Occam process. With the Occam process, components are interconnected using copper plating after the components are assembled into their final positions in an encapsulated module. With the Occam process, an assembly containing components is metalized with copper using standard printed circuit build-up processing methods, with circuit patterns created to form the required interconnections between leads of all the components.
As discussed herein, top plating layer of flexible conductor 510 will accept solder—e.g., gold, copper, platinum, nickel, or combination thereof—while the bottom of the flexible conductor 510 will not—e.g., titanium tungsten. If the bottom of the flexible conductor 510 accepted solder, solder would be able to flow or reflow all the way around the flexible conductor 510 due to air gap 610 when being soldered to a circuit board, causing a loss in compliancy. Additionally, for one embodiment, barrier metal 410 exposed by gap 610 is also resistant to solder.
In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will be evident that various modifications may be made thereto without departing from the broader spirit and scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense.
