Impedance compensation of flip chip connection for RF communications

A flip chip IC device utilized in RF transceivers includes a bare die having a number of metalized pads and each metalized pad has a solder ball deposited thereon. The flip chip IC device further includes a substrate having a number of connector pads corresponding to the metalized pads. The connector pads are connected to one or more electronic components disposed on the substrate via a number of connector strips. The bare die is flipped up-side-down such that the metalized solder pads are aligned and connected with the connector pads of the substrate via the solder balls. At least one of the connector strips includes a strip section having an uneven strip width configured to compensate an impedance of a transmission line formed based on a connection between a metalized pad of the bare die and a connector pad of the substrate to match predetermined impedance.

FIELD OF THE INVENTION

Embodiments of the present invention relate generally to radio frequency (RF) communications. More particularly, embodiments of the invention relate to impedance compensation of flip chip connection for RF communications.

BACKGROUND

Processing a flip chip is similar to conventional IC fabrication, with a few additional steps. Near the end of the manufacturing process, the attachment pads102of chip101are metalized to make them more receptive to solder as shown inFIG. 1A. A small dot of solder such as solder ball103is then deposited on each metalized pad such as pad102as shown inFIG. 1B. The chips are then cut out of the wafer as normal.

To attach the flip chip101into a circuit, the chip101is inverted to bring the solder dots down onto connectors (e.g., connector pad105) on the underlying electronics or circuit board104as shown inFIG. 1C. The solder is then re-melted to produce an electrical connection as shown inFIG. 1D. This also leaves a small space between the chip's circuitry and the underlying mounting. In most cases an electrically-insulating adhesive is then “underfilled” to provide a stronger mechanical connection, provide a heat bridge, and to ensure the solder joints are not stressed due to differential heating of the chip and the rest of the system. The underfill distributes the thermal expansion mismatch between the chip and the board, preventing stress concentration in the solder joints which would lead to premature failure.

Flip chip connection is the most popular for commercial integrated circuit package to date; conductive bumps which are connected from bare die side to substrate side for electric signal propagation. The advantage of flip chip interconnection is short signal propagation path, low loss and impedance controllable. However, in millimeter wave frequency range, flip chip interconnection still has certain significant parasitic effect and therefore proper impedance compensation for achieving desire impedance is necessary.

DETAILED DESCRIPTION

According to some embodiments, a flip chip IC device utilized in RF transceivers includes a bare die having a number of metalized pads and each metalized pad has a solder ball deposited thereon. The flip chip IC device further includes a substrate having a number of connector pads corresponding to the metalized pads. The connector pads are connected to one or more electronic components disposed on the substrate via a number of connector strips. The bare die is flipped up-side-down such that the metalized solder pads are aligned and connected with the connector pads of the substrate via the solder balls. At least one of the connector strips includes a strip section having an uneven strip width configured to compensate an impedance of a transmission line formed based on a connection between a metalized pad of the bare die and a connector pad of the substrate to match a predetermined impedance.

In one embodiment, the connector strips include a first ground strip, a second ground strip, and a signal strip disposed between the first ground strip and the second ground strip. The signal strip includes a first section coupled to a first connector pad of the substrate, a second section coupled to the first section, and a third section coupled to the second section and one or more electronic components disposed on the substrate. The second section has a second strip width that is different than a first strip width of the first section. In one embodiment, the second strip width is different than a third strip width of the third section.

In one embodiment, when an impedance of the transmission line is associated with capacitive impedance, the second strip width of the second section of the signal strip is wider than the first strip width or the third strip width. The length of the second section is shorter than or equal to a quarter of a wavelength associated with an operating frequency of the RF transceiver. Alternatively, a gap between the signal strip and the first ground strip or the second ground strip is wider in at least a portion of the first and second ground strips, for example, by cutting out a cavity on the first strip and/or the second strip.

According to another embodiment, when the impedance of the transmission line is associated with inductive impedance, the second strip width of the second section of the signal strip is narrower than the first strip width or the third strip width. Alternatively, the first ground strip and/or the second ground strip include a cavity cut out to allow the second section of the signal strip to expand into the cavities of the first ground strip and the second ground strip. The width of the cavity is shorter than or equal to a quarter wavelength associated with an operating frequency of the RF transceiver.

FIG. 2Ashows a perspective view of a flip chip IC device for RF transceivers.FIG. 2Bshows a top view of the IC device. Referring toFIGS. 2A and 2B, in this example, chip101includes at least a signal die pad201, a first ground die pad202, and a second ground die pad203. Substrate104includes a signal connector pad211, a first ground connector pad212, and a second ground connector pad213. Chip101is then flipped and aligned with substrate104such that signal die pad201, first ground die pad202, and second ground die pad203are aligned and connected with signal connector pad211, first ground connector pad212, and second ground connector pad213via respective solder balls or solder bumps, respectively. Each of the signal connector pad211, first ground connector pad212, and second ground connector pad213extend to be connected to external electronic components disposed on substrate104via respective strips (not shown). A strip extended from signal connector pad211is referred to as a signal strip. A strip extended from first ground connector pad212is referred to as a first ground strip. A strip extended from second ground connector pad213is referred to as a second ground strip.FIG. 3shows a side view of the IC device.

FIG. 4shows an equivalent circuit representing the IC device as shown inFIGS. 2A-2B. Referring toFIG. 4, CG1and CG2represent the discontinuous junction effect. Resistor R and inductor L represent bump length and loss effect.

According to one embodiment, a transmission line is formed between a connector pad of substrate104and a die pad (e.g., metalized pad) of die101due to high frequency signals. The impedance of the transmission line may not match the desired impedance of die101and/or substrate104. Such desired impedance is approximately 50 ohms. In order to compensate the mismatched impedance, the signal strip on substrate104may be configured with uneven strip width, which in turn transforms into different impedance, either high impedance or low impedance. In one embodiment, if the transmission line impedance is substantially inductive impedance, high impedance compensation is needed.

Referring now toFIG. 5, the impedance of a transmission line500between die101and substrate104can be determined using a Smith chart algorithm. The equivalent circuit as shown inFIG. 4contributes significant effect especially at millimeter wave frequency. Depends on the geometry structure, every elements in the equivalent circuit will have different values consequentially. The inductive and/or capacitive impedance properties will be shown from the equivalent circuit model. Thus, dependent upon the impedance of the transmission line is inductive impedance or capacitive impedance in nature, high impedance/inductive impedance compensation or low impedance/capacitive impedance compensation may be deployed. The type of transmission line impedance can be determined using a Smith chart as shown inFIG. 5.

Smith chart a graphical aid or nomogram designed for electrical and electronics engineers specializing in RF engineering to assist in solving problems with transmission lines and matching circuits. The Smith chart can be used to simultaneously display multiple parameters including impedances, admittances, reflection coefficients, scattering parameters, noise figure circles, constant gain contours and regions for unconditional stability, including mechanical vibrations analysis. The Smith chart is most frequently used at or within the unity radius region. However, the remainder is still mathematically relevant, being used, for example, in oscillator design and stability analysis.

Referring back toFIG. 5, the Smith chart shows the impedance changes from point A to point D along the equivalent circuit of the transmission line500. When the impedance of point ends at the top half of the Smith chart the transmission line impedance is in an inductive impedance type. When the impedance of point D ends at the lower half of the Smith chart, the transmission line impedance is in a capacitive impedance type. In the example as shown inFIG. 5, point D ends at the lower half of the Smith chart, so the transmission line impedance is capacitive impedance. Therefore, it requires an inductive impedance compensation (also referred to as high impedance compensation), which is represented by a dash line from point D back to point A on the Smith chart.

According to one embodiment, a strip width of a signal strip connecting a connector pad to an electronic component disposed on a substrate is altered to achieve higher impedance on the signal strip to compensate capacitive impedance as a result of a transmission line.FIG. 6Ashows a perspective view of an IC device with high/inductive impedance compensation according to one embodiment. Referring toFIG. 6A, in this example, substrate104includes first ground strip212, signal strip211, and second ground strip213. In one embodiment, the strip width of signal strip211is altered to alter the impedance of signal strip211(e.g., uneven strip width).

In one embodiment, signal strip211includes a first strip section601, a second strip section602, and a third strip section603. The first section601(e.g., connector pad) is coupled to a solder ball or solder bump that connects to a corresponding die pad201. The third section603is coupled to an electronic component disposed on substrate104(not shown). In one embodiment, the strip width of second section602is narrower than the strip width of first section601or third section602. As a result, the impedance of signal strip211is increased to become relatively high impedance. By narrowing the width of the second section602, a gap (e.g., a non-conductive or dielectric gap) between signal strip211and ground strips212-213has been enlarged to achieve the same result. In one embodiment, the length of second section602is shorter than or equal to a quarter wavelength (wavelength/4) associated with an operating frequency of a corresponding RF transceiver. The narrowed strip width of the section602is configured dependent upon the amount of mismatched impedance.

Alternatively, according to another embodiment as shown inFIG. 6B, the gap between signal strip211and ground strips212-213can also be increased by cutting out a cavity on ground strips212-213. Referring toFIG. 6B, in this example, cavity611is cut out from ground strip212and cavity612is cut out from ground strip213. Cavities611-612are configured symmetrically on both sides of signal strip211. In one embodiment, the width of cavities611-612are shorter than or equal to a quarter of wavelength (wavelength/4) associated with an operating frequency of a corresponding RF transceiver.

According to another embodiment, when the Smith chart shows that the transmission line impedance is a type of inductive impedance as shown inFIG. 7, capacitive impedance compensation may be employed to compensate the transmission line impedance to match a predetermined impedance (e.g., 50 ohms). As shown inFIG. 7, the point D ends at the upper half of the Smith chart, which indicates the impedance on the transmission line is inductive impedance (e.g., high impedance).

Referring now toFIG. 8A, in one embodiment, a portion of signal strip211has been configured wider than the remaining strip to lower the overall impedance on the signal strip, which in turn compensates the high transmission line impedance. In this example, signal strip211includes a first section801(e.g., connector pad), a second section802, and a third section803coupled to an electronic component disposed on substrate104(not shown). The width of second section802is wider than the width of first section801and/or third section803. The specific wider width is configured dependent upon the mismatched overall impedance. In one embodiment, the length of second section802is short than or equal to a quarter wavelength (wavelength/4) corresponding to an operating frequency of the associated RF transceiver. By widening the signal strip211, the impedance on signal strip211has been reduced. It in turn also narrows the gap between signal strip and ground strips212-213.

Alternative, according to another embodiment as shown inFIG. 8B, each of the ground strips212-213includes a cavity cut out from the strip. In addition, the wider section802of signal strip is further widened to form a first wing portion802A and a second wing portion802B. The wing portions802A-802B extend into at least a portion of the cavities formed from ground strips212-213, without electrically contacting the ground strips212-213.