PRESSURE SENSOR DEVICE AND ASSEMBLY METHOD

A semiconductor sensor device is assembled using a pre-molded lead frame having first and second die flags. The first die flag includes a cavity. A pressure sensor die (P-cell) is mounted within the cavity and a master control unit die (MCU) is mounted to the second flag. The P-cell and MCU are electrically connected to leads of the lead frame with bond wires. The die attach and wire bonding steps are each done in a single pass. A mold pin is placed over the P-cell and then the MCU is encapsulated with a mold compound. The mold pin is removed leaving a recess that is next filled with a gel material. Finally a lid is placed over the P-cell and gel material. The lid includes a hole that that exposes the gel-covered active region of the pressure sensor die to ambient atmospheric pressure outside the sensor device.

BACKGROUND OF THE INVENTION

The present invention relates generally to semiconductor sensor devices and, more particularly to a method of assembling a semiconductor pressure sensor device.

Semiconductor sensor devices, such as pressure sensors, are well known. Such devices use semiconductor pressure sensor dies. These dies are susceptible to mechanical damage during packaging and environmental damage when in use, and thus they must be carefully packaged. Further, pressure sensor dies, such as piezo resistive transducers (PRTs) and parameterized layout cells (P-cells), do not allow full encapsulation because that would impede their functionality.

FIG. 1(A)shows a cross-sectional side view of a conventional packaged semiconductor sensor device100having a metal lid104.FIG. 1(B)shows a perspective top view of the sensor device100partially assembled, andFIG. 1(C)shows a perspective top view of the lid104.

As shown inFIG. 1, a pressure sensor die (P-cell)106, acceleration-sensing die (G-cell)108, and master control unit die (MCU)110are mounted to a lead frame flag112, electrically connected to lead frame leads118with bond wires (not shown), and covered with a pressure-sensitive gel114, which enables the pressure of the ambient atmosphere to reach the pressure-sensitive active region on the top side of P-cell106, while protecting all of the dies106,108,110and the bond wires from mechanical damage during packaging and environmental damage (e.g., contamination and/or corrosion) when in use. The entire die/substrate assembly is encased in mold compound102and covered by the lid104, which has a vent hole116that exposes the gel-covered P-cell106to ambient atmospheric pressure outside the sensor device100.

One problem with the sensor device100is the high manufacturing cost due to the use of a pre-molded lead frame, the metal lid104, and the large volume of pressure-sensitive gel114. Accordingly, it would be advantageous to have a more economical way to package dies in semiconductor sensor devices.

DETAILED DESCRIPTION OF THE INVENTION

Detailed illustrative embodiments of the present disclosure are disclosed herein. However, specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments of the present disclosure. Embodiments of the present disclosure may be embodied in many alternative forms and should not be construed as limited to only the embodiments set forth herein. Further, the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments of the disclosure.

One embodiment of the disclosure is a method for manufacturing a semiconductor sensor device, and another embodiment is the resulting semiconductor sensor device. At least two dies, comprising (i) a pressure sensor die having a pressure-sensitive active region and (ii) at least one other die, are die-bonded to a lead frame. The at least two dies are wire-bonded to corresponding leads of the lead frame using bond wires. A mold pin is placed over the pressure sensor die and its bond wires. Mold compound is applied to encapsulate the at least one other die and its bond wires. The mold pin is removed leaving a recess in the mold compound surrounding the pressure sensor die and its bond wires. Pressure-sensitive gel is applied in the recess to cover the active region of the pressure sensor die and its bond wires.

Another embodiment of the disclosure is a semiconductor sensor device comprising (i) a pre-molded lead frame, (ii) two or more dies including a pressure sensor die and at least one other die mounted to the lead frame, (iii) bond wires electrically interconnecting the two or more dies and the lead frame, (iv) mold compound encapsulating the at least one other die and its associated bond wires, and (v) pressure-sensitive gel covering an active region of the pressure sensor die and its associated bond wires. At least one lead of the lead frame is wire bonded to both (i) the pressure sensor die and (ii) the at least one other die, and the pressure sensor die is located in a cavity of a flag of the lead frame.

FIGS. 2(A) and 2(B)respectively show a cross-sectional side view and a top plan view of a packaged semiconductor sensor device200in accordance with an embodiment of the disclosure. The exemplary configuration of sensor device200forms a no-leads type package such as a quad flat no-leads (QFN) package. Note that alternative embodiments are not limited to QFN packages, but can be implemented for other package types, such as (without limitation) ball grid array (BGA) packages, molded array packages (MAP), and quad flat pack (QFP) or other leaded packages.

Sensor device200includes a pressure sensor die202and an ASIC die204mounted to (e.g., physically attached and electrically coupled to) a pre-molded lead frame206, and an acceleration-sensing die208mounted to ASIC die204. Pressure sensor die (aka P-cell)202is designed to sense ambient atmospheric pressure, while acceleration-sensing die (aka G-cell)208is designed to sense gravity or acceleration in one, two, or all three axes, depending on the particular implementation. ASIC die204functions as the master control unit (MCU) for P-cell202and G-cell208by, for example, controlling the operations of and processing signals generated by those two sensor dies. ASIC die204is synonymously referred to herein as MCU204. Note that, in some embodiments, ASIC die204may implement both the functionality of an MCU and that of one or more other sensors, such as an acceleration-sensing G-cell, in which latter case, G-cell208may be omitted.

Pre-molded lead frame206comprises electrically conductive leads210embedded in an electrically insulating mold compound212. Lead210may be formed of copper, an alloy of copper, a copper plated iron/nickel alloy, plated aluminum, or the like. Often, copper leads are pre-plated first with a nickel base layer, then a palladium mid layer, and finally with a very thin, gold upper layer. Mold compound212may be an epoxy or other suitable material.

Conventional, electrically insulating die-attach adhesive224may be used to attach (i) P-cell202and MCU204to lead frame206and (ii) G-cell208to MCU204. Those skilled in the art will understand that suitable alternative means, such as die-attach tape, may be used to attach some or all of these dies. P-cell202, MCU204, and G-cell208are well known components of semiconductor sensor devices and thus detailed descriptions thereof are not necessary for a complete understanding of the disclosure.

The electrical interconnection between P-cell202and MCU204is provided via one or more shared lead(s)210A of lead frame206by respective, associated bond wires214wire-bonded between (i) bond pads on P-cell202and MCU204and (ii) lead(s)210A using a suitable, known wire-bonding process and suitable, known wire-bonding equipment. Similarly, the electrical interconnection between MCU204and G-cell208is provided by wire-bonding between other bond pads on MCU204and bond pads on G-cell208. Furthermore, the electrical interconnection between MCU204and the outside world is provided via one or more lead(s)210B of lead frame206by bond wires214wire-bonded between still other pads on MCU204and lead(s)210B. Bond wires214are formed from a conductive material such as aluminium, gold, or copper, and may be either coated or uncoated. Note that, in alternative designs, G-cell208can be electrically connected to MCU204using suitable flip-chip, solder-bump techniques instead of or in addition to wire-bonding.

MCU204, G-cell208, and their associated bond wires214are encapsulated within a suitable mold compound216. The mold compound may be a plastic, an epoxy, a silica-filled resin, a ceramic, a halide-free material, the like, or combinations thereof, as is known in the art.

A pressure-sensitive gel material218, such as a silicon-based gel, is deposited over P-cell214and its associated bond wires214, filling most of the recess formed in mold compound216around P-cell214. Note that, in alternative implementations, less of gel material218may be applied within the recess as long as the pressure-sensitive active region (typically on the top side) of P-cell214and its associated bond wires are covered by the gel. Pressure-sensitive gel material218enables the pressure of the ambient atmosphere to reach the active region of P-cell202, while protecting P-cell202and its associated bond wires214from (i) mechanical damage during packaging and (ii) environmental damage (e.g., contamination and/or corrosion) when in use. Examples of suitable pressure-sensitive gel material218are available from Dow Corning Corporation of Midland, Mich. The gel material may be dispensed with a nozzle of a conventional dispensing machine, as is known in the art.

A lid220having an opening or vent hole222is mounted over the gel-covered P-cell202fitting snugly into a seat formed within mold compound216, thereby providing a protective cover for the P-cell. Vent hole222allows the ambient atmospheric pressure immediately outside sensor device200to reach (i) the pressure-sensitive gel material218and therethrough (ii) the active region of P-cell202. Although shown centered inFIG. 2, vent hole222can be located anywhere within the area of lid220. Vent hole222may be (pre-)formed in the lid by an suitable fabrication process such as drilling or punching.

Lid220is formed of a durable and stiff material, such as stainless steel, plated metal, or polymer, so that P-cell202is protected. Lid220is sized and shaped depending on the size and shape of P-cell202mounted to the lead frame under the lid. Accordingly, depending on the implementation, the lid may have any suitable shape, such as round, square, or rectangular.

Sensor device200can be manufactured with less cost than comparable sensor devices, like those based on the conventional design of sensor device100ofFIG. 1, because sensor device200can be manufactured with a smaller lid and with less pressure-sensitive gel.

FIGS. 3-7illustrate one possible process for manufacturing sensor device200ofFIG. 2.

In particular,FIGS. 3(A),3(B), and3(C) respectively show a cross-sectional side view, a top plan view, and a three-dimensional (3D) perspective view of pre-molded lead frame206having electrically conductive leads210embedded in electrically insulating mold compound212. Lead frame206also has a shallow recess302for receiving P-cell202ofFIG. 2. The purpose of recess302is to prevent the die-attach material (e.g.,224inFIG. 2) from flowing out to the wire-bonding area of leads210.

FIGS. 4(A) and 4(B)respectively show a cross-sectional side view and a top plan view of (i) P-cell202and MCU204mounted on and wire-bonded to lead frame206ofFIG. 3and (ii) G-cell208mounted on and wire-bonded to MCU204. Note that the attachment or die-bonding of all of P-cell202, MCU204, and G-cell204can be achieved in a single die-bonding process step that includes the curing of the epoxy or other substance (e.g., die-attach tape) used to mount all of those dies in a single pass through a curing cycle (e.g., comprising heating and/or UV irradiation). Furthermore, (i) P-cell202and MCU204can be electrically connected to lead frame206and (ii) G-cell208can be electrically connected to MCU204all in a single pass though a wire-bonding cycle (or in a single wire-bonding process step).

FIGS. 5(A) and 5(B)respectively show a cross-sectional side view and a partial X-ray, top plan view of the sub-assembly ofFIG. 4with mold pin502placed over P-cell202. Mold pin502comprises (i) a lower portion504defining a cavity506that accommodates P-cell202as well as its associated bond wires214and (ii) an upper portion508whose outer dimensions are slightly larger than the outer dimensions of lower portion504. InFIG. 5(B), the outline labeled504represents the periphery of the lower portion of mold pin502resting on lead frame206. Note that the existence of the larger, upper portion508is optional.

FIGS. 6(A) and 6(B)respectively show a cross-sectional side view and a partial X-ray, top plan view of the sub-assembly ofFIG. 5after the addition of mold compound216to encapsulate everything in the sub-assembly ofFIG. 5that is outside of the cavity defined by mold pin502. One way of applying the mold compound216is using a mold insert of a conventional injection-molding machine, as is known in the art. The molding material is typically applied as a liquid polymer, which is then heated to form a solid by curing in a UV or ambient atmosphere. The molding material can also be a solid that is heated to form a liquid for application and then cooled to form a solid mold. Subsequently, an oven is used to cure the molding material to complete the cross linking of the polymer. In alternative embodiments, other encapsulating processes may be used. The mold pin502prevents mold compound216from seeping inside the cavity506and reaching the P-cell202.

In the implementation shown inFIG. 6, the mold compound216is applied to a height that is slightly higher than the lower portion504of the mold pin502, such that the mold compound216extends past the bottom of the upper portion508of the mold pin502. After encapsulation, the mold pin502is removed from the sub-assembly ofFIG. 6, leaving behind a recess or cavity within the mold compound216surrounding the P-cell506and its associated bond wires214.

FIGS. 7(A) and 7(B)respectively show a cross-sectional side view and a partial X-ray, top plan view of the sub-assembly ofFIG. 6after the removal of the mold pin502and after the subsequent addition of pressure-sensitive gel material218, which covers the P-cell202and its associated bond wires214. In the implementation shown inFIG. 7, the gel material218is applied up to the top of the bottom, smaller-dimensioned portion of the recess formed by the lower portion504of the mold pin502, leaving the top, larger-dimensioned portion of the recess formed by the upper portion508of the mold pin502unfilled.

Referring again toFIG. 2, after the application of gel material218, the lid220is mounted over the P-cell202and gel material218to form the final assembly of the sensor device200. Note that the upper portion508of the mold pin502has substantially the same outer dimensions as the lid220so that the lid220fits snugly within the seat formed in mold compound216by the upper portion508. The lid220preferably lies flush with a top (outer) surface of the mold compound216. Note that, for implementations in which the mold pin502does not have a larger-dimensioned, upper portion, but rather only a single-dimensioned portion, the lid220is fabricated to allow it to be press-fit into the recess formed over the P-cell202.

The shapes of the leads210of the lead frame206, specifically lead(s)210A, enable the indirect electrical interconnection of the P-cell202and MCU204by wire bonding both dies202,204to one or more shared lead(s)210A. This lead sharing, in turn, allows the mold pin502to be placed over the P-cell202in a way that does not impinge on either the bond wires214connecting the P-cell202to shared lead(s)210A or the bond wires214connecting the MCU204to the same shared lead(s)210A. In this way, the bond wires associated with the MCU204can be encapsulated by the mold compound216, while the bond wires associated with the P-cell202are covered with the gel material218. These features enable the sensor device200to be manufactured with only a single die-bonding cycle and only a single wire-bonding cycle.

Although not depicted in the drawings, in practice, a plurality of sensor devices are formed simultaneously by using a lead frame sheet that has a two-dimensional array of the lead frames, and then the die bonding and wire bonding steps are performed on all of the lead frames in the array. Similarly, all of the separate devices are encapsulated with the molding compound at the same time too. After assembly, e.g., using the process depicted inFIGS. 3-7, the multiple sensor devices are separated, e.g., in a singulation process involving a saw or laser, to form individual instances of the sensor device200.

As used herein, the term “mounted to” as in “a first die mounted to a lead frame” covers situations in which the first die is mounted directly to the lead frame with no other intervening dies (as in the mounting of P-cell202to lead frame206inFIG. 2) as well as situations in which the first die is directly mounted to another die, which is itself mounted directly to the lead frame (as in the mounting of G-cell208to lead frame206via MCU204inFIG. 2). Note that “mounted to” also covers situations in which there are two or more intervening dies between the first die and lead frame.

AlthoughFIG. 2shows sensor devices200having a P-cell and a G-cell, those skilled in the art will understand that, in alternative embodiments, the G-cell and its corresponding bond wires may be omitted.

AlthoughFIG. 2shows an embodiment in which a G-cell is mounted to the MCU with the electrical interconnection provided by wire-bonding, those skilled in the art will understand that the electrical interconnection between such dies can, alternatively or additionally, be provided by appropriate flip-chip assembly techniques. According to these techniques, two semiconductor dies are electrically interconnected through flip-chip bumps attached to one of the semiconductor dies. The flip-chip bumps may include solder bumps, gold balls, molded studs, or combinations thereof. The bumps may be formed or placed on a semiconductor die using known techniques such as evaporation, electroplating, printing, jetting, stud bumping, and direct placement. The semiconductor die is flipped, and the bumps are aligned with corresponding contact pads of the other die.

By now it should be appreciated that there has been provided an improved packaged semiconductor sensor device and a method of forming the improved packaged semiconductor sensor device. Circuit details are not disclosed because knowledge thereof is not required for a complete understanding of the invention.

Although the invention has been described using relative terms such as “front,” “back,” “top,” “bottom,” “over,” “above,” “under” and the like in the description and in the claims, such terms are used for descriptive purposes and not necessarily for describing permanent relative positions. It is understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the disclosure described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein.

The embodiments covered by the claims in this application are limited to embodiments that (1) are enabled by this specification and (2) correspond to statutory subject matter. Non enabled embodiments and embodiments that correspond to non statutory subject matter are explicitly disclaimed even if they fall within the scope of the claims.