Electronic device

Abstract

Electronic device (100) comprising: - a carrier board (1) with an upper surface (11), - an electronic chip (2) mounted on the upper surface of the carrier board, the electronic chip having a mounting side (21) facing the upper surface of the carrier board, a top surface (22) facing away from the upper surface of the carrier board, and side walls (23) connecting the mounting side to the top surface, wherein - the electronic chip has on the mounting side equal to or less than 5 stud bumps (24) per square millimeter of the base area of ​​the mounting side, - a laminated polymer cover (4) that at least partially covers the top surface of the electronic chip and extends onto the upper surface of the carrier board, and - the carrier board having at least one recess in its upper surface.wherein at least one stud bump extends into the depression with a projection and has a form-fitting profile, the projection being designed as a hook or a holder or being shaped into a mushroom form.

Description

Embodiments of the present invention relate to an electronic device, in particular an electronic device, comprising a carrier and an electronic chip mounted on the carrier. The electronic device may be, for example, a sensor device including a microelectromechanical system (MEMS) chip. The electronic device may be, for example, a MEMS microphone.MEMS microphones use microstructured, mostly silicon-based acoustoelectric transducers. These high sensitive sensor chips employ thin movable diaphragms, which also makes them very sensitive to mechanical stress from their placement in a protective housing. The most common way to keep such stress at a low level is to use conventional die attach with a soft and thick bond joint. In this case, the membrane and thus the electrical contacts are positioned upright and the internal electrical interconnections to the package and / or to a complementary ASIC (application specific integrated circuit) are performed by wire bonding. This leads to two major drawbacks: in the case of the dominant bottom opening configuration, the internal MEMS cavity cannot contribute to the (useful) acoustic back volume, but sums to the (parasitic) front volume, both of which degrade the performance of the microphone. Moreover, the bond wires require a lot of lateral space (for the substrate landings) and clearance (for the wire loop and a safety distance to the cap), both of which increase the dimensions of the component, which affects the general miniaturization efforts.Flip-chip placement on solder bumps is a general trend in microelectronics that could provide a solution to both problems. But in this case, the sensor chip is in turn firmly coupled to the package substrate, making it prone to be affected by static device stress, offset from reflow of solder, temperature induced stress due to CTE discrepancy between sensor and package materials, and dynamic stress from external influences.Thus, for example, for the microphone application described, a packaging solution for a bottom port microphone that uses an internal flip chip arrangement for the MEMS sensor without requiring adverse stress effects is needed.US 2012 / 0 093 346 A describes a microphone chip having contact bumps, by means of which the chip is fastened on a substrate. A cover sheet is laminated to the chip.US 2012 / 0 039 056 A1 describes a component arrangement having a first substrate and a second substrate, which can be an electronic component or element and which has contact elements by means of which the second substrate is fastened to contact bumps of the first substrate.The document DE 10 2013 106 353 A1 describes a method for producing a device having a MEMS component which is mounted on a carrier via soldered connections such as, for example, stud bumps. A film cover covers a part of the component and a part of the carrier plate.Mounted electronic chips are likewise known from the publications JP 2009-267 067 A and U.S. Pat. No. 2015 / 0 091 160 A1.An object to be solved is to provide an electronic device which avoids or at least reduces the above mentioned problems.This object is achieved, among other things, by an electronic device according to independent claim 1. Further embodiments and configurations are subject of the dependent claims.According to at least one embodiment, an electronic device includes a carrier board having a top surface. An electronic chip is mounted on the upper surface of the carrier board. The electronic chip may be a sensor chip, for example. In a preferred embodiment, the electronic chip may be a MEMS chip.The carrier board may preferably provide electrical connections for connecting the electronic chip. In particular, the carrier board may be a laminated multilayer board that may be based on HTCC (high temperature burn-in ceramic), LTCC (low temperature burn-in ceramic), polymeric materials, and / or glass. Conductive traces and / or vias may provide electrical routing between the elements mounted on the carrier board and external solder pads.The electronic chip may have a mounting side facing the top surface of the carrier board. In particular, the chip may comprise connection elements on the mounting side, wherein the connection elements are configured for electrically connecting and mounting the chip on the upper surface of the carrier board. Furthermore, the electronic chip may have a top side facing away from the top surface of the carrier board and side walls connecting the mounting side to the top side.According to a further embodiment, the electronic chip has stud bumps as connecting elements on the fastening side. The stud bumps may be located on electrode pads of the electronic chip, for example. To create a stud bump, a ball is formed using a standard ball jumping process. After the ball is bonded to the electrode pad, wire terminals are closed and the bonding head is moved to cut the wire. If the movement is directed substantially perpendicular to the attachment side, a "tail" may be left on the bump. Depending on the movement parameters, the width and the length of the tail and thus the size and shape of the stud bump can be adjusted. Each of the stud bumps may form an electromechanical connector that may combine inherent flexibility, low profile, and small footprint.The mounting side may be bounded by the chip outline, such that the area enclosed by the chip outline may be referred to as a footprint of the electronic chip. In particular, the electronic chip may preferably have equal to or less than 5 stud bumps per square millimeter of a base area of the mounting side. Simply stated, the electronic chip may be equipped with only a few stud bumps, for example equal to or less than 10 stud bumps. By way of example, a square-shaped or rectangular chip may have four stud bumps, each located in one of the corners of the square-shaped or rectangular base. Furthermore, for example, two stud bumps may be applied in each of the corners, resulting in an electronic chip with eight stud bumps.According to a further embodiment, the stud bumps are connected to the upper surface of the carrier by means of an electrically conductive adhesive. The conductive adhesive may be applied by dipping the stud bumps into a "wet" controlled thickness adhesive layer before the electronic chip is placed on the carrier board. Furthermore, the deposition of the conductive adhesive by printing, jetting or dispensing can provide higher volumes with better reproducibility. Advantageously, the use of the conductive adhesive provides a moderate temperature profile. While lead-free solder bumps typically require temperatures of about 250° C. for reflow processes, typical conductive adhesives cure at about 150° C. or less, and even temperatures of about 60° C. are possible. Therefore, only a low initial stress level is introduced during chip mounting.The conductive adhesive may be a thermoplastic or thermosetting resin comprising silicone and / or acrylate and / or epoxy and / or other resin materials filled with electrically conductive particles. Typically, Ag and / or Pd may be used. In the case of MEMS microphones or other capacitive sensors, a relatively high connection resistance of up to a kOhm range may be tolerable. In this case, other filling materials such as carbon may be possible as well. Additionally, inherently conductive polymers may be selected as a conductive adhesive.According to a further embodiment, the carrier board has at least one depression in the upper surface, wherein at least one stud bump extends into the depression. In particular, the tail of the stud bump can be accommodated in the depression. Preferably, the carrier board has a recess for each of the stud bumps. The one or more recesses may be filled by the conductive adhesive. Furthermore, at least one or more or all of the stud bumps have a positive profile. Such a form-fitting profile of a stud bump and in particular of its tail can provide stronger anchoring in the relatively soft conductive adhesive. For example, the tail may be bent at the end or may be formed as a hook or a holder. In addition, it is possible to make the tail into a mushroom shape.Preferably, at least one or more or all of the stud bumps have a height of equal to or greater than 30 μm and equal to or less than 180 μm. The height can in particular include the tail and an optional portion which is accommodated in a depression of the carrier board. The stud bumps may be made of, for example, a gold wire having a diameter of equal to or larger than 12 μm and equal to or smaller than 30 μm. Such stud bumps may be fabricated by a modified wirebonder as discussed above and may be placed directly on electrode pads such as Al pads at high rates of up to ~30 / s and do not require costly UBM (solderable metallization) processing, which is often undesirable in connection with MEMS wafer processes, for example.According to another embodiment, the electronic device includes a laminated polymer cover. The polymer cover at least partially covers the top surface of the electronic chip and extends onto the top surface of the carrier board. This may mean, in particular, that the polymer covering is firmly bonded, for example by an adhesive force, to a part of the upper surface. The polymer cover may be configured to mechanically hold the electronic chip on the carrier board.Furthermore, the polymer cover may cover a majority of the sidewalls of the electronic chip. In the case of a square or rectangular chip, the polymer cover may preferably cover at least three of the four side walls of the electronic chip.The polymer cover may be manufactured by at least partially laminating a polymer film on the electronic chip mounted on the carrier board and partially on the upper surface of the carrier board. The lamination process may preferably be based on a heat assisted deep drawing process driven by an air pressure differential above and below the foil. The polymeric material of the laminated film and thus the polymeric cover can be selected from a wide variety of thermoplastic or thermosetting polymers. Particularly preferably, the polymer cover comprises a B-state material, which is known to a person skilled in the art and combines a processability under thermoplastic conditions with a transformation to a thermosetting state during a final thermo- and / or radiation curing. Preferably, the polymer cover has a thickness of equal to or greater than 10 μm and equal to or less than 80 μm. A preferred thickness may be about 20 μm. In order not to impair the flexibility of the assembly, i.e., the electronic device, the polymer cover, i.e., the laminated film in the cured and finalized state constituting the polymer cover, preferably has an elastic modulus equal to or less than 1 GPa at room temperature.According to a further embodiment, the mounting side of the electronic chip is at a distance from the top surface of the carrier board, thereby defining a clearance between the mounting side and the top surface. The height of the clearance may be determined substantially by the height of the stud bumps minus the height of the portion of the stud bumps that is in a recess, if any. In particular, the free space can be free of underfill material. In other words, the electronic chip may be fixed and fixed on the carrier plate only by means of stud bumps with the conductive adhesive and the polymer cover, wherein any other underfill material, such as adhesive materials often applied between a chip and a substrate, may be avoided.Further features, advantages and functionalities will become apparent from the following description of the exemplary embodiments in conjunction with the figures. FIG. 1 illustrates an electronic device according to an embodiment, FIG. 2 illustrates an electronic device according to another embodiment, FIGS. 3A and 3B illustrate further features of an electronic device according to further embodiments, FIG. 4 illustrates a detail of an electronic device according to another embodiment; and FIGS. 5A and 5C illustrate details of stud bumps of an electronic device according to further embodiments.In the figures, elements of the same design and / or function are identified by the same reference numerals. It is understood that the embodiments depicted in the figures are illustrative representations and are not necessarily drawn to scale.FIG. 1 illustrates an electronic device 100 according to an embodiment. The electronic device 100 is formed as an electronic package and comprises a carrier board 1 having an upper surface 11 on which an electronic chip 2 is mounted and a lower surface 12 which is remote from the upper surface 11 and on which contacts are provided for external contacting with the electronic device 100. The electronic chip 2 may be, for example, a sensor chip and preferably a MEMS chip, such as a MEMS microphone. Although the following description refers to such an application, the present invention is not limited to MEMS microphones, but may also be useful for other types of voltage sensitive sensors as well as electronic components in general.A laminated multilayer board based on, for example, HTCC, LTCC, polymer, or glass serves as the carrier board 1 and thus as a package substrate. Conductive traces and vias provide electrical routing between the elements and the external solder pads. Ground planes may improve the electromagnetic shielding in combination with a cap 3, preferably a metal cap, attached to the carrier board 1 by means of a suitable connection material 30, for example a solder or a conductive adhesive.The electronic chip 2 has a mounting side 21 facing the top surface 11 of the carrier board 1. Furthermore, the electronic chip 2 has a top side 22 facing away from the top surface 11 and side walls 23 connecting the mounting side 21 to the top side 22. The electronic chip 2, which is configured as a MEMS microphone chip as explained above, is mounted on the support plate 1 such that a diaphragm and a back plate of the chip 2 face an opening 13 in the support board 1, which serves as a sound opening. As shown in FIG. 1, a barrier layer 14 may be deposited as an additional and optional element on the opening 13. The barrier layer 14 may be a particle and / or moisture barrier that can seal the opening 13 to prevent ingress of dust or water. In particular, the optional barrier layer 14 can be formed by a network. Other embodiments include structures attached to the electronic chip 2 rather than the carrier board 1. Alternatively, no barrier layer 14 may be present.For acoustic reasons, the sound opening on the uppermost carrier board layer should be as wide as possible. This drastically restricts the remaining area under the electronic chip 2. Therefore, the electronic chip 2 comprises stud bumps 24 on electrode pads, wherein the stud bumps 24 form electromechanical connection elements with inherent flexibility, low profile and small footprint. Each of the stud bumps 24 has a tail on a bump and is at least partially accommodated in a drop of conductive adhesive 5 on the top surface 11 of the carrier board 1, thereby providing voltage decoupling between the carrier board 1 and the electronic chip 2. Preferred heights of stud bumps including the tail and an optional portion housed in a recess as explained below are between 30 μm and 180 μm, including the limits. The stud bumps 24 may be formed using an Au wire having a diameter of equal to or larger than 12 μm and equal to or smaller than 30 μm. Stud bumps 24 are made by a modified wire bonding apparatus. They can be placed directly on Al pads at high rates (up to ~30 / s) and do not require costly UBM processing, which is often undesirable in MEMS wafer processes.All surfaces relevant to good electrical contact at the interface to the conductive adhesive 5 are preferably coated with a noble metal, for example Au, Pt, Pd, Ag or alloys thereof, in order to provide a low and stable resistance. The conductive adhesive 5 may be a thermoplastic or thermosetting resin, such as silicone, acrylate, epoxy or other suitable resin filled with conductive particles, for example Ag and / or Pd particles, or, in the case of MEMS microphones or other capacitive sensors, with other filler materials, such as carbon. Additionally, inherently conductive polymers are an alternative. The conductive adhesive 5 may be applied by dipping the stud bumps 24 in a "wet" layer of controlled thickness before the electronic chip 2 is placed on the carrier board 1. In addition, a deposition by printing, jetting or dispensing may be used that can provide a higher volume with better reproducibility.The desired mechanical properties of the cured adhesive 5 depend widely on parameters such as expected stress level and expected stress sensitivity, CTE (coefficient of thermal expansion) discrepancy of materials involved, number of stud bumps per chip, height of stud bumps, and others. In many configurations, an elastic modulus of equal to or less than 500 MPa, or preferably equal to or less than 100 MPa, is suitable for the adhesive 5. However, even a low performance substrate material having a CTE of up to 14 ppm / K may be used.Due to stud bumps 24, problems of other connection methods can be avoided. For example, it is practically impossible to implement metal spring elements for implementing the required voltage decoupling. In such a case, conventional chip solder bumps would be attached to the cantilevered terminals of the spring elements while the other terminals of the spring elements are fixed to the carrier board contacts. The gap height between the mounting surface of the chip and the top surface of the carrier board is limited to about 100 μm, which does not allow for complex spring designs in the vertical direction. Furthermore, it could not be avoided that parts of the spring structures would protrude from the chip outline, so that they would be obstructed by any means used for separating the front and rear volumes. Furthermore, conventional flip-chip underfilling would be even worse in this respect. Of course, a full underfill would be prohibitive for a MEMS microphone, but even an underfill in the form of a dam along the chip edges would completely block the springs.Compared to conventional solder bumps, the stud bumps 24 form interconnects that show a lower bond strength, in particular in the case of a low number of interconnects per chip, as given for a MEMS microphone and explained below. However, boosting using an underfill process cannot be used for MEMS microphones, as flow and dissipation to functional parts during application of the underfill material are detrimental. Thus, the clearance 6 between the mounting side of the electronic chip and the top surface of the carrier board is free of any underfill material. Instead of using an underfill, the electronic device 100 includes a laminated polymer cover 4. The polymer cover at least partially covers the top surface 22 of the electronic chip 2 and extends onto the top surface 11 of the carrier board 1. in particular, the polymer cover 4 is firmly bonded to a part of the top surface 11 by adhesion, so that the laminated polymer cover 4 mechanically holds and supports the electronic chip 2. In the illustrated embodiment, the polymer cover 4 mechanically holds and supports the electronic chip 2 on all the side walls 23 of the chip 2. especially preferably, the polymer cover 4 includes a B-state material that combines processability under thermoplastic conditions with transformation to a thermosetting state during final thermo- and / or radiation curing, and has a thickness of equal to or greater than 10 μm and equal to or less than 80 μm. A preferred thickness may be about 20 μm. In order not to impair the flexibility of the electronic device 100, the polymer cover 4, i.e., the laminated film in the cured and finalized state constituting the polymer cover 4, preferably has an elastic modulus equal to or less than 1 GPa at room temperature.The lamination process is based on a heat assisted deep drawing driven by an air pressure difference above and below a polymer film. As a result, the only effective force at the stud bumps 24 is directed straight downward, i.e., toward the top surface 11 of the carrier board 1, this being its only elastic loading direction. The reason for this is that the tails of the stud bumps 24 may be located on the carrier board 1, thereby providing a mechanical stopper. Any other load scenarios, for example, forces caused by bending, shearing, and / or pulling, which could easily destroy the stud-bump bond, may be handled by the polymer cover 4 after the curing process. An additional advantage of this arrangement is its moderate temperature profile. While lead-free solder bumps require about 250° C. for reflow processes, typical conductive adhesives cure at about 150° C., with about 60° C. also being possible. Therefore, a low initial stress level is introduced in the mounting of the chip 2.The polymer cover 4 may also provide a separation between the front and back volumes of the electronic chip 2. In the case of the illustrated MEMS microphone, the polymer cover 4 may have an opening 40 in the area of the MEMS cavity as a connection to the extended back volume provided by the cap 3.FIG. 2 illustrates another embodiment of an electronic device 100 additionally comprising an ASIC 7 mounted on the carrier board 1, in contrast to the previous embodiment. The ASIC 7 may support and control the function of the electronic chip 2 and thus the electronic device 100. The ASIC 7 may be mounted next to the electronic chip 2 under the polymer cover 4 as shown in FIG. 2 or outside thereof. It may use the described connection elements in the form of stud bumps or any other attachment and connection method, for example flip-chip mounting with solder balls, conventional die mounting with wire bonding or others. In the illustrated embodiment, even in the presence of the ASIC 7 under the polymer cover 4, the polymer cover 4 covers at least three of the four side walls 23 of the electronic chip 2, thereby mechanically holding the electronic chip 2 sufficiently.In the embodiments of FIGS. 1 and 2, the electronic chip 2 has equal to or less than 5 stud bumps per square millimeter of the base area of the mounting side. Taking into account typical sizes of MEMS chips, this can mean, in particular, that the electronic chip 2 has stud bumps of equal to or less than 10. FIGS. 3A and 3B illustrate two exemplary embodiments for the layout of the electronic device 100. In both figures, the outline 25 of the electronic chip, which for example has a square shape, is illustrated by the dashed line. Also shown are the positions of stud bumps 24, conductive adhesive 5 and (optional) barrier layer 14 covering the opening in the carrier board. FIG. 3A illustrates a layout with four stud bumps 24 forced into the corners of the available field, constrained by the electronic chip outline 25, to lose as little of the area for the sound opening 13 and the barrier layer 14 as possible. The respective outermost circle around stud bumps 24 indicates the size of conductive adhesive 5 on the carrier board, which preferably does not contact the polymer cover to minimize possible leakage currents in this ultra high impedance circuit. FIG. 3B illustrates another embodiment with two stud bumps 24 forming a connection in each of the corners to strengthen the connection. More than two stud bumps are also possible in each of the corners.FIG. 4 illustrates a detail of an electronic device according to another embodiment. In this embodiment, the carrier board 1 has at least one depression 15 in which at least one stud bump 24 or at least its elongate tail wire is accommodated. The recess 15 is filled with the conductive adhesive 5. Alternatively to the illustrated embodiment, the depression 15 can be created by other means, for example by structures on the upper surface of the carrier board 1, in particular solder mask patterns.An additional improvement can be achieved if at least one or more or all of the stud bumps and in particular their extensions have a form-fitting profile for stronger anchoring in the relatively soft conductive adhesive. Exemplary embodiments for form-fitting profiles are illustrated in FIGS. 5A to 5C. For example, the tail may bend at the end as shown in FIG. 5A, or may be formed as a hook or a retainer as shown in FIG. 5B. Alternatively, embossing the tail into a mushroom shape as shown in Figure 5C is a preferred possibility.Alternatively or in addition to the features described in connection with the figures, the embodiments illustrated in the figures may have further features which are described in the general part of the description. Moreover, features and embodiments of the figures may be combined with one another even if such a combination is not expressly described.The invention is not limited by the description based on the exemplary embodiments. Instead, the invention includes any novel feature and also any combination of features, which in particular comprises any combination of features in the claims, even if this feature or this combination itself is not expressly specified in the claims or embodiments.List of reference symbols used1 Carrier board 2 Electronic chip 3 Cap 4 Polymer cover 5 Conductive adhesive 6 Clearance 7 ASIC 11 Upper surface 12 Lower surface 13 Opening 14 Barrier layer 15 Depression 21 Fastening side 22 Upper side 23 Side wall 24 Stud bump 25 Outline 30 Bonding material 40 Opening 100 Electronic device

Claims

An electronic device (100) comprising: - a carrier board (1) having a top surface (11), - an electronic chip (2) mounted on the top surface of the carrier board, the electronic chip having a mounting side (21) facing the top surface of the carrier board, a top side (22) facing away from the top surface of the carrier board, and side walls (23) connecting the mounting side to the top side, wherein - the electronic chip on the mounting side has equal to or less than 5 stud bumps (24) per square millimeter of the base area of the mounting side, - a laminated polymer cover (4) at least partially covering the top surface of the electronic chip and extending onto the top surface of the carrier board, and - wherein the carrier board has at least one depression in the top surface, wherein at least one stud bump extends with a tail into the depression and has a positive profile, wherein the tail is designed as a hook or a holder or is formed into a mushroom shape.The electronic device of claim 1, wherein the polymer cover covers at least three side walls of the electronic chip.The electronic device of any preceding claim, wherein the stud bumps are connected to the top surface of the carrier by a conductive adhesive.Electronic device according to the preceding claim, wherein the conductive adhesive has a modulus of elasticity equal to or less than 500 MPa, and preferably equal to or less than 100 MPa.The electronic device of any preceding claim, wherein the mounting side of the electronic chip is spaced from the top surface of the carrier board, thereby defining a clearance between the mounting side and the top surface, the clearance being free of any underfill material.The electronic device of any one of the preceding claims, wherein at least one of the stud bumps has a height equal to or greater than 30 μm and equal to or less than 180 μm.The electronic device of any preceding claim, wherein the polymer cover comprises a B-state material.The electronic device of any preceding claim, wherein the polymer cover has a thickness equal to or greater than 10 μm and equal to or less than 80 μm.The electronic device of any preceding claim, wherein the polymer cover has an elastic modulus equal to or less than 1 GPa at room temperature.The electronic device of any preceding claim, wherein the electronic chip is a sensor chip.The electronic device of any preceding claim, wherein the electronic chip is a MEMS chip.

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