Single line axis solder dispense process for a MEMS device

A microphone assembly includes a substrate defining a port, a MEMS transducer, a guard ring, and a can. The MEMS transducer is coupled to the substrate such that the MEMS transducer is positioned over the port. The guard ring is coupled to the substrate and surrounds the MEMS transducer. The guard ring includes a plurality of edges that further includes a first edge and an opposing second edge. A portion of the first edge and a portion of the second edge have a reduced thickness relative to adjacent ones of the plurality of edges. The can is coupled to the guard ring such that the substrate and the can cooperatively define an interior cavity.

FIELD OF THE DISCLOSURE

The present disclosure relates to microelectromechanical systems (MEMS) devices, and in particular, to the manufacture of MEMS devices that include MEMS transducers.

BACKGROUND

Compact components are desirable when building high-performance, high-density devices such as mobile communication devices, portable music players, and other portable electronic devices. One solution for providing high quality, compact devices is to use microelectromechanical systems (MEMS).

For example, microphone assemblies for many portable electronic devices include MEMS acoustic transducers, which convert acoustic energy into an electrical signal. The MEMS acoustic transducer includes a silicon die that is mounted onto a printed circuit board (PCB) to form the microphone assembly. Although the silicon die is small, existing manufacturing processes for these microphone assemblies are limited in terms of the overall size of the silicon die that can be accommodated within a given microphone package.

SUMMARY

A first aspect of the present disclosure relates to a microphone assembly. The microphone assembly includes a substrate defining a port, a MEMS transducer, a guard ring, and a can. The MEMS transducer is coupled to the substrate such that the MEMS transducer is positioned over the port. The guard ring is coupled to the substrate and surrounds the MEMS transducer. The guard ring includes a plurality of edges that further includes a first edge and an opposing second edge. A portion of the first edge and a portion of the second edge have a reduced thickness relative to adjacent ones of the plurality of edges. The can is coupled to the guard ring such that the substrate and the can cooperatively define an interior cavity.

A second aspect of the present disclosure relates to a populated printed-circuit-board. The populated printed-circuit-board includes a substrate and a plurality of guard rings. The plurality of guard rings are coupled to the substrate. Each of the plurality of guard rings includes a plurality of edges. A first edge of the plurality of edges and an opposing second edge of the plurality of edges have a reduced thickness relative to adjacent ones of the plurality of edges.

A third aspect of the present disclosure is a method. The method includes providing a substrate defining a plurality of ports. The method also includes coupling a plurality of guard rings to the substrate such that each of the plurality of guard rings is positioned to surround a respective one of the plurality of ports. The method further includes coupling a plurality of MEMS transducers to the substrate, where each of the plurality of MEMS transducers is positioned within a periphery of a respective one of the plurality of guard rings and at least partially isolates a respective one of the plurality of ports. The method additionally includes applying a solder along a single axis between each of the plurality of guard rings such that the solder is applied approximately equally to adjacent ones of the plurality of guard rings. The method also includes coupling a plurality of cans to the plurality of guard rings to form a plurality of coupled microphone assemblies in which each of the plurality of cans is configured to enclose a respective one of the plurality of MEMS transducers. The method further includes separating each of the plurality of coupled microphone assemblies from the substrate to form a plurality of individual microphone assemblies.

In the following detailed description, various embodiments are described with reference to the appended drawings. The skilled person will understand that the accompanying drawings are schematic and simplified for clarity and therefore merely show details which are essential to the understanding of the disclosure, while other details have been left out. Like reference numerals refer to like elements or components throughout. Like elements or components will therefore not necessarily be described in detail with respect to each figure.

DETAILED DESCRIPTION

In general, disclosed herein is a microphone assembly that is produced using a single line axis solder dispensing process. The microphone assembly includes a MEMS acoustic transducer, an integrated circuit, a substrate, and a can. The MEMS acoustic transducer may be a capacitive acoustic transducer including a stationary back plate and a movable diaphragm, which are configured to convert acoustic energy incident on the diaphragm into an electrical signal. The MEMS acoustic transducer and the integrated circuit are coupled to the substrate. An electrically conductive guard ring is also coupled to the substrate and surrounds the MEMS transducer and the integrated circuit. The can is coupled to the guard ring by solder to at least partially acoustically and electrically isolate the MEMS acoustic transducer from an environment surrounding the microphone assembly. Traditionally, the solder is applied to the guard ring by a dispensing needle in a picture frame pattern that extends along the entire guard ring (e.g., along an entire perimeter of the can and microphone assembly, along two axes, etc.). The solder dispensing process requires the needle to have a minimum clearance away from the MEMS transducer to avoid solder contamination (e.g., wetting) of the surfaces of the MEMS transducer and/or damage to the MEMS transducer due to contact from the needle. This clearance requirement, between the needle and the MEMS transducer, limits the minimum size of the microphone assembly package that can be achieved for the MEMS transducer.

The embodiments disclosed herein can reduce the overall size of the microphone assembly that can be achieved for a MEMS transducer of fixed dimensions, without altering the size of the dispensing needle. In particular, microphone assemblies disclosed herein are produced by dispensing the solder along only two sides of the guard ring. In other words, the can for the microphone assembly is coupled to the guard ring by solder that extends along an entire length of only two sides of the can. A tag of epoxy or another adhesive product is applied to the remaining sides of the can to maintain an air-tight seal between the MEMS transducer and an environment surrounding the microphone assembly.

During production, a plurality of microphone assemblies may be formed onto a single substrate to form a populated PCB. As used herein, the term “coupled microphone assembly” refers to a microphone assembly that is connected to other microphone assemblies on the populated PCB. The coupled microphone assemblies may be arranged in rows along a length of the PCB. The coupled microphone assemblies may be aligned in both an X-axis direction and a Y-axis direction forming aligned rows and columns of coupled microphone assemblies. To prepare the coupled microphone assemblies for placement of the cans, the solder is dispensed along a single axis between the guard rings for each one of the coupled microphone assemblies. More specifically, the solder is dispensed along the single axis extending parallel to a longest edge of each one of the guard rings (e.g., the X-axis).

Among other benefits, the single axis solder dispensing process allows for an increase in the overall size (e.g., footprint) of the silicon die that is used within a microphone assembly without increasing the overall package size of the microphone assembly. The details of the general depiction provided above will be more fully explained by reference toFIGS. 1-17.

FIG. 1shows an individual microphone assembly, shown as assembly100, according to an illustrative embodiment. The assembly100includes a substrate102; and a cap, cover, or lid, shown as can104coupled to the substrate. The can104defines a continuous surface that is sized to surround and enclose all of the internal components of the assembly100. In the embodiment ofFIG. 1, the can104is made from a metal material (e.g., aluminum, titanium, steel, etc.). The can104is coupled to the substrate along a perimeter of the can104using solder106(e.g., a re-meltable conductive metal alloy, lead free solder, etc.) and an adhesive or sealant (e.g., an epoxy, etc.), shown as tag108. The solder106electrically connects the can104to conductive material that is embedded within or otherwise coupled to the substrate102. Additionally, the solder106at least partially acoustically isolates electrical components contained within the can104from an environment surrounding the microphone assembly100.

During production, a plurality of microphone assemblies may be formed on (e.g., populated) or otherwise coupled to a single substrate (e.g., a substrate blank, etc.). The substrate may be a printed-circuit-board (PCB) that includes printed circuit traces or pads to facilitate electrical connections between components of each one of the microphone assemblies.FIG. 2shows a top view of a coupled microphone assembly, shown as full frame assembly200that is produced using a dual axis solder dispense process (e.g., a conventional solder dispensing process). The full frame assembly200is shown before separation from a larger, populated PCB (e.g., a PCB that includes multiple, interconnected/coupled full frame assemblies200).FIG. 3shows a top view of the assembly100ofFIG. 1, which is made using the single-axis dispensing process.

As shown inFIG. 2, the full frame assembly200includes solder206that extends along an entire (e.g., full) perimeter of the can204. A thickness of the solder206is approximately uniform along the perimeter of the can204. In contrast, for the assembly100ofFIGS. 1 and 3, the solder106extends along an entire length of only two sides of the can104. More specifically, the solder106extends along an entire length of the two longest sides110of the can104. Additionally, the solder106wraps around each one of a plurality of corner regions112of the can104, which connect the longest sides110with short sides114that are arranged in substantially perpendicular orientation relative to the longest sides110.

As shown inFIGS. 1 and 3, a tag108is applied to each of the short sides114of the can104, between the can104and the substrate102. More specifically, the tags108are applied at a central position116along the short sides114, approximately half-way between the corner regions112bounding the ends of the short sides114. The tags108fill any gaps in solder106coverage along the perimeter of the can104to ensure an air-tight seal along the entire perimeter of the can104. In some embodiments, the tag108includes a material with a higher melting point than the solder106. In some embodiments, the tag108is and/or includes an epoxy. The epoxy may include a non-conductive epoxy (e.g., a resin-based alumina-filled epoxy, a resin-based silica-filled epoxy, etc.) and/or a conductive epoxy (e.g., a resin-based silver-filled epoxy, a resin-based nickel-filled epoxy, etc.). In other embodiments, the tag108includes another thermoplastic, polymide, adhesive, etc.

The size of the tag108and/or number of tags108used on each of the short sides114varies depending on the length of the short sides114, the amount of solder106used on the longest sides110and the peak time and temperature that the assembly100is exposed to during the solder reflow operation. In the embodiment ofFIGS. 1 and 3, the width of the assembly100is approximately 2.5 mm and after the reflow operation (as will be further described) the solder106covers approximately 1 mm on either side of the tag108. A single tag108in the embodiment ofFIGS. 1 and 3covers a distance of approximately 600 microns. Thus, only a single tag108is required to ensure an air-tight seal between the can104and the substrate102inFIGS. 1 and 3. In other embodiments, the number of tags108and/or the quantity of adhesive or sealant applied with each tag108may be different.

FIG. 4shows a top perspective view of a populated PCB300during a dispensing operation in which solder106is applied to the coupled microphone assemblies100by a dispensing needle302(e.g., before cans104are placed over each of the assemblies100). The flow rate of solder106passing through the dispensing needle302is approximately constant throughout the dispensing operation. Each of the assemblies100shown inFIG. 4includes a MEMS acoustic transducer, shown as MEMS transducer118; and an integrated circuit120. The MEMS transducer118is configured to convert acoustic energy into an electrical signal. The MEMS transducer118may include a movable diaphragm and a perforated back plate. Sound energy (e.g., sound waves, acoustic disturbances, etc.) incident on the diaphragm causes the diaphragm to move toward or away from the back plate. The change in distance results in a corresponding change in capacitance between conductive materials disposed on or within the diaphragm and the back plate. An electrical signal representative of the change in capacitance may be generated and transmitted to other portions of the microphone assembly, such as the integrated circuit, for processing. The integrated circuit may be an application specific integrated circuit (ASIC) or another type of semiconductor die integrating various analog, analog-to-digital, and/or digital circuits. In other embodiments, the MEMS transducer118may be another type of MEMS device now known or hereafter devised. For example, the MEMS transducer118may be a non-capacitive type MEMS device such as a piezoelectric transducer, a piezoresistive transducer, an optical transducer, etc.

The assemblies100are aligned with one another in both an X-axis direction306and a Y-axis direction308forming aligned rows and columns of coupled microphone assemblies100. As shown inFIG. 4, the longest side of each of the assemblies100is oriented parallel to the X-axis direction306, and the short side of each of the assemblies100is oriented parallel to the Y-axis direction308. In other embodiments, the orientation of the assemblies100may be reversed. As shown inFIG. 4, the solder106is applied along a single axis in one pass between each pair of assemblies100. More specifically, the solder106is applied along an X-axis direction306between adjacent microphone assemblies100, along the longest side of each of the assemblies100. A total of two passes of the dispensing needle302are shown inFIG. 4, forming a generally “U” shaped dispensing path310. Although only two passes of the dispensing needle302are depicted inFIG. 4, it will be appreciated that the dispensing pattern may repeat in as many passes as needed, snaking between adjacent rows of assemblies100, across the entirety of the PCB300.

FIG. 5shows a top view of the full frame assembly200after the solder206has been dispensed onto the PCB. As shown inFIG. 5, the solder206is dispensed in a rectangular-shaped ring for each full frame assembly200. The solder206is applied to each full frame assembly200on the PCB individually (e.g., independently from other full frame assemblies200). Due to the fixed flow rate of solder206through the dispensing needle302, the amount of solder206applied to full frame assembly200, along the rectangular dispensing path, is greatest in the corner regions212, where the needle302moves between adjacent sides of the full frame assembly200. In some instances, the buildup (e.g., piling) of solder206in the corner regions212can cause the solder206to slump or flow toward the sensitive electronic components within the full frame assembly200. A uniform minimum clearance is required on either side of the rectangular dispensing path to prevent the needle302from contacting any of the electronic components mounted to the PCB. This clearance, in part, sets a maximum value of the ratio of the silicon die size to microphone package size, or DP ratio, that can be achieved during production.

FIG. 6shows a top view of assembly100after the solder106has been dispensed onto the PCB300. Unlike the full frame assembly200ofFIG. 5, the solder106for the assembly100is applied along a single axis, and in one pass between each pair of assemblies100, which greatly reduces the amount of time required to apply the solder106to the PCB300. For example, the single axis dispensing process may improve the amount of units per hour (UPH) produced by approximately a factor of 2.75 or more as compared to a dual axis dispense process used for the full frame assembly200. A greater amount of solder is applied between the assemblies100to eliminate the need for multiple passes (i.e., to provide a sufficient quantity of solder106for each pair of assemblies100in a single pass on either side of each pair of assemblies100). Among other benefits, because the dispensing needle302only passes along two sides of each assembly100, the single axis dispensing technique reduces the minimum clearance requirement in the area both above and below each individual assembly100, thereby providing additional real estate on the PCB300for the MEMS transducer118and integrated circuit120. In some instances, the additional real estate afforded by the single line axis dispensing process can increase the DP ratio to 50% or greater.

The solder106couples the can104to a conductive material that is embedded within or otherwise coupled to the substrate102(e.g., a silicon, silicon oxide, glass, Pyrex, quartz, ceramic, etc.). Referring toFIGS. 7 and 8A, the underlying PCB is shown for the full frame assembly200and the assembly100, respectively. The PCB includes conductive traces surrounded by and/or embedded in non-conductive substrate material. The conductive traces may be formed in sheets, strips, or individual boards as desired. In some embodiments, the PCB includes a solder mask layer and/or a metal layer. As shown inFIG. 8A, the PCB300includes one or more circuit traces or pads, shown as integrated circuit traces312; a substrate aperture, shown as port314, that extends through the PCB300and provides fluid communication therethrough; a component trace or pad, shown as microphone trace316, that is substantially annular and surrounds the port314; and a periphery trace or pad, shown as guard ring318. The PCB300may include other elements, traces or pads, and/or embedded components.

According to an exemplary embodiment, the integrated circuit traces312are configured (e.g., arranged, positioned, etc.) to couple the integrated circuit120to the PCB300(see also,FIG. 4). In some embodiments, the integrated circuit traces312are configured to receive flux and/or solder to electrically couple the integrated circuit120to the PCB300. In other embodiments, the integrated circuit traces312include depressions or locations on the PCB300configured to receive adhesive and/or another coupling mechanism. The layout and/or configuration of the integrated circuit traces312may be different and arranged to suit the particular integrated circuit120employed in the MEMS device (e.g., MEMS transducer118ofFIG. 4). By way of example, the PCB300may include more than or less than three integrated circuit traces312(e.g., one, two, four, five, etc.). In other embodiments, the PCB300does not include the integrated circuit traces312.

As shown inFIG. 8A, the port314is a substantially round through-hole defined by (e.g., formed through, etc.) the PCB300. The port314may facilitate communication (e.g., audible communication, etc.) between the MEMS transducer118and an environment surrounding the microphone assembly100(e.g., the MEMS transducer118receives acoustic energy through the port314, etc.). The MEMS transducer118is a bottom-port MEMS device (i.e., the PCB300defines the port314). In other embodiments, the port314has a different shape, diameter, and/or is otherwise positioned on the PCB300. In an alternative embodiment, the MEMS transducer118is a top-port MEMS device (e.g., the can104defines the port314, etc.).

According to an exemplary embodiment, the microphone trace316is configured (e.g., arranged, positioned, etc.) to couple the MEMS transducer118to the PCB300. In some embodiments, the microphone trace316is configured to receive flux and/and solder to electrically couple the MEMS transducer118to the PCB300. In other embodiments, the microphone trace316includes depressions or locations on the PCB300configured to receive adhesive and/or another coupling mechanism. The layout and/or configuration of the microphone trace316may be different in various illustrative embodiments and may be arranged to suit the particular MEMS transducer118employed in the microphone assembly100. By way of example, the microphone trace316may have a different shape and/or a different diameter. In other embodiments, the PCB300does not include the microphone trace316.

As shown inFIG. 8A, the guard ring318is embedded or otherwise coupled to the PCB300and substantially surrounds the integrated circuit traces312, the port314, and the microphone trace316(e.g., the guard ring318extends along and/or around at least a portion of the periphery of each microphone assembly100of the PCB300, etc.). In some embodiments, the guard ring318is sunken in or recessed relative to the surface (e.g., the solder mask, etc.) of the PCB300(e.g., approximately twenty micrometers, etc.). The guard ring318may be formed as a part of the PCB300and/or embedded within the PCB300. In some embodiments, the guard ring318is formed of (e.g., manufactured from, etc.) a metal material (e.g., copper, steel, iron, silver, gold, aluminum, titanium, etc.). In other embodiments, the guard ring318may be formed of another material (e.g., a thermoplastic material, a ceramic material, etc.). In some embodiments, the guard ring318is adhered, fused, and/or otherwise coupled to the PCB300without the use of solder (e.g., adhesively coupled thereto, etc.). Solder may be applied to an outward facing surface of the guard ring. In some embodiments, the outward facing surface of the guard ring includes a trace and/or is tinned.

As shown inFIG. 8B, the guard ring318includes a plurality of edges including a first pair of parallel edges, shown as first edge324and second edge326, and a second pair of parallel edges that are adjacent to the first pair of edges (and perpendicular to the first pair of edges), shown as third edge328and fourth edge330. As shown inFIG. 8B, a length331of the second pair of edges is greater than a length333of the first pair of edges. Together, the plurality of edges define a frame. The second edge326is disposed on an opposing side of the guard ring318as the first edge324. As shown inFIG. 8B, a portion332(e.g., a central portion approximately half-way between the third edge328and the fourth edge330) of both the first edge324and the second edge326has a reduced thickness relative to the third edge328and the fourth edge330. Additionally, the portion332has a reduced thickness relative to each corner region334of the guard ring318. Arrows are provided inFIG. 8Bto indicate a thickness336of the portion332, a thickness338of the third edge328and the fourth edge330, and a thickness340of each corner region334. Among other benefits, the reduced thickness of the portion332of the first edge324and the second edge326ensures that a minimum amount of guard ring318material will be visible beneath the can104after the solder reflow operation (e.g., after heating the PCB300to reflow the solder along the first edge324and the second edge326, as will be further described).

Referring toFIG. 9, a method400of making a populated PCB is shown, according to an illustrative embodiment. Method400may be implemented with the assembly100and PCB300ofFIG. 4. Accordingly, method400may be described with regards toFIGS. 1, 3, 4, 6, 8A and 8B. Additionally, various steps of the method400are illustrated conceptually inFIGS. 10-17.

At402, a substrate (e.g., the substrate102, etc.) is provided (seeFIG. 10). The substrate may define a plurality of ports (e.g., the ports314, etc.). At404, a plurality of guard rings (e.g., the guard rings318, etc.) are coupled to the substrate (seeFIG. 11). Each of the plurality of guard rings includes a portion (e.g., the portion332, etc.) along a first edge and an opposing second edge that has a reduced thickness relative to the remaining edges. Block404may additionally include positioning each of the plurality of guard rings to surround a respective one of the plurality of ports and coupling each of the plurality of guard rings to the substrate with an adhesive or solder.

At406, a plurality of MEMS transducers (e.g., MEMS transducer118, etc.) are coupled to the substrate within a periphery of each of the plurality of guard rings (seeFIG. 12). For example, block406may include soldering the MEMS transducer to a microphone traces (e.g., the microphone traces316, etc.). Block406may additionally include aligning each of the MEMS transducers (e.g., an opening in each of the MEMS transducers) with a respective one of the ports. In some embodiments, block406may further include coupling an integrated circuit (e.g., the integrated circuit120, etc.) to the substrate (e.g., the integrated circuit traces312, etc.) within the periphery of each of the plurality of guard rings. In some embodiments, the plurality of MEMS transducers may be coupled to the substrate before coupling the plurality of guard rings to the substrate. In other embodiments, the plurality of MEMS transducers may be coupled to the substrate after coupling the plurality of guard rings.

At408, a solder (e.g., the solder106, etc.) is applied to the substrate along a single axis between each of the plurality of guard rings. In the embodiment shown inFIG. 13, solder is applied to the substrate in a line along an X-axis direction (e.g., X-axis direction306, etc.) to a second pair of edges (e.g., long edges, the third edge328and the fourth edge330, etc.). The solder is applied at a location that is spaced equally from a longest edge of adjacent guard rings318such that an equal amount of solder is provided to each guard ring318. Block408may include positioning a dispensing needle (e.g., the dispensing needle302, etc.) above the substrate, initiating the flow of solder through the needle, and moving the needle at a continuous rate along a serpentine path between adjacent rows of guard rings.

At410, a can (e.g., the can104, etc.) is coupled to each of the plurality of guard rings to enclose each of the plurality of MEMS transducers and to form a plurality of coupled microphone assemblies (seeFIG. 14). Block410may include providing a plurality of cans and positioning each of the plurality of cans over a respective one of the plurality of MEMS transducers such that the lower edges of each of the plurality of cans is aligned with a respective one of the plurality of guard rings (e.g., such that each can covers at least a portion of a first edge and a second edge of a respective one of the plurality of guard rings). Block410may further include pressing each of the plurality of cans into the solder to at least partially secure the cans in position relative to the substrate.

At412, heat is applied to the solder to reflow the solder along the first edge and the second edge of each of the plurality of guard rings (e.g., to reflow the solder into a gap between each of the plurality of cans and a respective one of the plurality of guard rings). Block412may include heating the entire populated PCB including the substrate, the MEMS transducer, the integrated circuit, the can, and/or other assembly components to a peak temperature that is just above the melting temperature of the solder. Block412may include dwelling at the peak temperature for a predefined period of time. For example, the populated PCB may be placed into an oven or passed beneath an infrared lamp.FIGS. 15-16show a top view of a single coupled microphone assembly before and after the reflow process, respectively. As shown inFIG. 16, after heating, solder reflows along the first edge324and the second edge326of the guard ring. The solder along the first edge and the second edge are concealed beneath the cover due to the reduced thickness of the first edge and the second edge. In other embodiments, the solder along the first edge and the second edge may protrude a distance outwardly beyond an outer edge of the can (e.g., a distance that is less than a distance between the outer edge of the can and an outer edge of the solder along the third edge and the fourth edge).

At414, a plurality of tags (e.g., the tags108, etc.) are applied to the substrate. As shown inFIG. 17, each of the plurality of tags is applied to a respective one of the first edges and the second edges, and is shared between adjacent assemblies. Block414may include positioning a dispensing head for the tags at a central position along a respective one of the first edges and the second edges and dispensing at a predefined flow rate for a predefined period of time. Block414may additionally include curing the tags (e.g., dwelling for a predefined time period) to ensure that each of the tags is structurally robust. Further details regarding the application of tags to the substrate may be found in U.S. Pat. No. 10,227,232, which incorporated by reference herein in its entirety. At416, each one of the plurality of coupled microphone assemblies is separated (e.g., singulated, diced, etc.) from the populated PCB to form a plurality of individual microphone assemblies. In some embodiments, the method400may further include coupling the microphone assemblies to an end-user device (e.g., a smartphone, a tablet, a laptop, etc.). In some embodiments, the method400may include additional, fewer, and/or different operations.

Further, unless otherwise noted, the use of the words “approximate,” “about,” “around,” “substantially,” etc., mean plus or minus ten percent.