Patent Application
20100221860
The present invention relates, generally, to the packaging of micro-electromechanical (MEMS) devices and, more specifically, to the mounting of precision MEMS sensors in three dimensions.
Modern consumer and commercial electronic device design has undergone a trend towards miniaturization and the incorporation of multiple disparate functionalities. In particular, single-chip-scale packages increasingly tend to include multiple small devices for a plurality of functions. Many applications call for the incorporation of two- and three-dimensional navigation technology into such compact devices. Two- and three-dimensional sensors suitable for that purpose include magnetic sensors (also referred to as magnetometers) and/or tilt sensors (also referred to as accelerometers). Preferably, these sensors are of minimum height along an axis perpendicular to the chip surface (the Z-axis) and compact in directions parallel to the chip surface (i.e., parallel to the XY-plane). Vertical mounting is needed to align sensors along the X-, Y-, and Z-axes. However, mounting Z-axis accelerometers or magnetometers along the z-axis can be very challenging for the packaging industry, especially for mass market applications that have space limitations. Current cost-sensitive, high-volume, standard mounting processes fail to facilitate mounting vertical sensors for applications with limited space. Accordingly, there is an emerging need for the simple, low-cost vertical and horizontal mounting of MEMS devices at precise angles in a chip-scale package.
The present invention provides, in various embodiments, processes and methods for integrating MEMS sensors with precise location and/or orientation in X-, Y-, and Z-direction into low-cost organic chip-scale packages. In certain embodiments, the method attains structures having an overall height of less than about 0.8 mm, a width of less than about 4 mm, and a length of less than about 6 mm. Various embodiments exploit the surface tension of solder to align a Z-axis-mounted MEMS device onto, or into a hole formed in, the X-Y surface plane of a substrate.
In a first aspect, the invention provides an apparatus for precision MEMS mounting in organic packaging. In various embodiments, the apparatus includes a vertical sensor circuit assembly and a horizontal circuit assembly. The vertical sensor circuit assembly includes a MEMS device surface-mounted to a substrate. The substrate has an array of connection pads on a first side, a dummy set of connection pads on the opposite side, and conductive leads between the array and bottom edge lead pads of the substrate. The horizontal circuit assembly includes a horizontal die and the vertical sensor circuit assembly, both surface-mounted to a substrate having conductive leads with each of the horizontal die and the vertical sensor circuit assembly. Solder surface tension is used to align the horizontal die and the vertical sensor circuit assembly to the horizontal circuit assembly. In some embodiments, the distance between the bottom edge and the top edge of the vertical sensor circuit assembly is less than about 0.8 mm, and the height of the horizontal circuit assembly is less than about 0.8 mm. The MEMS device may be a hermetic sealed cavity device.
In a second aspect, the invention provides a method for precision MEMS mounting in organic packaging. The method includes patterning a first side of a Z-axis substrate with bond pads corresponding to the MEMS device; patterning a first side of the MEMS device with metal contacts for mounting to the first side of the Z-axis substrate; placing the first side of the MEMS device in contact with the first side of the Z-axis substrate; reflowing the MEMS device and the Z-axis substrate at a first temperature, and patterning the first side of an XY-axis substrate with bond pads corresponding to the Z-axis substrate. The patterning on the first side of the XY-axis substrate is spread slightly outward from the center of mass of the Z-axis substrate. The MEMS device may be a hermetic sealed cavity device. While placing the MEMS device, optical pattern recognition may be used to orient the device.
In various embodiments, the method further includes patterning the opposite side of the Z-axis substrate with dummy lead pad; reflowing the Z-axis substrate and the XY-axis substrate at a temperature lower than the first temperature; and/or adding solder mask to one or both sides of the Z-axis substrate. In certain embodiments, the method moreover includes patterning a second side of the MEMS device with metal contacts for mounting to the first side of a second Z-axis substrate; and patterning a first side of a second Z-axis substrate with bond pads corresponding to the second side of the MEMS device for mounting, wherein the reflowing of the MEMS device at a first temperature causes the MEMS device to assume a perpendicular orientation between the first and second Z-axis substrates. In these embodiments, a hole may be cut in the XY-substrate, and the MEMS device may be positioned in the hole in the XY-substrate.
The foregoing discussion will be understood more readily from the following detailed description of the invention when taken in conjunction with the accompanying drawings, in which:
Another exemplary apparatus (150) in accordance with various embodiments is illustrated in
Apparatus 100, 150 and similar structures may be built according to the method illustrated in
The vertical MEMS devices (e.g., magnetometers 154 and/or 156) are patterned and stud-bumped or plated with gold or copper (step 304), and then aligned and pick-and-placed onto the Z-axis substrate 110 (step 306). Optical pattern recognition may be used during placement to recognize and orient the rotation of each device around an axis normal to side Az within less than one degree. A vacuum head may be used to pick up the MEMS devices and place them onto the Z-axis substrate 110, maintaining a Z-tilt of less than one degree with respect to the substrate surface. Next, the Z-axis substrate 110 with the aligned devices is reflowed at a temperature between 260° C. and 340° C. (step 308), allowing for later end-customer green process assembly with SnAgCu solder having a liquidus temperature of from about 220° C. to about 230° C. The Z-axis substrate 110 with aligned MEMS devices is then diced, and transfer-rotated 90 degrees into holding trays in preparation of the pick-and-place onto the XY-substrate.
In step 310, the XY-substrate 102 is patterned with bond pads corresponding to the horizontal MEMS devices (e.g., magnetometers 104 and/or accelerometer dies 108) and the Z-substrates 110 to be mounted. Then, solder mask is added to sides A and B to isolate the wicking action of the solder. Solder paste is applied to side A of the XY-substrate (step 312), and the XY- and Z-mounted MEMS devices are pick-and-placed into position (step 314). Again, optical pattern recognition may be employed with the pick-and-place to recognize and orient the rotation of the devices and Z-substrates in X- and Y-direction (i.e., around an axis normal to side A) within less than one degree. The vacuum head picks up the devices and places them onto the XY-substrate, maintaining a tilt of less than 1 degree with respect to the substrate surface. In step 316, the XY-substrate and devices mounted thereon is reflowed at a temperature between about 220° C. to about 230° C. (step 316).
The apparatus and method described above may be modified in various ways. For example, the Z-mounted MEMS device may be a hermetic sealed cavity device, such as an accelerometer. The vertical mounting of a hermetic sealed cavity device may be accomplished as described above, using additional weights to balance the pick-and-place Z-substrate so that the solder wicking process results in perpendicular alignment to the XY-substrate.
In an alternative embodiment, a Z-axis MEMS device may be directly soldered to the XY-substrate, without using a Z-axis substrate. The resulting structure 400 is illustrated schematically in
In another embodiment, the Z-mounted MEMS device is integrated into a hole in the substrate, resulting in a structure 500 shown in
In yet another embodiment, illustrated in
Having described certain embodiments of the invention, it will be apparent to those of ordinary skill in the art that other embodiments incorporating the concepts disclosed herein may be used without departing from the spirit and scope of the invention. Accordingly, the described embodiments are to be considered in all respects as only illustrative and not restrictive.
This application claims priority to and the benefit of U.S. Provisional Application No. 61/074,051, filed on Jun. 19, 2008, the entire disclosure of which is hereby incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61074051 | Jun 2008 | US |
