Gold stud bump flip chip with adhesive assembly is a well-proven technology that solves a wide range of microelectronics packaging problems. MEMS pressure sensors, which have their pressure sensor opening or diaphragm on the top side, may be flip chip mounted by their backside contacts in a variety of ways. However, MEMS motion sensors, with moving elements on the contact surface, cannot easily be flip chip mounted on that surface by any method requiring underfill, because of interference with the mechanical elements.
Combining gold stud bump flip with thermosonic, instead of adhesive, assembly provides a fast, flexible, low cost way to assemble these MEMS devices to boards or substrates. Because of the compliance and strength of the gold bumps, gold stud bump thermosonic assembly may not require underfill, so the mechanical integrity and capability of the MEMS device is not compromised.
For the work reported here, MEMS angular rate sensors were so mounted. The resulting MEMS die shear strengths provide adequate ruggedness without compromising device performance. They also match theoretical predictions and other recently reported thermosonic flip chip assembly results.
KEY WORDS: Flip Chip, Stud Bump, Thermosonic, MEMS
INTRODUCTION
Many of the growing number of applications for MEMS sensors take advantage of the small physical size of the sensor. The marriage of MEMS with flip chip, the smallest form of electrical interconnection, is then as inevitable as was the 1863 marriage of Charles Stratton with Lavinia Warren. [1] These two tiny technologies were born for each other.
Kim reports on MEMS pressure sensors stud bump mounted as a pressure sensing belt on the outside of aircraft during certification testing. [2] Since these pressure sensors have the sensing diaphragm on their top surface, and the electrical contacts on the bottom surface, they could be flip chip mounted with the ancient and unhealthful solder bump flip chip process, including the required underfill adhesive.
Unfortunately, MEMS motion sensors frequently have their moving elements located on the same surface as their electrical bond pads. While this does not preclude flip chip assembly, it certainly precludes face down solder bump flip chip assembly. Underfill adhesives are designed to wick into every little crevice and fill it with cured epoxy. Solder bump assemblies are underfill junkies -- they can't live very long without it.
As has been earlier reported in this forum [3], gold stud bump (more correctly known as gold ball bump) flip chip has many advantages over its ancient ancestor, solder bump. The gold ball contacts are placed with a wire bonder, and can be put onto any bond pad which can be wire bonded. The gold bumps can be placed onto bond pads smaller than 100 microns, on pitches below 150 microns, beyond the ken of most solder mavens. The gold bump is also more compliant than the cold and brittle solder.
Gold stud bump is most frequently assembled with conductive or non-conductive adhesives. [4] These could disable a MEMS just as effectively as underfill. However, when the stars are in proper alignment, gold stud bump flip chip assemblies may be made without adhesive, and without underfill, by gold thermosonic bonding.
Cheah [5] most recently reported on gold thermosonic flip chip assembly as a practical manufacturing technology. In gold thermosonic flip chip assembly, the gold bumped die is placed, under heat and pressure, with a collet driven by longitudinal or transverse acoustic energy. Conceptually, the connection is made as is the single connection in a wire bonder, except that all the connections on the die are made simultaneously, not serially. Tan [6] reported making up to 256 simultaneous connections, and summarized earlier work in this field.
Thermosonic bonding eliminates the need for conductive adhesive. Careful choice of substrate and pad layout can eliminate the need for underfill. The "double elimination" gives a potentially feasible method for flip chip assembly of single-surface MEMS sensors. In particular, since compensating for thermal expansion (CTE) mismatch between die and substrate is a major reason for underfill, matching the CTE by using a silicon substrate for a silicon die alleviates this problem. Having large bumps, and an adequate number of bond pads relative to die size, can provide enough mechanical strength to survive normal handing, e.g. the "gravity test."
ASSEMBLY PROCESS
The MEMS angular rate sensor chosen for this development was a 3.5 mm square silicon chip, with 18 aluminum bond pads surrounding the mechanical elements. The 100 micron square pads were at varied spacing, with the closest pitch being 140 microns.
The primary substrate was silicon, 2.5 cm square, with wire-bondable sputtered thin film nickel-nichrome-gold pads and traces. This developmental substrate was only a fan-out pattern, without other components. Mechanical test substrates were also made from resistor grade alumina, and from glass, with the same metallization.
The bumps were made from 1 mil (25 micron) wire, gold with 2% palladium, in a Hughes 2460 wire bonder, modified for stud bumping. Bump sizes were approximately 75 microns diameter at the base, and 50 microns high. They were not coined before assembly.
Assembly was in an SEC 401 Flip Chip Aligner-Bonder, with a transverse ultrasonic head. Various bonder settings and temperatures were used for process optimization. Glass test substrates were used to permit microscopic viewing of the MEMS structures after assembly. Ceramic substrates were used for mechanical samples and electrical tests. Functional assemblies were made on silicon substrates.
Fig. 1. Patterned alumina substrate with sputtered gold tracks and pads.
Testing included contact to contact resistance measurements, and mechanical die shear testing. Functional testing was performed elsewhere to verify assembled device operation.
RESULTS
About thirty total assemblies were completed, including the test chips. Figure 2 is a view through a glass substrate of a portion of the assembled MEMS.
FIG. 2. Flip chip MEMS assembly viewed through glass substrate
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