This is the third in a series of flip chip tutorials intended for new flip chip users and potential users. Tutorial #1 presented the basics: an overview of what flip chip is and does, and how it is made. Further tutorials, found in the Archives, explain the topics in more detail. Concurrently, FlipChips Dot Com’s Technology Updates present industry experts describing the newest developments in their fields; our Literature and Photo pages give supplemental material.

The gold stud bump flip chip assembly process creates conductive gold bumps on the die bond pads, and connects the die to the circuit board or substrate with adhesive or ultrasonic assembly. Stud bumping requires no under-bump metallization (UBM), and thus does not require wafer processing; individual die can be stud bumped as easily as they can be wire bonded.

BUMPING

Gold stud bumps are placed on the die bond pads through a modification of the "ball bonding" process used in conventional wire bonding. In ball bonding, the tip of the gold bond wire is melted to form a sphere. The wire bonding tool presses this sphere against the aluminum bond pad, applying mechanical force, heat, and ultrasonic energy to create a metallic connection. The wire bonding tool next extends the gold wire to the connection pad on the board, substrate, or lead frame, and makes a "stitch" bond to that pad, finishing by breaking off the bond wire to begin another cycle.

For gold stud bumping, the first ball bond is made as described, but the wire is then broken close above the ball. The resulting gold ball, or "stud bump" remaining on the bond pad provides a permanent, reliable connection through the aluminum oxide to the underlying metal. Figure 1 (13k jpg) shows a gold stud bump on a bond pad. The bump diameter at the base is about 75 microns.

After placing the stud bumps on a chip, they may be flattened (or "coined") by mechanical pressure to provide a flatter top surface and more uniform bump heights, while pressing any remaining wire tail into the ball. Each bump may be coined by a tool immediately after forming, or all bumps on the die may be simultaneously coined by pressure against a flat surface in a separate operation following bumping.

Gold stud bumped die may be attached by conductive or non-conductive adhesives, or by ultrasonic assembly without adhesive. Conductive adhesive may be isotropic, conducting in all directions, or anisotropic, conducting in a preferred direction only.

Isotropic Conductive Adhesive Assembly
Isotropic conductive adhesives consist of an adhesive binder filled with conductive particles that are normally in contact with each other, providing a low electrical resistance in all directions. The ancient Greeks called this kind of conductive adhesive "isotropic." The adhesive may be dispensed by stencil printing onto the substrate bond pads, or the bumped die may be dipped into a thin layer of adhesive, coating only the bumps with adhesive.

Stenciled isotropic adhesive assembly provides a larger quantity of adhesive than dipped assembly, making a mechanically stronger bond. The additional adhesive compensates to some degree for bump height variations. A panelized array of substrates may be simultaneously stenciled in one operation, speeding up assembly. The stenciled adhesive can be inspected or measured before die mount to insure uniformity. However, stenciling requires a high-precision stencil printer and stencils. Stenciling limits minimum pad pitch to about 90 microns, to allow adequate conductive adhesive transfer. Figure 2 (13k jpg) shows stenciled adhesive on a bond pad.

Dipping requires a thin, precisely controlled layer of adhesive, and coplanarity of the die and adhesive during the dipping process. Since dipping places adhesive only on the bump surface, the minimum bump spacing is smaller than for stenciling, to pad pitches of 60 microns or less. Dipping does not require additional equipment as stenciling does, since the die mount aligner-bonder can be used for dipping. However, dipping requires careful control of the adhesive layer thickness, and dipping is a serial process, lengthening throughput time. Figure 3 (13k jpg) shows a coined stud bump after dipping in conductive adhesive.

After the isotropic conductive adhesive is heat cured, a non-conducting underfill adhesive is applied to completely fill the under-chip space. The underfill adds mechanical strength to the assembly and protects the connections from environmental hazards. Underfill adhesive may be dispensed along one or more edges of the die, being drawn into the space under the die through capillary action. Heat-curing the underfill adhesive completes the assembly process.

Figure 4 (9k jpg) shows a cross-section of a connection from the die (top) to the substrate. Conductive adhesive surrounds the connecting surfaces, with non-conductive underfill beyond.

Non-Conductive Adhesive Assembly
Non-conductive adhesive assembly is in some ways similar to anisotropic adhesive assembly. A non-conductive adhesive is dispensed or stenciled at the die location on the substrate. The bumped die is pressed against the substrate pads with enough force give compressive dispersion of the adhesive, allowing no adhesive to remain between the stud bump and substrate pad mating surfaces. This pressure is maintained while the assembly temperature is elevated for sufficient time to at least partially cure the adhesive. The chip is mechanically bonded to the substrate by the cured adhesive, with metal to metal contact between the bumps and substrate pads. No separate underfill adhesive is required.

Non-conductive adhesive has advantages for assembly onto flexible substrates, since the adhesive is cured while in the aligner-bonder, keeping the die fixed in location thereafter. Dispensing the adhesive properly and repeatably requires automated equipment, and the aligner-bonder throughput is determined by curing time, including ramping up and down from the curing temperature.

Ultrasonic Assembly
A non-adhesive assembly for gold stud bumped die results from pressing the bumped chip onto gold substrate pads, applying heat, pressure, and sonic energy sufficient to form gold to gold metallic bonds as in thermosonic wire bonding. Depending on the die size and the application temperature requirements, assemblies with these gold to gold connections may not require undefilling. This allows flipchip assembly for applications such as MEMS, MOEMS, and SAW filters, which cannot tolerate adhesives against their active surfaces.

Gold stud bump flip chip offers several advantages. The bumping equipment, a wire bonder or dedicated stud bumper, is widely available and well characterized. Since stud bumps are formed by wire bonders, they can be placed anywhere a wire bond might be placed. They can easily achieve pitches of less than 100 microns and be placed on pads of less than 75 microns.

Since stud bumping can be done on a wire bonder, it does not require wafers or under-bump metallization (UBM). Single, off-the-shelf die can be bumped and flipped without pre-processing. This makes stud bump flip chip fast, efficient, and flexible for product development, prototyping and low to medium volume production, while easy to scale up to high volume wafer-based production with automated equipment.

Because stud bumping is a serial process, the bumping time required increases with the number of bumps. However, high speed equipment now can place as many as 12 bumps per second. Stud bump assemblies demand more precise die placement equipment and are less tolerant of placement errors than self-aligning solder assemblies.

Each of the stud bump assembly processes has advantages and limitations that suit it for specific applications. Stud bump assembly has been successful in a wide range of applications, including the fetal monitor and the 3-D memory pictured in our Photo Gallery. Selecting the most appropriate assembly process depends on the application, the die size and number of bumps, the substrate, equipment availability, cost, and other considerations. Choosing the proper process is the best assurance of success.

Book, "Flip Chip Technologies," John H. Lau, Editor
McGraw-Hill, NY, 1995. ISBN 0-07-036609-8
An excellent survey of the field as of 1995, including many of the basic processes.

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