This is the second in a series of flip chip tutorials intended for flip chip users and potential users. Tutorial #2 presents an overview of solder bump flip chip bumping and assembly processes. Concurrently, FlipChips Dot Com’s Technology News Updates present industry experts describing the newest developments in their fields; our Literature and Photo pages give supplemental material.

Flip chip assembly by means of solder connection to the bond pads was the first commercial use of flip chip, dating to IBM's introduction of flip chip in the 1960's. Solder bump has the longest production history, the highest current and cumulative production volumes, and the most extensive reliability data of any flip chip technology. Delco developed their solder bump processes in the 1970's; Delco Delphi now assembles over 300,000 solder bumped die per day for automotive electronics.

More recent solder bump flip chip process variations have lowered the manufacturing cost, widened flip chip applicability, and made solder bumped die and wafers available from several suppliers to the commercial market. This introductory survey discusses the operations performed in solder bumping and assembly, and describes several of the solder bump processes now commercially available. The references listed at the end of the tutorial provide details.

The solder bump flip chip process may be considered as four sequential steps: preparing the wafer for solder bumping, forming or placing the solder bumps, attaching the bumped die to the board, substrate, or carrier, and completing the assembly with an adhesive underfill.

The first step in solder bumping is to prepare the semiconductor wafer bumping sites on the bond pads of the IC's. This preparation may include cleaning, removing insulating oxides, and providing a pad metallurgy that will protect the IC while making a good mechanical and electrical connection to the solder bump and the board.

This under-bump metallization (UBM) generally consists of successive layers of metal with functions described by their names. The "adhesion layer" must adhere well to both the bond pad metal and the surrounding passivation, providing a strong, low-stress mechanical and electrical connection. The "diffusion barrier" layer limits the diffusion of solder into the underlying material. The "solder wettable" layer offers an easily wettable surface to the molten solder during assembly, for good bonding of the solder to the underlying metal. A "protective layer" may be required to prevent oxidation of the underlying layer. The Photo Gallery shows a photomicrograph of UBM on a wafer.

Solder bumps may be formed or placed on the UBM in many ways, including evaporation, electroplating, printing, jetting, stud bumping, and direct placement. As discussed below, the results of these methods may differ in bump size and spacing ("pitch"), solder components and composition, cost, manufacturing time, equipment required, assembly temperature, and UBM. The Photo Gallery includes a photomicrograph of reflowed solder bumps on a wafer.

Assembly operations include handling, placing, fluxing, and solder joining. The assembly process is influenced by the bumped die packaging, the solder bump, the substrate or board material and size, the assembly equipment, the end product, and the cost.

Bumped die may be transported in waffle packs or tape and reel. Presentation may be either bump-up or bump-down, depending on the bumping and the equipment. Tape and reel requires special tapes designed for carrying flip chips. Placing the bumped die may be by fine-pitch surface-mount equipment, or by high-accuracy flip chip placement equipment. In either case, the die must be aligned with the bond pads on the board before placement. Fluxes may be conventional or no-clean fluxes, with differing application and cleaning requirements. Soldering may be in a belt furnace or by hot gas or other local means.

One function of the solder bump is to provide a space between the chip and the board. In the last stage of assembly, this under-chip space is usually filled with a non-conductive "underfill" adhesive joining the entire surface of the chip to the substrate.

The underfill protects the bumps from moisture or other environmental hazards, and provides additional mechanical strength to the assembly. However, its most important purpose, particularly with solder bumps connections on large die or to organic substrates, is to compensate for thermal expansion differences between the chip and the substrate. Underfill mechanically "locks together" chip and substrate so that differences in thermal expansion do not break or damage the electrical connection of the bumps. Underfill has been found to increase the fatigue life of solder bumps by at least an order of magnitude.

Underfill must bond well to both the chip passivation and to the substrate. It must also be compatible with the flux. A cleaning step to remove flux residues may be required before underfilling. No-clean epoxy fluxes are generally compatible with underfills, fusing or reacting with the epoxy-based underfill. Underfill may be needle-dispensed along one or two edges of each chip. It is drawn into the under-chip space by capillary action, and heat-cured to form a permanent bond. Flow characteristics, adhesion to both chip and board, and cure time are key concerns. Newer developments in underfill include "no-flow" and solid underfill and reworkable underfills. For more details see "Underfill Update: NUF, MUF, WUF and Other Stuff" by Dr. Ken Gilleo, in Archives.

SOLDER BUMPING PROCESSES

Solder bumping processes may be differentiated by their UBM and method of depositing solder, as in the following sections:

The pioneering IBM C4 process was based on evaporation of the UBM and the solder onto the cleaned wafer pads. The UBM consists of successive thin film evaporated layers of Cr, Cr-Cu, Cu, and Au. Evaporated deposits are defined by a metal mask aligned with and mechanically clamped to the wafer at a controlled spacing.

The UBM evaporation is followed by evaporation of high lead solder to form the bumps. Low lead eutectic solder cannot be evaporated because of the difference in vapor pressures of lead and tin. Bump size and shape is determined by the mask openings and spacing. The evaporated solder is reflowed to form a spherical solder bump.

The evaporated bump process has a long manufacturing history with good reliability. The evaporated bump allows good control of the alloy and uniform bump heights. The process is limited to high lead solders with binary (two-component) alloys. It cannot be easily scaled up to larger wafers, and has a limited throughput, high capital equipment costs and high licensing fees.

Sputtered UBM/Electroplated Solder

Electroplating of solder was developed as a less costly and more flexible method than evaporation. The UBM is typically an adhesion layer of titanium tungsten (TiW), a copper wetting layer, and a gold protective layer. The UBM is sputtered or evaporated over the entire surface of the wafer, providing a good conduction path for the electroplating currents.

Bumping begins with photopatterning and plating a copper minibump on the bump sites. This thick copper allows the use of high-tin eutectic solders without consuming the thin copper UBM layer. A second photopatterning and plating of the solder alloy over the minibump forms the solder bump. The photoresist is then removed from the wafer and the bump is reflowed to form a sphere.

Electroplated bumping processes generally are less costly than evaporated bumping. Electroplating in general has a long history and processes are well characterized. The UBM adheres well to the bond pads and passivation, protecting the aluminum pads. Plating can allow closer bump spacing (35 to 50 microns) than other methods of bump formation. Electroplating has become more popular for high bump count (>3,000) chips becasue of its small feature size and precision.

Plating bath solutions and current densities must be carefully controlled to avoid variations in alloy composition and bump height across the wafer. Plating generally is limited to binary alloys.

Sputtered UBM/Printed Solder

The combination of thin film UBM with printed solder bumps was developed in processes by Delco and others to overcome some of the limitations of evaporated and electroplated bumps.

In a typical process, the UBM consists of sputtered aluminum as the adhesion layer, nickel as a solder-wettable barrier layer, and copper as wetting layer. Photopatterning and etching is used to remove these layers except over the bond pad openings. Solder is deposited by printing solder paste onto the UBM by stencil or other means, and reflowing the solder paste to form a spherical solder bump.

The resulting process is less costly than evaporated processes, and comparable in cost to electroplating. The UBM adheres well to the bond pad and passivation, protecting the aluminum pad. Delco has demonstrated excellent reliability for their process in high volume automotive applications. Solder paste gives good control of the bump composition, and allows a variety of alloys to be used, including eutectic, lead-free, non-binary, and low alpha particle solders. Printed solder bumps cannot achieve the close spacing of plated bumps, with current production typically limited to 150 micron or greater spacing. Processing of larger (300mm) wafers will require more costly sputtering equipment.

Electroless plated UBM promises to be the lowest cost approach to solder bumping, because it eliminates thin film and masking steps and permits batch processing.

The UBM is formed by selective chemical plating in a wet chemical, maskless process. The wafer backside is first covered by a protective layer of resist. An alkaline zincate solution removes the bond pad aluminum oxide layer and etches the surface. An electroless plated nickel layer forms the barrier and wettable layer. It is protected from oxidation by an immersion gold layer. The UBM photo in the Photo Gallery shows electroless Ni/Au UBM. The electroless nickel may also be plated higher to form a bump for adhesive or other non-solder flip chip assembly. This will be covered in more detail in a subsequent tutorial.

Solder bump deposition is by stencil printing of solder paste. The solder bump is reflowed and flux residues are removed. The Photo Gallery shows reflowed solder bumps on electroless nickel UBM.

The cost advantages of the electroless UBM solder bump process result from eliminating the masking and metal sputtering required by other methods, and allowing parallel batch processing of multiple wafers, which increases throughput and reduces costs. Pitch is limited by the solder dispensing process, not the UBM process.

Electroless Ni-Au UBM requires that all exposed metal other than the pads be passivated or covered with resist. The plating baths must be carefully controlled and kept free of contaminants to insure uniform plating.

Other Solder Bumping Methods

Less common methods of solder bumping include solder bump bonding with solder wire, solder jetting with molten solder, and solder ball placement directly onto the UBM.

Solder bump bonding (SBB) uses solder wire in a modified wire bonder to place a ball of solder directly onto the bond pad. The scrubbing action of the wire bonder causes the solder ball to bond to the bond pad. The solder wire is broken off above the bump, leaving the bump on the pad, where it can be reflowed.

Solder bump bonding is a serial process, producing bumps one by one at rates up to about 8 per second. It has advantages in allowing closer spacing than printed bumps.

Solder jetting places solder bumps on Ni-Au UBM by controlling a stream of droplets of molten solder. Demand mode jetting systems use piezoelectrics or resistive heating to form droplets in much the same manner as an ink-jet printer. Mechanical positioning directs the droplet placement. Continuous mode jetting systems use a continuous stream of solder droplets with electostatic deflection of the charged droplets to control placement.

Solder ball placement bumping depends on directly placing micro-spheres of solder on the UBM, similar to methods well developed for ball-grid array (BGA) and chip scale packages (CSP).

CONCLUSIONS

Solder bump flip chip, the oldest flip chip assembly method, has evolved over several generations into a variety of species which may be distinguished by their under-bump metallurgy and solder placement methods. Each has a differing set of strengths and limitations which suits them for differing applications.

FOR FURTHER INFORMATION:

"Low Cost Flip Chip Technologies," John H. Lau
McGraw-Hill, NY, 2000. ISBN 0-07-135141-8
A broad survey and comparison of flip chip bumping and assembly techniques.

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