Leif G. Fredin
Copper bumping in flip chip assembly offers increased reliability, extended temperature range, greater mechanical strength, higher connection density, improved manufacturability, and better electrical and heat-dissipating performance.
The thorn in the rose of flip chip assembly has always been the mechanical stress caused by the difference in thermal expansion coefficients between the chip and the substrate materials. As the assembly changes temperature, this differential thermal expansion creates shearing forces on the bumps. These forces increase with the distance from the chip center, or “neutral point,” so that corner bumps and larger chips are most vulnerable to damage. Repeated thermal expansion and contraction with unprotected bumps leads to open connections from fatigue cracking of the bumps, especially with solder bump flip chip assembly. Unprotected gold or polymer bumps are more compliant than solder bumps, but still risk damage from thermal stress.
Underfill continues to be the primary defense against thermal expansion damage. Underfill generally is an adhesive that “locks together” the die and substrate, reducing or eliminating differential expansion. Production managers hated early-generation underfills, which required slow, painstaking application and relatively long thermal cure times. Newer underfills are moving to “snap cure” or to “no-flow” pre-application, sometimes of a combined flux and underfill.
The damage to flip chip connections from differential thermal expansion can be reduced by increasing the vertical distance between the chip and the substrate. The connecting bump acts like a tiny lead in absorbing the shear forces. Higher bumps act like longer leads, with proportionately increased protection. Some approaches to flip chip assembly use larger solder bumps, or even include non-melting high-lead solder bumps to increase the chip to board distance. The non-melting bump does not reflow and change shape, so it maintains distance, while a lower-temperature eutectic solder on the high-lead bump or on the substrate pad reflows to complete the connection.
Unfortunately, using larger solder bumps to increase chip to board distance has undesirable side effects. The larger diameter spheres require larger spacing between the die bumps, reducing the interconnection density. The longer conductive path through the larger lead sphere increases both the electrical and the thermal resistance of the connections. And, of course, more lead is as welcome as a space alien in today’s ever-greener factory environment.
The growing commercial availability of relatively tall, cylindrical copper bumps solves many of these problems. The non-melting electroplated copper cylinder maintains greater chip to substrate distance, providing stress relief. The high aspect ratio columns permit closer bump spacing for a given height, increasing connection density. Where underfill is still required, the greater height allows faster underfill flow and more uniform distribution, especially with high viscosity “thermally loaded” underfills. The greater mechanical shear strength of copper compared to lead both strengthens the connections and increases the overall ruggedness of the assembly. The electrical connection may be made by reflowing a tiny solder cap, which limits solder wicking during assembly, permits the use of less aggressive solder fluxes, and minimizes lead content. To entirely eliminate lead, the cap may be lead-free solder, or indium, or the final connection may be with conductive adhesives. Finally, the increased thermal and electrical conductivity of copper compared to lead improves both the electrical and the thermal performance of the assembly.
A variety of copper bumping processes have been reported over the past decade. At TLMI, we use the well-established, well-characterized microelectronic packaging technologies of sputtering, electroplating and photolithography, to create well-defined copper cylinders of uniform height with a variety of conductive caps. Figure 1 shows the process flow.
Figure 1 Process flow for forming copper bumps on a silicon wafer with aluminum pads and conventional passivation.
1. Pre-Clean The pre-cleaning step assures a properly prepared pad surface for the under-bump metal (UBM) deposition.
2. Sputter The sputtered UBM forms a good mechanical and electrical connection to the pad, including an adhesion / barrier layer. The UBM also hermetically seals the passivation opening, protecting the aluminum. For standard aluminum pads, the adhesion/barrier layer is generally titanium, or a titanium-tungsten alloy. A sputtered copper second layer provides a plating seed, and an electrical connection to the wafer for electroplating the bumps. Sputtering insures the best adhesion and electrical continuity for these layers.
3. Resist Coat The plated wafer is coated with a carefully controlled, pinhole-free layer of photo resist, to define the patterned plating of the bumps. Ideally, the resist should be slightly thicker than the required plating height, including all of the layers. Solder may be plated thicker than the resist, provided a subsequent solder reflow gives the required hemispheric final bump shape.
4. Resist Expose The photo resist is exposed and developed to define the bump. The cleanliness of these small resist openings is assured by a brief de-scumming step before initiating electroplating.
5. Bump Plating Electroplating, with precise control of the current density and the plating solution composition, brings the copper cylinder and its cap to the desired height. The cap material and thickness vary with the intended application and the assembly method. For adhesive assembly, a thin layer of gold over the copper provides a long-lasting oxidation-free surface. Other applications require a nickel-gold cap.
6. Resist Strip After plating the bumps and caps, the remaining photo resist is stripped from the wafer, and the conducting metal layer that was required for electroplating is etched away.
7. Metal Etch After plating the bumps and caps, the remaining photo resist is stripped from the wafer, and the conducting metal layer that was required for electroplating is etched away.
The result is a high aspect-ratio copper cylinder, with a cap suitable for flip chip assembly. Our standard process produces bump heights from 10 to 70 micrometers. Bump height uniformity is plus or minus 5 micrometers in die, 15 across a wafer, and 20 wafer to wafer. Our Design Rules give layout details and tolerances.
A wide variety of solder caps can be plated onto the copper bump for solder assembly. The amount that is plated is carefully controlled to give the thickness of solder required for the final assembly. The solder cap may be reflowed to provide a dome shape before assembly. Figure 2 shows a test coupon with eutectic solder caps that have been reflowed to form smooth domes. Lead-free solders, or even indium, may also be deposited as a cap.
Figure 2. Copper bumps with reflowed eutectic solder caps
TLMI offers a proven, volume production process for high aspect ratio copper bumping, customized for a variety of flip chip assembly methods. The resulting bumps offer significant advantages in reliability, ruggedness, connection density, electrical and thermal performance, and ease of assembly over solder bumps.
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