CU Pillar Solder Reflow

Device package dimensions have shrunk, necessitating the reduction of the pitch for device to package interconnections. This need has led to copper pillar “bumping” in which a pillar of copper is plated into a mold typically formed of photoresist.  After the deposition of the copper pillar, a solder alloy is plated on the top surface and the photoresist mold is stripped away.  Finally, the top layer of solder alloy is reflowed creating a spherical surface of solder alloy atop the copper pillar.

During reflow it is necessary to limit copper oxidation, so the solder alloy can wet and bond to the copper while providing the collapsible “ball” for interconnection. Different conventional methods used to reduce oxidation include:

• Application of flux with reflow in a N² rich atmosphere followed by a warm water spray/stream
  • Oxygen level under 100 ppm during reflow
• Flux free reflow in formic acid/N² mixed atmosphere in a chamber or in a conveyor oven
• Reflow in a “hard” vacuum or a partial vacuum with formic acid

While the first method is the simplest, the handling necessary between the spin on of flux, reflow oven, and clean is inconvenient and costly. S-Cubed’s new integrated hot plate reflow system is a single system. With dry-in/dry-out solder flux apply, reflow, and cleaning in the same system. This eliminates the inter process handling of the first method and the cost, complexity, and danger that comes with higher levels of formic acid in the other two methods. As seen in Figure 1B multiple hot plate and clean modules can be stacked to improve throughput while keeping a small foot print.

Reflow process considerations

During the reflow process, the time spent above the liquidus phase of the heating profile needs to be minimized, to the extent possible, to prevent the formation of intermetallics. Over time these unwanted intermetallics can render the solder joint brittle and unstable. Minimizing the time spent above the liquidus phase is accomplished by creating a temperature “spike”; minimizing the dissolution of contact metals(s) and the resultant formation of intermetallics.

It is necessary to maintain the solder at the top of the pillar and to minimize the wetting of the solder down the side walls of the pillar. This requires exquisite timing of the heating profile which is facilitated in a hot plate oven using pin height to control heating and cooling. The pin height is repeatable within 10 microns in the S-Cubed hot plate system.  Controlled rapid cooling is made possible using a liquid cooled chamber lid, the wafer is heated by descent to the hot plate and cooled by ascent to the cooled lid.  The cooling is enhanced by convection currents of purge N² gas to maintain the O² levels under 100ppm, in proximity to the cooled lid.

Reflow on an advanced hotplate* vs in a conveyor oven or vacuum system

Conveyor oven and vacuum systems

• The open-ended conveyor ovens require approximately $350K/year in Nitrogen when in operation on a 24x7x365 basis (This cost is often higher in Asia)
  • If wafer throughput is 30 WPH (262,000 wafers per year) the cost is at least $1.30 per wafer
  • Assumes nitrogen cost at $.50 per 100 ft³
• Conveyor ovens cannot be utilized in clean rooms suitable for semiconductor production, as they are dirty systems
• Conveyor ovens occupy a vast footprint
• Formic acid conveyor ovens consume large quantities presenting costly safety issues
• Flux-less systems using Vacuum are expensive to purchase and costly to maintain

S-Cubed’s advanced solder reflow and clean system for copper pillar solder reflow*

• Dry-in/Dry-out cassette or FOUP processing in a single system
  • Provides reflow and clean capability in the same small and clean tool
• Programmable control of heating and cooling profiles
• Chill plate/convection enhanced cooling
• Nitrogen gas consumption per wafer on the order of 20 SCF
  • $0.10 per wafer (Assumes nitrogen cost at $.50 per 100 ft³ )
• Closed loop control of oxygen concentration through purge and exhaust controls
• Tool is clean room compatible
• Footprint approximately one fifth of the older technology
• If flux is applied the reflow and cleaning is in the same tool (no external inter-process handling)
• Enhanced control of pillar side wall wetting with programable temperature profile
• Flux-less Formic acid-based processing can be carried out in the hot plate module
  • Our design minimizes the use of formic acid by only introducing it during heating as the wafer descends to the hotplate

^*Patent applied for