Solder Ball Reflow

The melting and adhesion of solder balls dropped on prepared screen-printed pads of flux is a standard industrial process generally referred to as solder ball reflow.  The reflow process is typically carried out in conveyor ovens using forced convection for uniformity. The concentration of ambient oxygen during reflow should be minimized to prevent oxidation of the solder and if applicable the pad material below the ball. This is achieved by a constant flow of nitrogen gas into the tunnel called nitrogen purging.

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 unwanted intermetallics. Over time these 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 metal(s) and the resultant formation of intermetallics.

The conveyor oven tunnel is open at both ends so the required flow rate of Nitrogen is relatively high, between 1000 and 1500 SCFH.  This figure can rise during high throughput of material into the conveyor oven owing to “air drag in” as the material enters from the air ambient into the tunnel. A further concern is the generation of particulate matter as the belt of the conveyor is dragged over the fixed bottom of the tunnel. While such particle generation is not a severe problem in circuit board manufacturing, it is inappropriate in a semiconductor cleanroom. Therefore, the conveyor oven needs to be in a remote location from the wafer processing area, which requires a cleanroom. After the reflow process the, now melted, spherical balls and the substrate are cleaned by a spray/stream application of warm water followed by a spin dry, which necessitates a return to the cleanroom.

Issues with the current reflow processes utilizing a conveyor oven and/or fluxless 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)
  • Using a wafer throughput of 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
• Conveyor ovens that use formic acid consume large quantities presenting costly safety issues
• Flux-less systems using Vacuum are expensive to purchase and costly to maintain
• When the balls are dropped on the flux, a subsequent cleaning step must be carried out in a separate tool

Innovative technology offered by S-Cubed’s solder reflow and clean system for dropped ball systems

• Flux and solder ball applied 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, Figure 1
• Chill plate/convection enhanced cooling
• Nitrogen gas consumption per wafer is approximately 20 SCF (Tests, Figure 2, carried out at 10 CFH for 5 minutes)
  • $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
• Formic acid can be used, though it is not necessary for balls dropped on flux
  • • Reflow and cleaning in same location, no inter-process handling

 

 

*1: Modules can have multiple process zones, spin modules can have 2 stacked bowls, pillar reflow thermal modules can have 2 stacked.

Document # 0-00088-10