Ultra-Low Residue Flip-Chip Flux NC-26-A
Ultra-Low Residue Flip-Chip
Customer-proven in copper-pillar flip-chip processes for mass reflow (MR)
- High yield soldering to
copper OSP traces
- No wetting onto the die
- Designed for copper pillar
- Dipping with minimal bridging
- Holds large die in place during reflow (NC-26S)
- Customer-proven residue compatibility with CUF and MUF without delamination
Copper pillars topped with solder micro-bumps are now the standard flip-chip solder bump in the semiconductor assembly industry. The relentless drive towards finer pitch, combined with reduced copper-pillar height, makes aqueous cleaning of flip-chip residues more difficult. An emergent failure mode is joint damage during aqueous jet impingement.
The move towards semiconductor-grade ultra-low residue no-clean fluxes, and away from cleaning processes is therefore inevitable.
Ultra-low no-clean fluxes have several characteristics critical to their functionality, such as rheology, residue level, solderability, the ability to retain die in place, and compatibility of the final residue with molded underfill (MUF) and capillary underfill (CUF).
Substrate warping challenges
Comparative Viscosities as a Function of Time
Importance of rheology for fine-pitch dipping processes
Rheology (viscosity/tack) must be adjusted based on the process needs, especially for fine pitch, without sacrificing the ability to hold the die in place.
- High viscosity can lead to bridging and inability to pick die from the dip tray
- Low viscosity can lead to flux wicking and die-surface contamination
Thermogravimetric analysis (TGA) is unrepresentative of real situations for flux. The exposed surface area is much smaller in TGA than for real flip-chip applications. In real applications, flip-chip fluxes will have lower residue levels than shown here.
Solder wetting, also called "solderability" must be controlled and balanced. Excessive wetting can lead to bridging. Poor or insufficient wetting can lead to weak solder joints.
Wetting for ultra-low residue fluxes
Holding Die in Place
MDR test method measures the movement of reflowed solder from its original position during reflow.
Compatibility with MUF and CUF
SEM X-section Check on Flux Thin-Film
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