Industry News

Optimizing Reflow Profiling in Lead-free SMT Assembly

Edward Briggs
ebriggs@indium.com

Outline

  • Reflow phases
  • Max slope vs. ramp rate
  • Factors affecting reflow process window
  • Solder defects
  • Conclusion

Reflow Phases

Preheat

  • Minimize thermal shock
  • Drive off volatiles
  • Ramp rate

Pre-reflow

  • Flux activation/oxide removal
  • Void minimization

Reflow

  • Intermetallic formation
  • Peak
  • TAL

Cooling

  • Grain structure
  • Minimize CTE

Preheat Ramp Rate

  • Max (+) Slope 1.46°C/s
  • Ambient to peak 4min 40s (280s)
  • 1.46°C/s x 280s = 408.8°C increase from ambient to peak
  • 25°C (ambient) + 408.8°C = 433.8°C peak

Preheat-Ramp-Rate

  • True ramp rate or slope is from ambient to peak (RTP profile)
  • Ramp rate considerations for soak profile
  • Ramp rate = ΔTemp/ Δtime
  • 214°C/280s = 0.76°C/s

Preheat-Ramp-Rate-2

Solder Beading/Balling

Profile recommendations

  • Ramp to peak profile
  • Ramp rate (ambient to peak) 1-1.5C/s

solder-beading-balling

Voiding

  • Components such as BGA, LGA, SGA, QFN, flip-chip
  • Out-gassing entrapped flux
  • Wetting
  • Particle size/ powder type

voiding-1 voiding-cale

Profile recommendations

  • Raising peak temperature
  • Lowering peak temperature
  • Soak
  • Mixed alloy

KIC-soak

Factors Affecting Reflow Process Window

  • Water soluble vs. RMA/no-clean
  • Powder type
  • Board size
  • Component density/diversity

Solder Defects

  • Tombstoning
  • Solder Balling/Beading
  • Voiding
  • Graping
  • Head-in-Pillow (HIP)

Tombstoning

Unbalanced wetting forces – leadless component

  • Weight of chip
  • Surface tension under chip
  • Surface tension side of chip

Tombstoning

Profile recommendations

  • Minimize thermal gradient before reflow
    • Slow ramp rate (belt speed)
    • “Shoulder” soak
  • Effect of nitrogen

profile-recommendations

Solder Beading

  • Isolated paste aggregates
  • Low stand-off components
  • Pad design

solder-beading

Solder Balling

Small particles isolated from solder joint

  • Origin
  • Flux spattering
  • Flux slump
  • Solder mask
  • Oxidation

solder-balling

Graping

  • Small print deposits translate to large surface-area-to flux ratio
  • Fluxing capacity diminished
  • Flux “run-away”
  • Oxidized solder particles
  • Mask defined vs. non-solder mask defined
  • Resistors more prone than capacitors

graping