Optimizing Reflow Profiling in Lead-Free SMT Assembly by Indium Corporation

Edward Briggs
[email protected]

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

• 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

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

Profile recommendations

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

Solder Beading

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

Solder Balling

Small particles isolated from solder joint

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

Solder Beading/Balling

Profile recommendations

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

Voiding

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

Profile recommendations

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

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