Lead-free fluxes used in solder paste, liquid flux for wave soldering, flux gels and wire solder are available today. These flux systems are designed to enhance the soldering process and are formulated to give excellent solder wetting performance with the added thermal stability of the chemistry, required with lead-free assembly. Traditional fluxes used with tin-lead alloys may not be adequate to circumvent the slower wetting of lead-free alloys and the higher temperatures normally associated with lead-free solders. Flux systems specifically formulated for lead-free soldering will require new activator packages and heat stable gelling and wetting agents to avoid solder defects. Due to the slower wetting and higher surface tension of many lead-free alloys, choosing the right flux for lead-free soldering will prevent the increase of solder defects and greatly assist in maintaining production yields. Typical defects, which can show an increase when transitioning to lead-free assembly are detailed below. These defects can be eliminated with proper flux selection and process control.
• Potential Defect Increase – Lead-free SMT Assembly
◦ Bridging – Paste with poor hot slump behavior
◦ Solder balls – Paste with poor slump properties
◦ Tombstoning – Thermal differences across board
◦ Non-wetting – Excessive preheating or inadequate flux activity
◦ Poor wetting – Poor flux activity or excessive preheating
◦ Solder Voids – Thermal profile too low, or inadequate flux chemistry
◦ Solder beading – Paste with poor hot slump or excessive preheating
◦ Potential Defects Increase – Lead-free Wave Soldering
◦ Bridging – Flux deactivation during preheating or solder contact
◦ Icicling – Flux too low in activity or preheating temperature to high
◦ Solder Balls – Insufficient preheat or flux-solder mask incompatibility
◦ Insufficient Hole-Fill – Flux activity too low, too low solids, or excessive preheat temperature or too low a contact time with molten solder
•Requirements for a Lead-Free Flux:
◦ Low-activation temperature
◦ Adequate shelf life
◦ High activity level
◦ High reliability
◦ Residues benign or easily remove with water if the paste is a water washable type
•Other Considerations for the Lead-free Flux :
◦ Is the paste for dispensing or printing?
◦ Note manufacturers use different types of activators for different alloys
◦ Select flux carefully to balance activation temperature with thermal profile
◦ What is the compatibility of the flux with the alloy selected?
◦ What are the reliability properties (SIR, electro-migration, corrosion)?
Considerations for Lead-free Solderpaste
•Important properties to consider during selection: ◦Solder Balling Test activity
◦ Wetting Test , specific finishes and solder atmosphere (air or nitrogen)
◦ Voiding Potential, lead-free alloys are more prone to solder voids
◦ Tack Life Over Time
◦ Stencil life and abandon time
◦ Cold Slump
◦ Hot Slump tested to higher temperatures 180-185 C ° .
◦ Shelf-life Testing
•Properties to evaluate in-process:
◦Printability
◾Relax/Recovery
◾Print Speed
◾Durability
◦ Component Placement
◾ Drop back for tack
◦ Reflow
◾ Examine solder joint formation on a variety of leads and PWB finishes
• Properties to evaluate after reflow
◦ Thermal Shock
◦ Thermal Cycling
◦ Impact Resistance
◦ Reliability (SIR)
Technical Considerations for Wave Soldering Fluxes Designed for Lead-free Assembly
• Ability to be evenly applicable by spray, wave, or foam applications
• Activator package able to sustain higher preheating temperatures
• Able to be used with a variety of lead-free finishes, bare copper OSP, gold nickel, tin, silver immersion, tin-copper
• Sustained activity, the flux should remain active throughout the contact time with the molten solder, insuring good peel back of the solder
• Low dross potential, the flux must not react excessively with the molten solder as to create large amounts of dross
• The flux must not discolor or char at the higher soldering temperatures associated with lead-free wave soldering
• The flux should not decompose at the higher solder temperatures
• The flux residues must be benign if it is a no-clean flux, and easily washed in hot water if it is a water washable flux type
Technical Considerations for Lead-free Cored Solder Wires
•The flux should not spatter or fume excessively at the slightly higher soldering temperatures associated with lead-free soldering
•The flux should have activator systems designed to solder a variety of lead-free board and component finishes
•The flux must be active enough and remain active enough during tip contact to compensate for the reduced wetting of lead-free alloys
•The flux residues must be benign if it is a no-clean type, or easily removed in hot water if it is a water washable type cored solder
•The residue should not char or darken in color when using slightly higher solder tip temperatures
Source: Kester Solder Co.