SMT in Line
www.smtinline.com

Overview

• Impact of SMT process equipment is a function of selected solder paste
  • Paste Technology
  • Reflow temperature
  • Flux activity
• In general equipment is capable and compatible
• This presentation examines each SMT process step with a view to the impact of no lead materials

• Matching performance with new powder and flux formulations is as any solder paste.
• For the printing process the lead free alternatives' factors of impact are similar to 63Sn/37Pb:
  • Stencil life
• Environmental conditions(temperature and humidity)
• Response to pause
• Squeegee Speed
• Stencil wiping frequency
• Compatibility with pump head and other pump head technologies

The Component Placement Process

• For the placing process the lead free alternatives' factors of impact are similar to Sn/Pb:
  • Tack (the ability of the solder paste to hold any particular component in position before the solder paste is reflowed)
  • Environmental conditions (temperature and humidity)

Thermal Energy Transfer Comparison

• Radiant (IR) heat transfer: Temperature differential between source and target is 4x the setpoint.
• Forced convection heat transfer: Temperature differential between source and target is directly proportional.

Effect of Convection on Profiles

• More energy is transferred to the product
• Minimal shadowing effect
• Lower heating element temperatures
• Setpoint = board temperature
• Soaking without overheating smaller components
• Small δT between small and big components

Effect of Nitrogen on Profile

• Nitrogen is thermally invisible to the profile
• Convection blowers deliver heat transfer the same in oxygen as in nitrogen
• Nitrogen affects wetting and finish not temperature

Effect of N2 on Solder Joints

• Higher wetting forces when no air is present resulting in shorter wetting time
• No re-oxidation in the oven with nitrogen
• Beneficial for OSP's where the coating volatilizes and breaks down at temp
• Best used with no clean fluxes
• Benefits fine pitch for bridging
• Enhanced cosmetics
  • Bright, shiny solder joints
  • No laminate discoloration

Reflow

• Thermal management
• Flux recovery
• Equipment reliability

The Reflow Soldering Process

• Greatest impact of all the process
• All currentlu preferred no lead solder pastes reflow at higher temperatures (Peak: 215°C) than leaded solder pastes
  • 63/37 reflow takes >45 seconds time above liquidous with a peak 215~220°C (Melting point 183°C)
• Ovens with higher temperatures capabilities may be needed
• Ovens with high mass profile capabilities may be needed
• Due to the nature of the condensate material produces during the reflow process it may be necessary to use a flux management system (active intermal exhaust) to remove the residue - minimizing the oven cleaning requirements

Wave Soldering Process

• Thermal management
• Dross control
• Inert atmosphere - retrofit?
• Flux
• Corrosion of stainless steel - may require a retrofit kit.
  • Tin, at elevated temperatures is corrosive to the stainless steel.
    • MELONITE™ treatment
    • Grey cast iron with Chromium as an additive.
• The introduction of no lead solder will have some level of impact on the wave soldering process and wave soldering equipment:
  • Most common no lead material is 99.3%Sn/0.7%Cu & 96.5%Sn/3.5%Ag may need to be processed at a higher temperature than leaded solders affecting:
    • Solder pot materials of construction
    • Pre heat considerations (Thermal shock)
  • Alloy wetting
• Cost implications
  • Nitrogen soldering
  • Additional heat costs money
  • Equipment life cycle of current equipment

Inspection

• Lead-free solders do not wet as well as traditional solders
• Many lead-free alloys yield a duller surface finish (i.e. less shiny joints)
• Pass/fail criteria may need to be reviewed

Cleaning

• Baked-on fluxes, higher temperature reflow profiles aggregate the cleaning process of "no cleans" materials
  • Many users still clean no-clean materials
    • Improve conformal coating adhession
    • Improve casmetics
    • Meet customer specifications

Process Impact Overview

• No "drop-in" lead free process available
• There may be some equipment considerations in the reflow, wave solder cleaning and inspection processes.
• Definite impact of higher processing temperatures on soldering materials, boards, components and subassemblies.

(10/30/2000)