Lead Free Surface Mount Technology Ian Wilding BSc Senior Applications Engineer Henkel Technologies
Overview of the Presentation First contact: Impact on the production operator Packaging Labelling Impact on the process Side-by-side comparison of tin/lead and Pb-free Switching to Pb-free Training the eye: Inspection of Pb-free solder
LF300 Designed specifically for Pb-free process Paste is to be a drop in replacement for MP100 For the printer NOT for reflow! Same shelf life as MP100 Same recommended storage conditions as MP100
First contact: Impact on the production operator Packaging Paste is still available in jars and semco cartridges Note the colour of packaging For LF300 jars, Green lids For LF300 cartridges, Green cartridge and caps
First contact: Impact on the production operator Packaging Mass of paste in the packaging For both LF300 and MP100: 500g in jars For MP100 650g in semco, for Pb-free 600g in semco» (roughly same volume!)
Impact on the production operator Labelling New motif Green stripe denotes Pb-free
Impact on the process What needs addressing on the production line when switching from MP100 to LF300? Solder Paste: alloy switch Solder Paste printing settings: no significant change Component placement: no change Reflow soldering temperatures of the solder paste: major zone changes Inspection of Pb-free joints: retraining of what is deemed a good finish
Loctite Multicore Alloy Specifications, % What is the alloy switch? Metal 96SC SN62 Sn Remainder (95.5) 61.5-62.5 Ag 3.6 4.0 1.8-2.2 Cu 0.6 0.8 - Pb < 0.1 remainder(36) Bi < 0.05 - Cd < 20 ppm -
Loctite Multicore Alloy Note the colour change! Sn62 ADP powder 96SC AGS powder
Printing Comparison LF300 is formulated with the same metal volume % as the MP100 paste lower density alloy LF300, 88.5% wt % metal paste lower density paste so paste roll needs to be larger to get same drop off as Sn/Pb pastes Powder size same as MP100 AGS paste for the same process LF300 print definition arguably better than MP100
Printing Comparison LF300 print speed range is comparable with MP100 Print pressure can be lowered if desired LF300 abandon time, open time, tack time is comparable with MP100
Printed Paste Properties Comparison Slump comparable with equivalent MP100 pastes additional slump may overcome reduction in alloy spread during reflow Tack force comparable with equivalent MP100 pastes
Switching to Pb-free Change in alloy impacts reflow temperature SN62 liquidus at 179C 96SC liquidus at 217C Reflow zone temperatures therefore have to be increased Higher temperatures has impact on surface cosmetics
Flux Exhaustion and Solder Spread Increasing Severity of Reflow
Pb-free Paste Process Issues Reduced spread - particularly on Cu OSP increase apertures to 100% pad size Mid-chip solder beads increased alloy surface tension and reduced wetting may reduce this defect
Training the eye: Inspection of Pb-free solder Sn62: shiny and smooth surface 96SC: dull and pitted marbled effect
Key inspection differences
Key inspection differences
Key inspection differences
Key inspection differences
Key inspection differences
Why there is no need to be afraid! A change in reflow process settings, but what has changed on the board? Paste Surface cosmetics method of assessment What hasn t changed? Printed deposits Inspection method Good reliable joints Your ability to recognise a change in alloy and process
Microstructure of 96SC Interdendritic Intermetallic Needle Intermetallic Interfacial Intermetallic Copper
Dull Joint Appearance SnAg3,8Cu0,7 Sn dendrites visible Rough surface Dull appearance
Surface Shrinkage on Pb-free Solder Joint Real process cooling rates produce noneutectic structure Tin dendrites surrounded by solidification contraction Cosmetic. Not a reliability issue
Wetting on Cut Terminations Flux reflowed in nitrogen Flux reflowed in air Possibly due to surface tension
Voids in Solder Joints Voids are present, but only at 7.5%
SEM sections
Example of Flux Voids
Micro-sections
Summary of Inspection of Pbfree Solder Joints Traditionally IF A JOINT LOOKS GOOD IT IS RELIABLE The evidence shows that the same applies to Pb-free solder joints what constitutes good is only slightly changed provided there is good wetting and no flux exhaustion, reliability is not sensitive to reflow profile
Conclusion: A direct comparison Printing definition pin-testing residue reliability 30 25 stencil life abandon time oven contamination reflow temperature window wetting power 20 15 10 5 0 fine pitch squeegee speed squeegee pressure mid-chip balling tack force solder balling tack time residue quantity slump resistance residue colour MP100 LF300
The Pb-free Reflow Process Challenge Achieve good wetting on the coldest part of the circuit board 235 C for 10 seconds is the minimum Minimise the temperature of the hottest part of the circuit board minimise T Keep the process stable Monitor the process accurately
Realistic Worse Case Reflow Board T +20 C Longer, Hotter Preheat Melting Temperature 217 C Stability Error +5 C Wetting Adequate +20 C Hot Spot 265 C Measurement Error +5 C Cold Spot 235 C
Consequences of Assembling Complex Boards Heat sinks, shielding, large components large T across the board longer time spent equilibrating temperature Greater total heat input hot spots get much hotter than necessary for wetting component and board damage flux exhaustion
Minimising Thermal Damage Improved oven designs to reduce T Improved measurement systems work closer to the limits Operate reflow in nitrogen less reoxidation of metal surfaces less flux exhaustion less oxidation of resins MORE EXPENSE Use Condensation Reflow Equipment Only economic for special applications
Reflow for Simple Boards Temperature, C 300 200 100 Very small T, little or no preheat soak required 200 400 600 800 1000 Time/s Hottest spot Coldest spot
Reflow for Relatively Complex Boards Temperature, C 300 200 100 Peak temperature may be higher Modest T, preheat soak required 200 400 600 800 1000 Time/s Hottest spot Coldest spot
Reflow of Heavy, Complex Boards Temperature, C 300 200 100 Large T, long preheat soak required Peak temperature higher than needed for wetting 200 400 600 800 1000 Time/s Hottest spot Coldest spot
Effect of pre-heat soak on flux medium Pre-heat Particles fully encased in flux medium Solvent loss Particles coated with protective resin film
Flux Exhaustion and Solder Reflow Increasing Severity of Reflow
Comparison of Alloy Properties Alloy Sn62 SnAg3.8Cu0.7 Melting point 179C 217C Density (g/mm2) 8.5 7.5 Surface tension @ 260C in air Wetting angle on Cu (deg) * Values for Sn60Pb40 ** Values for pure tin 481 mnm-1* 548 mnm-1** 12* 43
Wetting angle on copper 43 0 96SC 12 0 Sn/Pb Copper
Lower Density Lower metal content (mass%) for same rheology Same metal content (volume%) for same rheology Less ability to roll and drop-off squeegee Small differences to rheology required Less buoyancy for void removal Bigger volume and therefore more joints per Kg!!
Higher Surface Tension Increased wetting angle - less spread especially on OSP copper Print over whole pad area Or change inspection criteria More difficult for voids to escape One cause of higher voiding in lead-free joints
Voids Void levels significantly higher in lead-free joints Especially when using Pb-free solder on Sn/Pb BGA bumps Generally not a reliability problem Can cause shorts if large void in fine pitch BGA Can be affected by flux chemistry
Higher Melting Point Reflow Window Narrower process window with Pb-free High reflow temperature Peak T limited by components Low T across board needs effective heat transfer Longer, hotter preheat But more stress on paste Higher power, higher air flow ovens more protective fluxes required
96SCLF300AGS88.5 Paste Linear Reflow Profile Window 300 Temperature/C 250 200 150 100 50 0 Longer, hotter profile 0 200 400 600 800 1000 Time/s Good Poor
96SCLF300AGS88.5 Preheat Soak Reflow Process Window 300 Longer, hotter profile 250 Temperature/C 200 150 100 50 0 0 100 200 300 400 500 600 700 800 --- Time/s --- GOOD POOR
Pb-free Paste Reflow Issues Linear profile preferred for flux BUT high T? Component damage due to higher soldering Temperatures Lack of components with lead-free finishes Incomplete pad coverage - exposed Copper - Leaching of Ag/Pd finishes High Sn content faster dissolution Higher temperatures faster dissolution Dull joints - revise inspection criteria? -
Pb-free Paste Process Issues Mid-chip solder beads increased alloy surface tension and reduced wetting may reduce this defect Tombstone defects sensitivity may increase or decrease Lower wetting angle less wetting force for chip raising Larger T more likely uneven chip wetting anti-tombstone Pb-free alloy blends available
Summary of Pb-free Reflow Process Process window is smaller than that for Sn/Pb pastes expect to be more important to match paste and process industry will see developments over the next couple of years Hardware developments likely Component compatibility increasing
Any Questions?