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Ionic Cleanliness Testing Research of Printed Wiring Boards For Purposes of Process Control
Ionic Cleanliness Testing Research of Printed Wiring Boards For Purposes of Process Control
Dr. Mike Bixenman, Kyzen Corporation
Dr. Ning Chen Lee, Indium Corporation
Steve Stach, Austin American Technology
SMTAI
Dr. Mike Bixenman, Kyzen Corporation
Dr. Ning Chen Lee, Indium Corporation
Steve Stach, Austin American Technology
SMTAI
Agenda
• Background • Why is Cleanliness Testing Important?• New Fluxes Designs• Predicting Solvent Action • Hypotheses• Methodology• Data Findings • Accept / Reject Hypotheses• Inferences from the Data
Ionic Cleanliness Testing
• In use since the sixties• Ionic contaminants dissolved in the extract solution• Solubility of residue needed to measure ionic levels
Source: http://www.scscoatings.com/parylene_equipment/omegameter.aspx
Cleanliness Testing Importance
• Product reliability is directly relative to ionic cleanliness
Assessing Cleanliness Today
• Non-destructive testing often fails to detect …– Advanced flux designs– Residues under capacitors, resistors, and array components
R.O.S.E. Testing
• Production Floor Ionic Cleanliness Standard for 37 years
• IPA75% / H2O25% extract solution
Era of Change
Moore’s Law
Source: http://en.wikipedia.org/wiki/File:Transistor_Count_and_Moore%27s_Law_-_2008.svg
No-Clean Soldering
• Disruptive technology empowered by Ozone Depletion • Goal was to eliminate in-production cleaning
Source: http://en.wikipedia.org/wiki/Ozone_depletion
Miniaturization
• Most obvious electronics industry trend • Moore’s law serving as the engine• Feature size reduction
– Mechanical hole pitch and via reduction– Component reduction
• Drives multiple changes in flux technology
New Flux Designs
• Miniaturization Drives Change– Flux Consistency– Oxide– Oxygen Penetration Path– Flux Burn Off– Wetting Speed – Spattering– Soldering Under Air – Lead Free
• High Temperature • Poor Wetting • Large Dendrite
Source: Lee, 2009
Flux Consistency
• Solder power shifting from – Type 3 ~ 25 to 45 microns– Type 4 ~ 20 to 37 microns
• Flux requires changes for printing and soldering yields– More homogenous– Increased viscosity ~ Thixotrophic
• Assembly process is more vulnerable to – Bridging ~ requires more slump resistance
Source: http://www.indium.com/blogs/Solar-Blog/No-Slump-Metallization-Paste/20080730,38,2862/
Oxide Formation
• Volume reduction– Alloy and Flux – Reduced in proportion to decreasing pitch
• Solder materials shrink in proportion to pitch– Thickness of metal oxide does not shrink in proportion to pitch– Amount of oxide to re removed by unit volume of flux
• Increases with decreasing pad dimension
• Capacity per unit amount of flux needs to be increased
Oxygen Penetration Path
• While pitch and pad size decrease – Oxygen penetration path through flux and alloy also decrease– Results in rapid oxidation of
• Flux materials ~ increased cleaning difficulty
• Increased levels of flux
– Flux with greater oxidation resistance and barrier needed
Flux Burn Off
• Vaporization of solvent carriers in flux – Increases with increasing exposure area per unit volume– Flux burn off increases with decreasing flux quantity deposited
• Flux employed for finer pitch needs to be– More non-volatile– High molecular weight materials needed
• Changes solubility parameter
• More resistance to burn off
Wetting Speed
• Fast wetting can cause problems during reflow soldering• Defects due to unbalance wetting force
• Tombstoning
• Swimming
– Increase with decreasing component size • Sensitivity toward miniaturization
• Fluxes with slower wetting speed allow– More time for the wetting
force to be balanced – Decreases defect rate
Source: http://www.xs4all.nl/~tersted/PDF_files/Plexus/tombstoning.pdf
Spattering
• Caused by moisture pick-up in solder paste • Miniaturization brings
– Solder joint closer to gold fingers– Increased vulnerability toward solder spattering– Fast solder coalescence action increases the problem
• To minimize spattering– Fluxes with low moisture pickup and wetting speed are needed
Soldering Under Air
• Miniaturization causes – More oxides – Easier oxygen penetration
• Soldering in air increases the change in – Flux compositions
• Desired flux should provide– Oxidation resistance – Oxygen barrier to protect
parts during reflow
Source: http://www.circuitmart.com/pdf/nitrogen_effect_for_wave_soldering.pdf
Lead Free
• Main stream alloys – High tin compositions
• Surface finishes – OSP– HASL– Immersion Ag, ENIG, and Sn
• Complexities include– High Temperature– Poor Wetting – Large Dendrite
Lead Free High Temperature
• High tin alloy melting range– 217-227C– Soldering temp usually 20-40C higher than eutectic SnPb
• Higher temperature causes – Increased thermal flux decomposition– More flux burn off – Oxidation of fluxes and metals
• To avoid problems caused by high temperature soldering– Flux require higher thermal stability– Higher resistance to burn off – Higher oxidation resistance– Higher oxygen barrier
Poorer Wetting
• Surface tension of lead free alloys– SAC ~ 0.55-0.57 N/m– 20% higher than SnPb ~ 0.51N/m
• Results in poorer alloy – Wetting – Spreading
• Deficiencies are compensated by new fluxes – Lower surface tension to improve solder spread– Higher flux capacity ~ higher flux strength
Large Dendrite Formation
• Does not normally cause early failures• Tin crystalline lattice and unfavorable grain orientation
– Poses reliability concern
• Reliability concern can be alleviated by – Forming solder joint with refined grain structure– Improved flux compositions
Limitations of R.O.S.E. Testing
R.O.S.E. Problems
• Test method relies on dissolving flux residue• Many of the new flux compositions do no dissolve • Ionic contaminants in flux not detected
Hypotheses
H1: The extract solution (IPA75% /H2O25%) will not adequately dissolve many of today’s flux technologies
H2: A new test solvent that dissolves all flux technologies is needed
Predicting Solvent Action
• Hildebrand (1936)– Solubility of a solvent’s ability to dissolve a contaminant – Proportional to cohesive energy of the solvent– Solvent molecules overcome soils with similar solvency behavior
• Hansen (1966)– Hildebrand’s Theory broken into 3 – parts
• Dispersion Force
• Polar Force
• Hydrogen Bonding Force
Components of Hansen Space
• Dispersive Force– Predominates for non-polar soils
• Polar Force – Differences in electronic dipole differences – Positive and negative electron forces attract
• Hydrogen Bonding – Ability to exchange electrons
Source: Hydrogen Bonding, 2009, Wikipedia
Two Dimension View of Hansen Space
Teas Charts
3-D Plot of Solvent Properties
Research Methodology
Methodology
• 9 Flux Residue compositions evaluated – Rosin ~ 1 – No-Clean designed for Tin-Lead ~ 5– No-Clean designed for Lead-Free ~ 1– Water Soluble designed for Tin-Lead ~ 1 – Water Soluble designed for Lead-Free ~ 1
• 20 Solvents with known HSPs • Flux residues exposed for 1-hour to 20 Solvents• Solubility Graded for each Solvent • Data placed into HSPiP Software • HSP for flux residue calculated
Grading Scale
Interaction Zone
Relative Energy Difference
• Ra ~ Given solvent and its reference value • Ro ~ Interaction radius of the sphere
RED = Ra/Ro• 0 = No energy difference – Easily Dissolved• <1 = Inside Solvents – High Affinity • Close to 1 – Boundary Condition – Disperses Soil • > 1 = Outside Solvents – Low Affinity
Data Findings
Rosin
No-Clean #1
No-Clean #2
No-Clean #3
No-Clean #4
No-Clean #5
No-Clean # 6 ~ Lead-Free
Water Soluble #1 – Lead Free
Water Soluble #2
Flux Compositions are Different
Accept or Reject Research Hypotheses?
H1 - IPA 75% / H2O will not dissolve today’s flux residues
Second set of tests were run
Kinetics vs. Thermodynamics
• Thermodynamics– Solvency parameters for dissolving the soil
• Kinetics – Thermal – temperature affects– Impingement – energy affects
How does the HSP change when heat and impingement are applied?
IPA75% / H2O 25% - 72°F
IPA75% / H2O 25% - 100-110°F
IPA75% / H2O 25% - 175-180°F
Inferences From the Data
R.O.S.E. Test Method
• Dependent on extracting ionic residues– Method requires dissolving the flux residue
• The data suggests – ROSE method is not suitable for measuring ionic cleanliness of
flux residues outside the IPA/H2O interaction zone
• Controversial Statement in that ….– No-Clean encapsulates ionic residue
• But do they?
– Miniaturization reduces the distance between conductors• What about residues that bridge conductors?
Temperature Changes HSP of IPA/H2O
• At 72°F – IPA/H2O effective on few flux residues
• At 100-110°F – IPA/H2O effective on rosin and few no-cleans
• At 175-180°F – IPA/H2O effective on a majority and not effective on others
• Workable for many Ion Chromatography extractions but – Not effective for R.O.S.E. testing on many flux residue types
New Solvents and Test Equipment
• Data Suggests– Range of test solvents needed that match up with today’s soils– Test equipment needed with
• Higher energy capabilities
• Ability to remove residues under low clearance parts
• Site specific test capability
Follow On Research
• Characterize the HSP of today’s solder flux products
• IPA75%/H2O25% HSP on solder flux products
• Cleaning Agents that match up to flux soils – Data indicates no one test solvent will work
• New instrumentation that provides – Improved impingement energy – Higher temperature capability– Site specific
Acknowledgements
• Carolyn Leary, Cassie Leary, John Garvin, and Kevin Soucy of Kyzen Application Testing and Research for funning the data sets
• Steven Abbott of Hansen Solubility for help in learning and getting around in the HSPiP software
• Dr. Bill Kenyon and Dr. Ken Dishart for supplying ionic testing history and Hansen Solubility research methods
Questions or Comments
Dr. Mike Bixenman of Kyzen Corporation
Dr. Ning Chen Lee of Indium Corporation
Steve Stach of Austin American Technology
