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ELEC 6740 Electronics Manufacturing:
Chapter 13 – Flux & Cleaning
R. Wayne JohnsonAlumni ProfessorAuburn University
Outline
1. Introduction2. Flux3. Contaminants4. Selection of cleaning methods5. Cleaning processes & equipment6. Cleanliness Test Methods & Requirements7. Design for cleaning
Introduction
! Flux residue" Cleaning dependent on flux type" Clean:
# Water soluble organic acid fluxes# Rosin based fluxes with halides# Resin based fluxes with halides
" Cleaning optional# Rosin based fluxes without halides# Resin based fluxes without halides
Cleaning Difficulty
Cleaning Difficulty
Function of flux
! To chemically react with oxides and quickly produce a fresh, tarnish-free surface at soldering temperatures so that intermetallic bonding can take place
! To prevent re-oxidation during the sodlering cycle
Flux Classification
! L = low or no flux/flux residue activity! M = Moderate flux/flux residue activity! H = High flux/flux residue activity
Flux Classification
Inorganic fluxes
! Hydrochloric acid! Hydrofluoric acid! Stannous chloride! Sodium fluoride! Potassium fluoride! Zinc chloride! Not used for electronics assembly!!!!
Organic Acids
! Citric acid! Lactic acid! Oleic acid! Stronger than rosin fluxes! Require cleaning, aqueous or semi-aqueous! Can be used for military applications
Rosin Fluxes
! Extracted from the stumps or bark of pine trees! C19H29COOH! Abietic acid (70-85%)! Pimaric Acids (10-15%)! Rosin fluxes are inactive at room temperature but
become active when heated to soldering temperatures
! The melting point of the rosin is 172-175oC
Rosin Fluxes
RCO2H + MX $ RCO2M + HX
RCO2H = is the rosin in the flux
M = Sn, Pb or Cu
X = oxide, hydroxide, or carbonate
Rosin Fluxes
Rosin Fluxes
! Rosin fluxes (R and RMA) do not have to be cleaned, especially if halide free
! Sticky and can attract particles! Residue can impact bed of nails testing
" Penetration of probe" Coating probe
Low Residue or No-Clean Fluxes & Solder Pastes (Resin)
Low Residue or No-Clean Fluxes & Solder Pastes (Resin)
! Resin content varies from 1-35%" Solids content
! High solids content $ more residue! Low solids content resin fluxes leave no
visible residue
Low Residue or No-Clean Fluxes & Solder Pastes (Resin)
! Generally not as aggressive as rosin fluxes! Boards need to be handled properly to avoid
contamination! Boards should be used FIFO to minimize storage
time (aging of finish)! Solder balling more of a problem especially with
wave soldering! Compatibility of flux with solder mask and finish
must be determined
Low Residue or No-Clean Fluxes & Solder Pastes (Resin)
! Impact of residue on bed-of-nails testing" Similar to rosin fluxes
! Compatibility with conformal caotings must be determined" Conformal coatings are typically silicone or
urethane coatings dipped or sprayed onto the board surface after assembly to provide environmental protection in the field
Contaminants
Contaminants
! Particulates" Dust" Lint" Solder balls
! Nonpolar" Rosin residues" Synthetic resin" Organic compounds from
low residue/no-clean flux formulations
" Plasticizers from core flux
! Nonpolar (cont.)" Oil used in wave soldering" Greas or oil from palcement
of insertion equipment" Hand lotions or makeup" Release agents on
components" Insoluble inorganic
compounds (oxidation products)
" Rheologoical additives to solder paste
Contaminants & Solvents
Polar Contaminants
! Residues that form ions when dissolved in water" Salts
# NaCl
" Halides" Acids
! Increase the conductivity of water
Polar Contaminants
! Main cause of electromigration" Produces dendritic growth between conductors
and causes shorts" Requires:
# A voltage difference between conductors# The presence of moisture# Ionic material
Polar Contaminants
! Corrosion" Pb + ½O2 = PbO" PbO +2HCl = PbCl2 + H2O" PbCl2 + H2O + CO2 = PbCO3 + 2HCl
" CuO + 2HCl =CuCl2 + H2O
Environmentally Harmful Solvents
CFC Phase-out
Other Selection Considerations
! Economics of usage" Materials costs" Equipment cost" Energy cost" Disposal/treatment costs
! In-house technical expertise! Compatibility with equipment & components
Other Selection Considerations
! Performance! Surface tension
" Gaps and spacings! Streaking properties! Odor! Boiling point! Flash point! Fire hazard
Cleaning
Cleaning Chemicals
Batch Solvent Cleaner
Semi-aqueous Batch Cleaner
Three Stage Semi-aqueous Batch Cleaner
Semi-aqueous Batch Cleaner
In-line Semi-aqueous Cleaner
Effectiveness of Aqueous Detergents
Effectiveness of Aqueous Detergents
Effectiveness of Aqueous Detergents
Chelation
! Chelation is a process mwhereby an insoluble product is dissolved with the aid of a complexant
! EDTA (ethylenediaminetetraacetic acid) and its sodium salts are typical chelating agents
! Chelation is used to dissolve heavy metal salts which are otherwise insoluble in water
! The resultant complex or chelate is water soluble
Closed loop Design with/without Chelating Agent
Aqueous Cleaner
Cleanliness Test Methods
! Visual inspection" 2-10X microscope
! Solvent extraction" Test solvent alcohol and deionized water" Using agitation, submerge board into solvent mixture to
dissolve contaminants" Measure the NaCl equivalent in the solvent per square
inch of board area" J-STD-001 requires less than 10.06µg NaCl equivalent
per square inch
Cleanliness Test Methods
! Surface Insulation Resistance! J-STD-001! IPC-T-650 Test Method
SIR Test Patterns
SIR Test Patterns
SIR Test Patterns
Effect of Humidity on Insulation Resistance
SIR Test Conditions
Bare Board Cleanliness
Example Results
Example Results