ELEC 6740 Electronics Manufacturing:

Chapter 13 – Flux & Cleaning

R. Wayne JohnsonAlumni ProfessorAuburn University

334-844-1880johnson@eng.auburn.edu

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