Ionic Cleanliness Testing

Research of Printed Wiring

Boards for Purposes of Process

Control

Mike Bixenman, Kyzen Corp.Ning Chen Lee, Indium Corp.

Steve Stach, Austin American Technology

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

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

Dr. Mike Bixenman of Kyzen Corporation

Dr. Ning Chen Lee of Indium Corporation

Steve Stach of Austin American Technology