Cleaning Before Coating

Presented by

Jigar Patel, Senior Process Engineer

Influencing factors

Failure mechanisms

Coating failures

Cleaning before coating

Analytics and test methods

Cleaning Before Coating

Mechanical

Electrical

Chemical Thermal

Influencing Factors

Atmospheric change

Salt spray

Humidity Migration

Vibrations

Corrosion Corrosive gas

Temperature

Influencing Factors

Trend

Larger components

Low component densities

High standoff spaces

Simple component types

Miniaturized components

High component densities

Low standoff spaces <1mil

Complex component types

Industry Trends

Coating protects against failures due to

humidity:

Electrochemical migration

Leakage current

Signal distortion on high-frequency

circuits

Advantages of Coating

Manufacturing of

PCBs

Residues from acid

processes

Developer

chemistry

Alkaline cleaner

HAL-Residues

Residues of ENiG-/

CSN-Process

Rinse water quality

Manufacturing of

Components

Residues from

metallization baths

Rinse water quality

Deflashing Chemicals

Mold release agent

Flux residues

Assembly

Solder pastes

Wave soldering

Rinse water quality

Reflow soldering

Reflow condensates

Gas emission

Hand perspiration

etc.

Possible Contamination

Resin or rosin

Activator – organic or inorganic

Solvent (dilution)

Thixotropic agent

Fluxes/pastes

Contamination due to handling

i.e. Fingerprints, dust, oil, salts

Critical Residues

Influencing factors

Failure mechanisms

Coating failures

Cleaning before coating

Analytics and test methods

Cleaning Before Coating

Electrochemical Migration

Leakage current

Signal distortion on high-frequency circuits

Climatic Failure Mechanisms

Dendrite growth between solder joints

Climatic Failure Mechanisms

Influence factor:

Moisture

Material factor:

Fluxes

Influence factor:

Humidity

Electrochemical Migration

Vapor/Humidity

+

Ionic contamination

(salts, flux activators)

=

Conductive electrolytes

+

Stress voltage

=

Electrochemical migration

Ground

VCC

Influence factor:

Condensation

Influence factor: Stress voltage

Material factor:

Fluxes

Influence factor:

Humidity

Electrochemical Migration

Reaction rate

Ele

ctr

ode pote

ntial E

SnO2 – H

2O

Passivity

Passivity

Immunity

Sn0

Sn02

Sn

Corrosion

SnH4

Corrosion

Corro-

sion

Sn4 +

Sn02 –

3

Sn2 +

– HSn0

2

pH-value

2,0

V

1,5

1,0

0,5

0

-0,5

-1,0

-1,5

-2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

OH- alkalinity

Dissociation of Water and Tin Corrosion

Mechanisms of Dendrite Growth

Ion migration

MeMen+

+ ne*

V

ne* + Men+

Me

Anode PCB Cathode

Deposition

MeMen+

+ ne* ne* + Men+

Me

Anode PCB Cathode

Dissolving

MeMen+

+ ne*

Metal-ions

Anode PCB Cathode

Dendrite Growth

Lead free/Ag containing solder pastes

tend to build up short-lived dendrites

→ Electrochemical migration

→ Proven by continuous resistance

measurement

Climatic Failure Mechanisms

Electrochemical migration

Leakage current

Signal distortion on high frequency circuits

Leakage Current

Optically clean FR4 surface

Loading contrast of optical invisible,

conductive contaminations (SEM)

High Non-critical

Activator Residues

Danger of leakage current under conformal coating

After climate change

Cracks

Activator Residues

Important: no humidity, only temperature change – day/night

→ Activators influence leakage current and climate safety

Coating Resin layer

After soldering

Flux indicator

Climatic Failure Mechanisms

Electrochemical migration

Leakage current

Signal distortion on high frequency circuits

Conventional contact connections

HF contact connection

Influence of Geometry on the Complex Resistance

Inductance

Capacity

1

0

Signal

Time

Signal

1

0

Time

Bit Failures in HF Connections

Dielectricity of the rosin/resin disturbs transmission

Cleaned PCB no transmission failure

Bit failure

Cleaning Before Coating

Influencing factors

Failure mechanisms

Coating failures

Cleaning before coating

Analytics and test methods

Blister type failure Electrochemical migration underneath

protective coating

Coating Failures

Wetting failures under components Pore formation and delamination

Coating Failures

Coating Failures

Cracks due to temperature change and reaction of coating with resin

14000

12000

10000

8000

6000

4000

2000

0

20° C 30° C 40° C 50° C 60° C 70° C 80° C

68° F 86° F 104° F 122° F 140° F 158° F 176° F

PU-coating

AY-coating

Wax coating

Wate

r vapor perm

eability,

sta

ndardiz

ed [g / m

2 x h]

Temperature

Water Vapor Permeability

Contamination

Influences on Uncleaned Substrates

Contamination reduces adhesive forces and leads to delamination

Residues Consequences

Resin based residues as part of the flux

Subsequent delamination when exposed to

climatic changes

Cracks in coatings

Failures in HF connections

Flux activators

Hygroscopic – danger of failure

Corrosion on connections

Leakage currents

Organic tin-salts (reaction product from flux and

solder paste)

Insufficient adhesion and creeping underneath

coatings

Sulphur and ammonium compounds (can be

contained in fluxes)

Insufficient adhesion and creeping underneath

coatings

Chemical facilitators for component release Impacts wetting properties

Delamination possible

Oligomeric parts of substrates and solder mask

interaction

Contamination film

Similar effect as with chemical facilitators

Surface Tension and Cleaning

Higher surface tension = better coating adhesion

Cleaning Before Coating

Contamination under coating is hygroscopic

Limited adhesion on un-cleaned PCB

Coating alone is inadequate because of vapor permeability (even worse

than cleaning without coating)

Coating is applied to increase climatic reliability

→ Reliable coatings require cleaning confirmed by coating suppliers

Cleaning Before Coating

Influencing factors

Failure mechanisms

Coating failures

Cleaning before coating

Analytics and test methods

Partially removed

residues

Partially attacked

residues

CRITICAL!!!

Almost removed residues

LESS CRITICAL!

Flux Residues completely

removed

TARGET!

Stages of Flux Removal

Aqueous

cleaning

Spray-in-air Ultrasonic Spray-under-

immersion

H2O

Semi-

aqueous

Cleaning

Cleaning Before Coating

Ultrasonic Spray-under-immersion

Aqueous Spray-in-Air High Pressure Inline Process

Dosage

Station

Pre-

Wash

Chemical

Isolation Rinse Final Rinse Dryer Dryer Wash

Treatment system Treatment system

2

1

3

4 5

6

7

Pre-

Wash

Chemical

Isolation

Rinse Final

Rinse Dryer Dryer Wash

Process characteristics:

Medium to high throughput

Spray pressure 70 – 140 psi

Aqueous Spray-in-Air Batch Process

Chamber

tank

Rinsing 1

with tap water

Rinsing 2

with DI-water

Tank for

cleaning agent

Heating

Process characteristics:

For low to medium throughput

Spray pressure < 50-70 psi

Small footprint

Cleaning Before Conformal Coating

Influencing factors

Failure mechanisms

Coating failures

Cleaning before coating

Analytics and test methods

Requirements for Coating

Evaluation of cleanliness (prior to coating):

Test Required value

Ionic contamination < 0.4 µg/cm2

Surface tension > 40 mN/m

ZESTRON® Flux Test Residue-free

ZESTRON® Resin Test Residue-free

Test Method – Ionic Contamination

Measurement using an ionic contamination meter

Test Method – Ionic Contamination

Process schematic

VE - H 2 0 + IPA

PCB

Regeneration

Measuring cell

Pump

Bad wetting= unclean surface

Good wetting= clean surface

Wettability and Surface Cleanliness

Testing method – ink test

Activator Analysis

Application

Rinsing

Drying

Resin residues do not become colored

Samples of Results after Flux Test

No encapsulation of activators in the resin

Resin Analysis

Approval of synthetic and natural resins

Shows local dispersion

Based on color reaction

Samples of Results after Resin Test

Resin residues are colored

After Before

Test Method – Tin Test

Evidence of stannous components

Based on color reaction

Tin test positive

Tin test negative

Coating Approval

Coating

Test

Climatic Reliability

Coating Reliability Test

Climatic Reliability

Test according to IEC 68-2-3 Ca

40°C, 93% RF, 21 days

Ag-containing solder pastes have a tendency to illustrate

temporary dendrites

Gas development

at defects

+

– 1kOhm

Coating Analsysis

Coating Reliability Test (CoReTest)

Advantages of the Coating Reliability Tests

Weak point analysis

(Coating Reliability Test)

Life expectancy test

(for example IEC 68-2 Standard)

+ During development - After development (type approval)

+ Fast (max. 10 hrs) - Time consuming (about 6 months)

+ Low costs - High costs

+ Identification of all weak points - Failures could remain undiscovered

- Pseudo failure rate +No pseudo failure rate

Conclusion

Coating neccessary due to moisture / condensation

Residues harm the coating

Simple test methods for Q-analysis available

Cleaning increases reliability of coating

Different cleaning processes are available

Cleaning Before Coating

Presented by

Jigar Patel, Senior Process Engineer