Reliability Considerations from Flux Residues trapped ... · Reliability Considerations from Flux...
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Reliability Considerations from
Flux Residues trapped under
Component Terminations
Mike Bixenman, DBA, Kyzen Corporation
Bruno Tolla, Ph.D., Jennifer Allen, Kyle Loomis, Kester Corp.
Kester – Kyzen Joint Research Project
Originally published in the Proceedings of SMTA International, Rosemont,
Il, September 25-September 29, 2016
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Outline/Agenda
Introduction
Problem Statement
Purpose Statement
Research Hypotheses
Test Board
DOE
Responses
Conclusions
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INTRODUCTION
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Electro-Chemical Failures
Smaller form factors present
Limitations
Obstacles
Challenges
Time delayed effects
Residues trapped under components
Can be active
Mobilized
Metals can migrate
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Leadless Components
Low Standoff gaps + High soldering mass
Block flux outgassing channels
Residues accumulate and bridge conductors
Can still contain solvents and activators
Small moisture levels can lead to
Leakage currents
Dendritic growth
Shorts
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No-Clean Solder Pastes Engineered to render a
Benign post soldering residue
During reflow
Solvents are designed to outgas
Activators oxidize and reduce
Remaining activators and fluxing by-products
Designed to be encapsulated into a resin binder
The design is to yield an inert residue
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PROBLEM STATEMENT
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Flux Residues under BTCs
Several factors that can cause failures
Flux outgassing channel is compromised
Solvents do not totally evaporate
Live residue can be present
Polar
Hydroscopic
Acts as a media for corrosion
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Failed Devices
BTC devices fail due to
Dendritic growth under the component
termination
Location of dendrites correspond to
Trapped flux residues
Cleaning under BTCs is challenging
Residue remaining under the component post
cleaning may also be problematic
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PURPOSE STATEMENT
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Research Purpose
Research the effects of
Different solder paste activator packages
Cleaning
No Cleaning
Partial Cleaning
Total Cleaning
Reflow profiles
Ramp-to-Spike
Soak
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RESEARCH HYPOTHESES
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Research Hypotheses TestedH1: Flux residues trapped under the bottom termination
create the potential for ion mobilization and current leakage
H2: Flux activators can be designed to reduce current
leakage potential
H3: Process optimization helps to reduce current leakage
as longer profiles promote the conversion of activators into
inert residues
H4: Partial cleaning can expose flux constituents that can
increase leakage potential
H5: Total cleaning reduces current leakage potential
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TEST BOARD
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SIR Flux Reliability Test Board
A non-standard board design was
chosen to place the resistivity
‘sensors’ for a SIR test under various
low standoff devices
1. Board surface finish: OSP
2. Sensor traces: Variable
3. Copper weight: 1oz copper
4. Hole size under QFN: 20 mils or smaller non plated
5. Legend color ink: White
6. Solder mask: LPI, min thickness at the conductor
edge = 8μm (0.315 mil)
7. Fiducial: Qty 3, 0.050’’
8. Board thickness: 0.062”
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SIR Flux Reliability Test BoardSeveral low standoff devices
were selected to obtain a wide
variety of SIR data sets
Devices selected were:
• BGA100 with 0.8mm pitch
• Resistors 2512, 1210 &
0805 passives
• QFN44’s and QFN100’s
Pin out shown is for the A.S.R., 4 channel
B24 connector wiring harness (A,B,C,D)
– no hard wiring required
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SIR Flux Reliability Test BoardChannel D: To achieve a series
of sensors under the BGAs, the
board layout complements the
internal Daisy Chain of each
BGA to form the SIR electrical
gap spacings between selected
balls under each device
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SIR Flux Reliability Test BoardChannel C: To achieve a series of
sensors under the passives, the
board has traces under the central
body of the 12 devices to form the
SIR electrical gap spacings under
each device.
Paste was deposited on the sensor
traces to ensure flux connections
between the sensor traces.
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SIR Flux Reliability Test BoardChannel B: To achieve a series of
sensors under the QFN44 devices,
the board traces form a loop around
the central pad terminal and between
the loop and the perimeter leads to
form the SIR electrical gap spacings.
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SIR Flux Reliability Test BoardChannel A: To achieve a series of
sensors under the QFN100 devices,
the board traces form a loop around
the central pad terminal and between
the loop and the perimeter leads to
form the SIR electrical gap spacings.
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DOE
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SIR Flux Reliability Test BoardThe IPC SIR test method using the open format B24 pattern directs the
user to setup measurement systems to obtain an electrical field strength
between the positive and negative traces (gaps) to 5V for 200um of
spacing.
The table below shows a variety of field strength for the IPC standard
boards and the DoE board at different voltage conditions.
QFN100
Selected voltage Bias was 8V
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SIR Test Parameters
Test Coupon: Kester Flux Reliability Test Board
Bias: 8 volts
Test Voltage: 8 volts
Temperature: 85°C
Humidity: 85% RH
Measurement Interval: every 20 minutes at
condition
Test Duration: 7 Days (168 hours)
Temperature is ramped before humidity
elevated to avoid reaching the dew point.
Inverse applies to the recovery ramp down
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Reflow Profiles
Two different reflow conditions were used
with the intent to subject the flux resides to
low and high heating conditions
Ramp to
Spike
Soak
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Cleaning Tool Setup
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Cleaning Parameters
Cleaning Conditions
No-Cleaning
Partial Cleaning
Inline spray-in-air, 2 FPM, 3 min wash
Total Cleaning
Inline spray-in-air, 0.5 FPM, 10 minute wash
Wash Temperature: 65°C
Subset of parts where removed during
setup to assure partial and total cleaning
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Responses
Surface Insulation Resistance
Residues
Dye Pry Method
Ion Chromatography
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SURFACE INSULATION
RESISTANCE
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SIR Readings obtained
A wide variety of readings were obtained across the 96 unique DoE
combinations of paste activators, sensors and devices, reflow profiles,
and cleaning methods
QFN100
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Cleaning Effects
• The average logarithmic values based the response of each board
• A pattern emerged showing the effect of cleaning on the SIR
responses
QFN100
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Ramp Reflow Profile
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Soak Reflow Profile
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Device Responses
Analysis of all the SIR readings data by device show that the QFN100’s
have the largest variance in data
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QFN 100
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Activator 1 for QFN 100
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Activator 2 for QFN 100
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1.E+04
1.E+05
1.E+06
1.E+07
1.E+08
1.E+09
1.E+10
1.E+11
20
480
940
1,400
1,860
2,320
2,780
3,240
3,700
4,160
4,620
5,080
5,540
6,000
6,460
6,920
7,380
7,840
8,300
8,760
9,220
9,680
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Activator 3 for QFN 100
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Activator 4 for QFN 100
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SIR Results and Discussion
The backbone of flux composition
Solvents
Additives
Other types of activators
Greater decisive impact on reliability than
does Halogen-Free
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Activators 1, 2, 3, & 4 Activator 1 & 2
Halogen Free
Worse reliability performance
Activators 3 & 4
Halide activators
Failures identified by SIR spikes
Characteristic of ECM
Activator 3
Halide based activator
Interplays between chemical reactions, processing conditions
and end-usage environments are thoroughly understood
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VISUAL RESIDUES
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QFN 44
Part profile
Activator 1
Soak Profile
Uncleaned
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QFN 100
Part Profile
Activator 4
Soak Profile
Uncleaned
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QFN 100
Part Profile
Activator 4
Soak profile
Partial Cleaning
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QFN 100 & QFN 44
Part Profile
Activator 4
Soak profile
Total Cleaning
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ION CHROMATOGRAPHY
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Not Cleaned Boards
Only a few ions exceeded guidelines
Nitrate, Nitrite, Sulfate and Potassium
Come from the board
Not a part of the flux package
Flux specific components
Chlorides from Activator 4
Bromides from Activator 3
Weak Organic Acids from all activator packages
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Cleaned Boards
Partial and Total
Chlorides present in Activator 4 require an
extensive cleaning process
Bromides show a similar trend
Weak organic acid levels
Remain stable throughout the test
Similar across activator packages
Zero halogen fluxes
Significant differences in reliability
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HYPOTHESES TESTED
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Hypothesis 1
Flux residues trapped under the bottom
termination create the potential for ion
mobilization and current leakage
Accept
Data conclusive finds that flux residue trapped
under the component has the potential to drop
resistance and current leakage
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Hypothesis 2
Flux activators can be designed to reduce
current leakage potential
Accept
Data conclusively finds that the activator has a
significant effect on resistance and current leakage
Activator 3 was the safest activator package
Should flux residue not be cleaned
If flux residue was still present following the cleaning process
All activator types had high resistance values and
showed no current leakage when parts were totally
cleaned
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Hypothesis 3
Soak reflow profile reduces current leakage
as compared to the Ramp-to-Spike profile
Reject
Interesting finding
Some packages are more sensitive to heat
treatment than others
On QFN 100, soak profile found lower resistivity
values when residue was present
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Hypothesis 4
Partial cleaning can expose flux constituents
that can increase leakage potential
Undetermined
The data finds that partial cleaning be
detrimental for some classes of activators
More experiments are needed to demonstrate
a degradation between uncleaned and
partially cleaned conditions
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Hypothesis 5
Total cleaning reduces current leakage
potential
Accept
The data conclusively finds that total cleaning
improves resistance values
No SIR fails were detected
All activator packages and components types
passed when totally cleaned
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CONCLUSIONS
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Low Standoff Components Present dramatic impacts on the reliability of the
final assembly
Customized SIR test board is valuable in testing
Solder paste types
Cleaning material effectiveness
Cleaning equipment
Environmental conditions
Modeling end use environment
Best approach to studying reliability risks
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Flux compositions Key Finding
Broad class of chemicals used in flux activators have
different activities and moisture sensitivities
Not cleaned and partial cleaned boards can
have meaningful risk assessments
Thorough cleaning below published guidelines
found excellent reliability under all components,
regardless of activator package
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Thank you Mike Bixenman
Kyzen Corp.
Bruno Tolla, Ph.D.
Kester Corp.
Jennifer Allen
Kester Corp.
Kyle Loomis
Kester Corp.
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Acknowledgements Dale Lee from Plexus, for his contribution to the reliability
board layout
Denis Jean, Product Technology Manager at Kester
Chelsea Jewell, Process Development Engineer at
KYZEN Corporation to clean test boards.
Kevin Soucy, Application Manager at KYZEN
Corporation for his help in setting up the machine to run
test boards.
James Perigen, Chemist at KYZEN Corporation for
running the IC analysis on test boards
©2016 Kester Corp - Kyzen Corp.