Reliability Considerations from Flux Residues trapped ... · Reliability Considerations from Flux...

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

©2016 Kester Corp - Kyzen Corp.

Outline/Agenda

Introduction

Problem Statement

Purpose Statement

Research Hypotheses

Test Board

DOE

Responses

Conclusions

INTRODUCTION


Electro-Chemical Failures

Smaller form factors present

Limitations

Obstacles

Challenges

Time delayed effects

Residues trapped under components

Can be active

Mobilized

Metals can migrate


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


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


PROBLEM STATEMENT


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


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


PURPOSE STATEMENT


Research Purpose

Research the effects of

Different solder paste activator packages

Cleaning

No Cleaning

Partial Cleaning

Total Cleaning

Reflow profiles

Ramp-to-Spike

Soak


RESEARCH HYPOTHESES


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


TEST BOARD


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”

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


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


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.


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.


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.


DOE


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

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

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

Cleaning Tool Setup


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

effects ©2016 Kester Corp - Kyzen Corp.

Responses

Surface Insulation Resistance

Residues

Dye Pry Method

Ion Chromatography


SURFACE INSULATION

RESISTANCE


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

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

Ramp Reflow Profile


Soak Reflow Profile


Device Responses

Analysis of all the SIR readings data by device show that the QFN100’s

have the largest variance in data

QFN 100


Activator 1 for QFN 100




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





SIR Results and Discussion

The backbone of flux composition

Solvents

Additives

Other types of activators

Greater decisive impact on reliability than

does Halogen-Free


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


VISUAL RESIDUES


QFN 44

Part profile

Activator 1

Soak Profile

Uncleaned


QFN 100

Part Profile

Activator 4

Soak Profile

Uncleaned


QFN 100

Part Profile

Activator 4

Soak profile

Partial Cleaning


QFN 100 & QFN 44

Part Profile

Activator 4

Soak profile

Total Cleaning


ION CHROMATOGRAPHY


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


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


HYPOTHESES TESTED


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


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


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


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


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


CONCLUSIONS


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


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


Thank you Mike Bixenman

Kyzen Corp.

[email protected]

Bruno Tolla, Ph.D.

Kester Corp.

[email protected]

Jennifer Allen

Kester Corp.

[email protected]

Kyle Loomis

Kester Corp.

[email protected]


mailto:[email protected]




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


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