Advanced Flexible Substrate Technology for Improved Accuracy… … · · 2017-06-22Advanced...

Advances in Flexible Substrate Technology to Improve Resolution, Definition, Accutance, & Conductivity of Screen Printed ConductorsAdvanced Flexible Substrate Technology for Improved Accuracy, Definition, and Conductivity of Screen Printed Conductors

Advanced Flexible Substrate Technology for Improved Accuracy,

Definition, and Conductivity of Screen Printed Conductors

Art DobieDan Kamben

Chromaline Screen Print Products


Screen Printed Flexible CircuitsFlexible Flat CablesPrinted SensorsRFID TagsMedical ApplicationsDisplaysSmart Garmentsmore

Why Screen Printing? Screen Printing is a mature and easily scalable process which many circuit makers already use successfully in-house.


Flexible Substrate Printing LimitationsPrinting High Resolution on Flex is difficult!

Conductive ink spreads substantially on industry common flexible substrates.

Ink spread inhibits increasing circuit density or reducing circuit real estate without involving other, more involved and more costly patterning methods.

Many flexible circuit fabricators would need to subcontract parts out of house in order to incorporate other patterning methods and in-turn lose control of both cost and lead time.


20µ concentric circles20µ square serpentine Edge definition @ 500x

Develop a screen stencil material solution capable of reproducing printable image features with very high accuracy and definition, and high resolution capability.

Desire: improve print quality, accuracy, and resolution on flexible substrates

20µ square serpentine Edge definition @ 500x


Part B: Develop a substrate level solution capable of maintaining image size accuracy of printed circuit elements on flexible material by inhibiting ink spread.

Desire: improve print quality, accuracy, and resolution on flexible substrates


Newly developed Substrate (“F”) Coated PET

• Ink adhesion promoter• PSR – Print Spread Retention

– Pre-stabilized– Hazy / translucent

Part B: Develop a substrate level solution capable of maintaining image size accuracy of printed circuit elements on flexible material by inhibiting ink spread.


Materials Test Reporting: 3 years & ongoingScreen Stencil Materials: (emulsion, capillary film)

Screen Image:ResolutionAccuracyEdge DefinitionEOM (capillary film)

Substrate Material: (initially PET)Print Testing:

Printed AccuracyPrinted DefinitionPrinted Resolution

Measurements:ProfileResistance


Screen & Prepress for TestingFactors which influenced mesh selection

a) Minimum artwork size requirements(smallest lines & spaces)

b) Desired Wet Ink Film Thickness (“WIFT”)

c) Ink/mesh relationship (Can the ink pass thru the chosen mesh?)

d) Screen cost (capability vs dollars)


Artwork vs Screen Aspect Ratio

30µ Lines & Spaces:

Requires a thin mesh, which results from using mesh with small filament diameters.

For practical strength and function, mesh with smaller diameters require higher mesh counts

Higher mesh counts have smaller openings.

Screen & Prepress for Testing


The screen is elevated a specific height above the substrate. The squeegee moves downward, deflecting the screen into a single line of contact with

substrate, and then travels laterally. The mesh tension of the elevated screen “peels” up behind the traveling squeegee, leaving ink in position on substrate.

Screen Printing is the only “contact” printing process that requires the ink to pass thru the printing plate from back to front.

print composition

(ink, paste, etc.)

squeegee

Ink Transfer in Screen Printing

squeegee travel

direction

wet ink deposit

screen “peel” angle


“T” based on total screen thickness (mesh + emulsion thickness)

“W” is greater than “T”Optimal

“W” = “T”Practical Limit

“W” is smaller than “T”Very difficult, if not impossible

“T”

“T”

“T”

“W”

“W”

“W”

Printable Line Width vs. Screen Thickness: “Aspect ratio”


It’s easier to print a factory…

or a skyscraper!

…than a house,

Printable Line Width vs. Screen Thickness: “Aspect ratio”


Ink/mesh relationship (Can the ink pass thru it?)

For proper passage and to avoid sifting, the mesh openings should be approximately 4x the size of the largest particle in the paste.



The larger PET thread size interferes with paste flow through the line channel. SS alloy allows for thinner wire size and less obstruction to paste flow.

Ink/mesh relationship (Stainless Steel vs PET)



Screen & Stencil System

For the test printing we used the newly developed 25µ thick Capillary Film applied to Calendared 400-18µ stainless steel wire mesh (51% open area,

29µm thickness). This enabled us to reproduce the smallest features (30µ) of the test artwork in screen-making. The EOM typically measured at ~ 10µm.


Aspect Ratio: Overall Screen Thickness vs Feature Size

*Overall Screen Thickness = Mesh Thickness + EOM

EOM

Mesh ThicknessOverall Screen Thickness*

Smallest Image size


Aspect Ratio: Overall Screen Thickness vs Feature Size

• The calendared mesh thickness of the CAL 400-18µ mesh was 29µ.

• Overall screen thickness for the CAL 400-18µ mesh with 10µ EOM was 39µ.

• The smallest feature on our test artwork was 30µ.

10µm

29µm39µm

30µm


Screen & Printed Areas Typically Measured

100µ L/S50µ L/S

40µ L/S

30µ L/S


Flexible PET Substrates Tested• Substrate A (standard industry material)

– Print treated PET• Primer for ink adhesion

– Pre-stabilized– Optically clear

• Substrate B (common industry material)– Surface treated PET

• Ink adhesion• High surface tension


• Substrate F (newly developed)– Coated PET

• Ink adhesion promoter• PSR – Print Spread Retention



PrintScreenSubstrate A, printed image sizes

Average178.5μm79% spread

Range164μm - 189μm

Max Spread89%

Squeegee direction

100μm Traces/Spaces (Ag Paste)


PrintScreen Average142.7μm43% spread

Min. - Max.136μm - 148μm

Max Spread48%

Squeegee direction

Substrate B, printed image sizes



PrintScreen Average101.4μm< 2% spread


Max Spread9%

Squeegee direction

Substrate F, printed image sizes



Substrate A Substrate B

164μm - 189μm 136μm - 148μm 96μm - 109μm

Substrate F



Substrate A

Substrate B

Substrate F>50μm, shorting

>50μm, shorting

44um - 54um

Screen

49µm



Average49.1 um< 2% shrink

Min. - Max.44um - 54um

Max Spread8%

Squeegee direction

Substrate F, Paste Spread



Average40.9μm~ 2% spread


Max Spread15%

Squeegee direction




Average30.8 um~ 3% spread

Min. - Max.26um - 37um

Max Spread23%

Squeegee direction




50, 40, & 30um Concentric CirclesSubstrate A – 50µ

Substrate F – 50µ

Substrate A – 40µ


Substrate A – 30µ



3D Surface Micro-Texture Measurement & Analysis DataBruker Contour GT-K 3D Optical Microscope


Bruker Contour GT-K 3D Optical Microscope Measurement DataSubstrate A – Location 1 Squeegee direction


Bruker Contour GT-K 3D Optical Microscope Measurement DataSubstrate B – Location 1 Squeegee direction


Bruker Contour GT-K 3D Optical Microscope Measurement DataSubstrate F – Location 1 Squeegee direction


Substrate F, Bruker X-Axis Profile: 100µ lines & spaces


Substrate F, Bruker X-Axis Profile: 50µ lines & spaces


Substrate F, Bruker X-Axis Profile: 30µ lines, 50µ spaces


Profiles of Screen Printed Copper Conductors

Substrate F

Substrate A


0.00

200.00

400.00

600.00

800.00

1000.00

1200.00

1400.00

100 75 50 40

Aver

age

Cros

s Sec

tiona

l Are

a

Nominal Trace Width (microns)

Average Trace Cross-Sectional Area Comparison

Substrate A

Flextrace

• Average trace cross-sectional area is very similar between Substrate A and Substrate F

Substrate F


0.00

5.00

10.00

15.00

20.00

25.00

30.00

100 75 50 40

Trac

e Sp

read

% O

ver N

omin

al

Nominal Trace Size (microns)

Trace Percent Spread Comparison

Flextrace

Substrate A

• Percent Spread is less than 5% over nominal across all traces printed on Substrate F

• Substrate A spread varies from 8 to 28% over nominal with increasing spread as trace width decreases

Substrate F


0.00

2.00

4.00

6.00

8.00

10.00

12.00

100 75 50 40

Trac

e W

idth

Sta

ndar

d De

viat

ion

Nominal Trace Width (microns)

Trace Width Standard Deviation

Flextrace

Substrate A

• Substrate A has significantly higher standard deviation of trace width

• Standard Deviation increases as trace width decreases for Substrate A.

• Standard deviation for Substrate F is relatively constant

Substrate F


R² = 0.9741

R² = 0.9761

0

20

40

60

80

100

120

140

0 20 40 60 80 100 120

Resis

tanc

e (O

hms)

Nominal Trace Width (Microns)

Trace Resistance Values

Substrate AFlextrace T15

• Trace Resistance Values were higher on Substrate A

• Trace Resistance Delta decreases with increasing trace width

Substrate F


1.00

1.20

1.40

1.60

1.80

2.00

2.20

2.40

20 30 40 50 60 70 80 90 100 110

Volu

me

Resis

tivity

Nominal Trace Width (Microns)

Volume Resistivity Comparison

Flextrace 1.0mm trace



Substrate A 1.0mm trace

Substrate A 1.2mm trace

Substrate A 1.4 mm trace

• Volume resistivity is higher for Substrate A

• Volume resistivity delta remains relatively constant with decreasing resistivity as trace width increases.

• Substrate A has volume resistivity increase of 14% to 45% over Substrate F

Substrate F 1.0mmSubstrate F 1.2mm

Substrate F 1.4mm

Trace length


Lower Resistance Values

The migration of vehicle and solvents in the printed paste into the coated substrate inhibits the paste from spreading laterally. The

printed paste then retains position and the “draw down” of the ink’s fluids into the substrate causes a more confined packing of the

conductive particles resulting in higher conductivity.


Better Conductivity vs Paste Reduction• Based on achieving

the same conductivity on Substrate F as on Substrate A, the reduction in paste required increases as the trace width decreases.

• At 40µm trace width, ~33% less paste was used while at 100µm ~3% less paste was needed for equal functionality.

y = 0.005x2 - 1.1892x + 71.559R² = 1

0

5

10

15

20

25

30

35

0 20 40 60 80 100 120

Past

e Re

duct

ion

(%)

Conductive Trace Width (microns)

% Paste Reduction vs Trace Width (Resistance)


SummaryThe intended goal was to improve print accuracy and definition, and in turn increase resolution and reduce line resistance on flexible printed circuits. The combination of advanced screen stencil materials capable of reproducing sub-100 micron printable image features with high accuracy, and a new flexible substrate solution capable of inhibiting ink spread, permits reproduction of image sizes to an exceptional degree of accuracy.

Results of both field and in-house testing of the new screen stencil materials combined with the new advanced PET substrate suggest these goals have been realized. Greatly increased printed image accuracy provides the opportunity for sub-100 micron feature sizes. Print results very similar to those reported here were regularly achieved while using 10 different commercial Ag conductive ink formulations.

Test measurements also identified a significant reduction in line resistance when printed on Substrate F. This due to the improved 3-dimensional profile of the traces as printed on Substrate F, resulting in improved packing of the paste solids as it is drawn into the coated substrate.


AcknowledgementsChromaline would like to thank the following companies who were kind enough to provide sample materials for various print tests:

Conductive Paste:DuPontEMSHeraeusInsulectroPPGESLAGFA

Asada Mesh Co. for providing samples of ultra-fine mesh types.

Special thanks to those customers, industry allies, & Chromaline personnel for assisting with time, materials, and equipment during various stages of print testing.


Thank you for your kind attention!

Questions?

Art DobieDan Kamben

Chromaline Screen Print Products

Advanced Flexible Substrate Technology for Improved Accuracy… … · · 2017-06-22Advanced...

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