final report 04.15.2009 rev.: 22.05.2009 dr. lászló jakab, lászló milán molnár, olivér...

BUDAPEST UNIVERSITY OF TECHNOLOGY AND ECONOMICSELECTRONICS TECHNOLOGY DEPARTMENT

Final Report04.15.2009Rev.: 22.05.2009

Dr. László Jakab, László Milán Molnár, Olivér Krammer

FEM simulation of stencil deformation

BOSCH - STENCIL_FEM 2/59

WORKPLAN OF THE PROJECT

Phase one – simulation- measuring the necessary parameters – done- collecting the missing technical data – done- constructing the FEM model for the PCB – was not necessary according to kick-off meeting

Milestone 1 – simulation model of the PCB is ready – pilot stencil model was ready instead of the PCB model

- measuring the elastic properties of stencil – done- constructing the FEM model of the stencil – done, extended with the FEM model of a real squeegee- running simulations – done

Milestone 2 – Decision about the continuance depending on the results of the simulations – project is continued

Deadlines from the start of the project (15.10.2008)

End of 2nd month

End of 4th month

Phase two – Designing and carrying out experiments- setting up experiment – testboard is ready- carrying out stencil printing tests – done- measuring printing transfer efficiency – done

Milestone 3 – Comparing simulation and experimental results, evaluation the results of the project – done

End of 6th month


CONTENTS

1. Investigating stencil deformation in case of point loading:

Stencil is loaded at the center; different sizes of underside supports were used; pilot FEM model of the stencil was created according to deformation results.

2. FEM model of a real squeegee:

Finite Element model of a printing squeegee was created on the base of squeegee deformation experiments; squeegee is inserted into the stencil FEM model.

3. Stencil deformation by squeegee loading:

Stencil is loaded at different locations by a printing squeegee; different sizes of underside supports were used; final FEM model of the stencil was created (including the squeegee) according to deformation results.

4. Testboard for stencil printing experiment:

Testboard was designed according to BOSCH requirements; thick steps (with different distances from pads) were formed by selective electroplating.

5. Stencil printing experiment:

Stencil printing experiment was carried out using testboards with different step thicknesses; the deposited paste height was measured and simulated; keepout area rule was set up on the basis of simulation results.


1. Stencil deformation by point loading


STENCIL DEFORMATION EXPERIMENT

frame 58x58 cm

stencil 50x50 cm

underside support

30x30 cm16 cm

Stencil:

- stainless steel

- lasercut

- thickness: 175 µm

- ordered from DEK



free to movemeasuring probe

fixed micrometerclock

clock stand stencilclock fixing

m

loading armclock

stencilunderside support

365 mm953 mm

Clock:range: 0…1 mmaccuracy: 10 µm

Load: 2.26…7.5 kg / 22…131 N


STENCIL DEFORMATION RESULTS

0 20 40 60 80 100 120 1400

1

2

3

4

5

6

Measured 30 cm support Measured 16 cm support

Ben

ding

(m

m)

Force (N)

m21

m55


FEM MODEL OF THE STENCIL FOIL

FEM model has been created for both 16 cm and 30 cm

underside support, to match to both experimental results.

The material parameters obtained from Comsol library:

Steel AISI 4340 – E: 205·109 Pa, ν: 0.28, ρ:7850 kg/m3

stencil thickness: 175 µm


SIMULATION RESULTS

0 20 40 60 80 100 120 1400

1

2

3

4

5

6

Measured 30 cm support Measured 16 cm support Simulated 30 cm support Simulated 16 cm support

Ben

ding

(m

m)

Force (N)

measured: 2.84 µm/N, simulated: 3.03 µm/N

measured: 1.95 µm/N, simulated: 2.11 µm/N



frame 58x58 cm

stencil 50x50 cm

underside support

30x30 cm16 cm

Stencil deformation experiment has been extended by underside supports of 10x10 cm and 5x5 cm.


STENCIL DEFORMATION RESULTS

0 20 40 60 80 100 120 1400

1

2

3

4

5

6

Measured 30 cm support Measured 16 cm support Measured 10 cm support Measured 5 cm support

Ben

ding

(m

m)

Force (N)

Stencil thickness: 125 µm


2. FEM model of a real squeegee


DEFORMATION OF A REAL SQUEEGEE

The bending of the squeegee measured with the same loads as the stencil before to create the FEM model of the squeegee.


SQUEEGEE UNDER INVESTIGATION

Length: 300 mmBlade: stainless steelThickness: 200 µm

19

15

35

0 20 40 60 80 100 120 1400,0

0,2

0,4

0,6

0,8

1,0

1,2

Ver

tical

dis

plac

emen

t of

squ

eege

e (m

m)

Force (N)

BOSCH - STENCIL_FEM 15/59BOSCH - STENCIL_FEM

SIMULATING SQUEEGEE DEFORMATION

FEM parameters of the squeegee:

Length: 300 mm

Thickness: 200 µm

Height: 15 mm

Initial angle: 60 °

E: 195·109 Pa

ν: 0.28

ρ: 7850 kg/m3


SIMULATING SQUEEGEE DEFORMATION

0 20 40 60 80 100 120 1400,0

0,2

0,4

0,6

0,8

1,0

1,2 Measured deformation Simulated deformation

Ver

tical

dis

plac

emen

t of

Squ

eege

e (m

m)

Force (N)


3. Stencil deformation by squeegee loading


MEASURING STENCIL DEFORMATION PUSHED WITH SQUEEGEE

x

y

Squeegee length:300 mm

Stencil thickness:125 µm

Loads are the sameas previous:22…131 N

Underside support:31 cm, 20 cm, 10 cm


FEM MODEL OF THE STENCIL WITH SQUEEGEE

Stencil dimensions: real size – 580 mm x 580 mm x 125 µm

Mesh: 1:1:100 (x:y:z) ratio for numerical accuracy, finer mesh size at pressure area (see figures)

Squeegee is pressed from the top side, by uniform pressure

Material properties: steel, E=195 GPa, Poisson’s Ratio: 0,28.

Boundary conditions: surfaces inside the supported area can move and bend, other surfaces are fixed

Squeegee location: y=0 mmSupport size in example: 20 cm

Width of support system: 30 mmSupport size in example: 20x20 cm


DEFORMATION IN X DIRECTION, 30 cm SUPPORT

0 20 40 60 80 100 120 1400

1

2

3

X = 0, avg

=18 µm

X = 27 mm, avg

=14 µm

X = 54 mm, avg

=18 µm

X = 81 mm, avg

=20 µm

X = 0, simulated X = 27 mm, simulated X = 54 mm, simulated X = 81 mm, simulated

Ben

ding

(m

m)

Force (N)



0 20 40 60 80 100 120 1400

1

2

3

X = 0, avg

=18 µm

X = 27 mm, avg

=11 µm

X = 54 mm, avg

=11 µm

X = 81 mm, avg

=12 µm

X = 0, simulated X = 27mm, simulated X = 54mm, simulated X = 81mm, simulated

Ben

ding

(m

m)

Force (N)



0 20 40 60 80 100 120 1400

1

2

3 X = 0,

avg=13 µm

X = 27 mm, avg

=7 µm

X = 0, simulated X = 27 mm, simulated

Ben

ding

(m

m)

Force (N)


DEFORMATION IN Y DIRECTION, 30 cm SUPPORT

0 20 40 60 80 100 120 1400

1

2

3

Y = 0, avg

=18 µm

Y = 40 mm, avg

=29 µm

Y = 80 mm, avg

=36 µm

Y = 120 mm, avg

=28 µm

Y = 0, simulated Y = 40 mm, simulated Y = 80 mm, simulated Y = 120 mm, simulated

Ben

ding

(m

m)

Force (N)



0 20 40 60 80 100 120 1400

1

2

3 Y = 0,

avg=18 µm

Y = 40 mm, avg

=32 µm

Y = 80 mm, avg

=7 µm

Y = 0, simulated Y = 40 mm, simulated Y = 80 mm, simulated

Ben

ding

(m

m)

Force (N)



0 20 40 60 80 100 120 1400

1

2

3 Y = 0,

avg=18 µm

Y = 40 mm, avg

=5 µm

Y = 0, simulated Y = 40 mm, simulated

Ben

ding

(m

m)

Force (N)


MEASURING STENCIL DEFORMATION OF THE 150 µm TEST STENCIL

Squeegee length:300 mm

Stencil thickness:150 µm

Loads are the sameas previous:22…131 N

Point load and squeegee load is applied too

Underside support:20 cm


DEFORMATION OF 150 µm STENCIL IN CASE OF POINT LOADINGS

0 20 40 60 80 100 120 1400

1

2

3

4

On 25 mm squeegee line, avg

=17 µm

On 55 mm squeegee line, avg

=24 µm

On 25 mm squeegee line, simulated On 55 mm squeegee line, simulated

B

endi

ng (

mm

)

Force (N)


DEFORMATION OF 150 µm STENCIL IN CASE OF SQUEEGEE LODING AT 55 mm FROM CENTRE

0 20 40 60 80 100 120 1400

1

X = 0, avg

=36 µm

X = 27 mm, avg

=21 µm

X = 54 mm, avg

=6 µm

X = 81 mm, avg

=18 µm


Ben

ding

(m

m)

Force (N)


DEFORMATION OF 150 µm STENCIL IN CASE OF SQUEEGEE LODING AT 25 mm FROM CENTRE

0 20 40 60 80 100 120 1400

1

X = 0, avg

=18 µm

X = 27 mm, avg

=16 µm

X = 54 mm, avg

=25 µm

X = 81 mm, avg

=12 µm


Ben

ding

(m

m)

Force (N)Conclusion: including apertures in simulation is not necessary


4. Testboard for stencil printing experiment


THE TESTPATTERN

Step in different height from board to board:for example +20 µm, +40 µm, +60 µm

The height of steps is formed by selective electroplating

The clearance between the steps and the pads is varying from 300 µm to 5 mm

Stencil aperture for paste deposition (0.5x0.5 mm), the paste transfer efficiency is not affected by Area Ratio, base thickness 35 µm

Clear pad for reference thickness of paste measurement


THE TESTBOARD

The testboard was designed according to Bosch requirements.

Base thickness:contour and pads

Higher steps byselective electroplating


THE TESTBOARD

Nine pieces of testboard were made with immersion Ag finish;3-3 of each step thicknesses: +20 µm, +40 µm, +60 µm.


MEASURING THE STEP THICKNESSES

The thickness of the steps was measured with a Tencor Alphastep 500.

Horizontal range: 2 mm

Vertical range: 10 nm…300 µm

Vertical resolution: 0.1 µm or 2.5 nm

0,5 1,0 1,5-20

0

20

40

60

80

100

Ve

rtic

al (

µm

)

Horizontal (mm)


MEASUREMENT POINTS

narrow steps

square

wide steps


RESULTS OF ALPHASTEP MEASURING

0

20

40

60

80

100

120

Wide step

Ave

rage

(µ

m)

Narrow step Square

Wide step

SquareNarrow step

Wide step

SquareNarrow step


5. Stencil printing experiment


STENCIL PRINTING EXPERIMENT

Printer model: DEK 248

Accuracy: (achievable) ±25μm

Repeatability: ±10 μm

Printing speed: 10-70 mm/s

Squeegee force: 0-150 N

Experimental settings:

Printing speed: 30 mm/s, squeegee force: 92 N, blade length: 300 mm, separation speed: 6mm/s,5 testboards were used for process setup.

1. Print 1 testboard -> print 1 fake board -> dry clean of stencil underside (repeated for 3 testboards)

2. Stencil direction / board direction was changed, stencil cleaned by wet wipe and with pressured air

3. Same run steps as No. 1. for another 3 boards


TEST RUN

Board ID.: Direction of printing

Narrow step

height [µm]

Wide step

height [µm]

Square1

height [µm]

Square2

height [µm]

ID1 Vertical 27 20 25 50

ID2 Horizontal 29 24 30 18





vertical printing

horizontal printing

narrow steps

widesteps

square2

square1


MEASURING DEPOSITED PASTE HEIGHT

solder paste

solder pad

step

Measuring equipment:

ASC-Visionmaster 150

Maximum sample height: 5.1 cm

Resolution: 1.78 μm

Maximum measurable height: 365 μm

Field of view: 2.1x2.8 mm


ID. 2.: STEPS ARE PARALLEL TO SQUEEGEE

0,3 0,5 0,7 1 1,3 1,7 2 2,5 3 5 no step0

100

120

140

160

180

200

220

240

Pas

te h

eigh

t (µ

m)

Step distance (mm)

Stencil thickness

Step-pad: 29 µm

0,3 0,5 0,7 1 1,3 1,7 2 2,5 3 5 no step0

100

120

140

160

180

200

220

240

Pas

te h

eigh

t (µ

m)

Step distance (mm)

Stencil thickness

Step-pad: 24 µm

*Line is only for guide, not simulation result. Paste is higher than Cu step because if the stencil did not bend down to the pad, it lifted the paste during separation.




0,3 0,5 0,7 1 1,3 1,7 2 2,5 3 5 no step0

100

150

200

250

300

350P

aste

hei

ght

(µm

)

Step distance (mm)

Stencil thickness

Step-pad: 68 µm

0,3 0,5 0,7 1 1,3 1,7 2 2,5 3 5 no step0

100

150

200

250

300

350

Pas

te h

eigh

t (µ

m)

Step distance (mm)

Stencil thickness

Step-pad: 50 µm





0,3 0,5 0,7 1 1,3 1,7 2 2,5 3 5 no step0

100

150

200

250

300

350P

aste

hei

ght

(µm

)

Step distance (mm)

Stencil thickness

Step-pad: 75 µm

0,3 0,5 0,7 1 1,3 1,7 2 2,5 3 5 no step0

100

150

200

250

300

350

Pas

te h

eigh

t (µ

m)

Step distance (mm)

Stencil thickness

Step-pad: 53 µm




ID. 1.: STEPS ARE PERPENDICULAR TO SQUEEGEE

0,3 0,5 0,7 1 1,3 1,7 2 2,5 3 5 no step0

100

120

140

160

180

200

220

240

Pas

te h

eigh

t (µ

m)

Step distance (mm)

Stencil thickness

Step-pad: 27 µm

0,3 0,5 0,7 1 1,3 1,7 2 2,5 3 5 no step0

100

120

140

160

180

200

220

240

Pas

te h

eigh

t (µ

m)

Step distance (mm)

Stencil thickness

Step-pad: 20 µm





0,3 0,5 0,7 1 1,3 1,7 2 2,5 3 5 no step0

100

150

200

250

300

350P

aste

hei

ght

(µm

)

Step distance (mm)

Stencil thickness

Step-pad: 55 µm

0,3 0,5 0,7 1 1,3 1,7 2 2,5 3 5 no step0

100

150

200

250

300

350

Pas

te h

eigh

t (µ

m)

Step distance (mm)

Stencil thicknessStep-pad: 32 µm





0,3 0,5 0,7 1 1,3 1,7 2 2,5 3 5 no step0

100

150

200

250

300

350P

aste

hei

ght

(µm

)

Step distance (mm)

Stencil thickness

Step-pad: 92 µm

0,3 0,5 0,7 1 1,3 1,7 2 2,5 3 5 no step0

100

150

200

250

300

350

Pas

te h

eigh

t (µ

m)

Step distance (mm)

Stencil thickness

Step-pad: 58 µm




STENCIL DEFORMATION OF NARROW STEPS

0,3 0,5 0,7 1 1,3 1,7 2 2,5 3 5-60

-40

-20

0

20

Steps are perpendicular to squeegee, avg

=16 µm; ID. 1-5-6

Steps are parallel to squeegee, avg

=10 µm; ID. 2-7-8

Steps are perpendicular to squeegee, simulated, max. bending 12 µm Steps are parallel to squeegee, simulated, max. bending 38 µm

Pas

te h

eigh

t di

ffer

ence

(µ

m)

Step distance (mm)

Conclusion: if steps are perpendicular to the squeegee, the deposited paste has higher height with higher deviation, and the stencil can bend less.


MEASURING THE DEPOSITED PASTE AREA

Paste area was measured when the steps were perpendicular to printing directionThe results were averaged from the pads outlined by the red rectangle


MEASURING THE DEPOSITED PASTE AREA

Left:ID. 1 - no step

Right:ID. 6 – 0.5 mmstep distance

0 - no step ID1. - 27 µm ID5. - 55 µm ID6. - 92 µm0

200000

300000

400000

500000 r=12.1%

r=8.8%

r=5.7%

r=2.6%

Pas

te a

rea

(µm

2 )

Step-pad (difference in height)

Aperture area (500x500 µm)

BOSCH - STENCIL_FEM 50/59BOSCH - STENCIL_FEMBOSCH - STENCIL_FEM

3D SIMULATIONS FOR DIFFERENT DIRECTION OF PRINTING

Simulations showed basically different printing process depending on the printing direction. The bending of the stencil can be senn in the figures above.


DIFFERENT PLOTS IN DIFFERENT CASES

- The simulation data for slides (52-57) were extracted from 3D simulations

like on the previous slide

- BUT there are two different types of simulation plots:

- If the printing direction is parallel to the row of pads, the simulation

doesn’t show a cross-setion of the bending stencil.

- If the printing direction is perpendicular to the row of pads, the simulation

data is a cross-section of the stencil.

Example: cross-section data from

the left image on the previous slide


ID. 2.: PADS NEAR TO LARGE Cu SQUARE

0,6

1,6

2,6

3,6

4,6

5,6

6,6

7,6

8,6

9,6

10,6

11,6

12,6

13,6

14,6

15,6

0

100

120

140

160

180

200

220

240

Measured paste height Simulated paste height

Pas

te h

eigh

t (µ

m)

Step distance (mm)

Stencil thickness

Step-pad: 30 µm

0,6

1,6

2,6

3,6

4,6

5,6

6,6

7,6

8,6

9,6

10,6

11,6

12,6

13,6

14,6

15,6

0

100

120

140

160

180

200

220

240


Pas

te h

eigh

t (µ

m)

Step distance (mm)

Stencil thicknessStep-pad: 18 µm



0,6

1,6

2,6

3,6

4,6

5,6

6,6

7,6

8,6

9,6

10,6

11,6

12,6

13,6

14,6

15,6

20

40

60

80

100

120

140

160

180

200

220


Pas

te h

eigh

t (µ

m)

Step distance (mm)

Stencil thickness

Step-pad: 58 µm

0,6

1,6

2,6

3,6

4,6

5,6

6,6

7,6

8,6

9,6

10,6

11,6

12,6

13,6

14,6

15,6

0

100

120

140

160

180

200

220

240


Pas

te h

eigh

t (µ

m)

Step distance (mm)

Stencil thickness

Step-pad: 69 µm



0,6

1,6

2,6

3,6

4,6

5,6

6,6

7,6

8,6

9,6

10,6

11,6

12,6

13,6

14,6

15,6

20

40

60

80

100

120

140

160

180

200

220

240 Measured paste height Simulated paste height

Pas

te h

eigh

t (µ

m)

Step distance (mm)

Stencil thickness

Step-pad: 80 µm

0,6

1,6

2,6

3,6

4,6

5,6

6,6

7,6

8,6

9,6

10,6

11,6

12,6

13,6

14,6

15,6

0

100

120

140

160

180

200

220

240


Pas

te h

eigh

t (µ

m)

Step distance (mm)

Stencil thickness

Step-pad: 80 µm



0,6

1,6

2,6

3,6

4,6

5,6

6,6

7,6

8,6

9,6

10,6

11,6

12,6

13,6

14,6

15,6

0

100

120

140

160

180

200

220

240


Pas

te h

eigh

t (µ

m)

Step distance (mm)

Stencil thickness

Step-pad: 25 µm

0,6

1,6

2,6

3,6

4,6

5,6

6,6

7,6

8,6

9,6

10,6

11,6

12,6

13,6

14,6

15,6

0

100

120

140

160

180

200

220

240


Pas

te h

eigh

t (µ

m)

Step distance (mm)

Stencil thickness

Step-pad: 50 µm



0,6

1,6

2,6

3,6

4,6

5,6

6,6

7,6

8,6

9,6

10,6

11,6

12,6

13,6

14,6

15,6

0

100

120

140

160

180

200

220

240


Pas

te h

eigh

t (µ

m)

Step distance (mm)

Stencil thickness

Step-pad: 53 µm

0,6

1,6

2,6

3,6

4,6

5,6

6,6

7,6

8,6

9,6

10,6

11,6

12,6

13,6

14,6

15,6

0

100

120

140

160

180

200

220

240


Pas

te h

eigh

t (µ

m)

Step distance (mm)

Stencil thickness

Step-pad: 56 µm



0,6

1,6

2,6

3,6

4,6

5,6

6,6

7,6

8,6

9,6

10,6

11,6

12,6

13,6

14,6

15,6

0

100

120

140

160

180

200

220

240

260

280

Pas

te h

eigh

t (µ

m)

Step distance (mm)

Stencil thickness

Step-pad: 120 µm

0,6

1,6

2,6

3,6

4,6

5,6

6,6

7,6

8,6

9,6

10,6

11,6

12,6

13,6

14,6

15,6

0

100

120

140

160

180

200

220

240


Pas

te h

eigh

t (µ

m)

Step distance (mm)

Stencil thickness

Step-pad: 90 µm


SIMULATING THE KEEPOUT AREA

In the simulations 92 N squeegee force and perpendicular steps (as the worst case) were used.

0

20

40

60

80

100

0 5 10 15 20 25 30 35

Observed solder mask thickness,which caused solder bridging

Step distance [mm]

Stencil thickness 175 µm Stencil thickness 150 µm Stencil thickness 125 µm Stencil thickness 100 µm Stencil thickness 75 µm

Ste

p he

ight

[µm

]

Usual soldermask thickness

distance [µm]= 1.6·h·dd - foil thickness [µm]h - step height [µm]


SUMMARY

- FEM model of the stencil is created including a real squeegee

- Printing experiments were carried out.

- Steps, perpendicular to squeegee line cause higher paste deposit

with higher deviation, and the stencil can bend less.

- According to the simulations, the minimum recommended keepout

area is 1.6*step_height*stencil_foil_thickness.

- This recommendation can be used for step stencils as well (for PIP

technology or for mixed-pitch applications) instead of the IPC-7525

standard ’36*step_height’ rule.

final report 04.15.2009 rev.: 22.05.2009 dr. lászló jakab, lászló milán molnár, olivér...

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