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1 PRIVILEGED AND CONFIDENTIAL MATERIALS

1991 – 2014 TechLead Corporation

“Stencil Design, Materials & Processes for

Today’s Miniaturized Electronics Assembly”

Bruce Barton, Alent

Jim Price, ASM/DEK

Ben Scott, Datum Alloys

Sue Holmes, Photo Stencil

Tony Lentz, FCT Assembly

Ahne Oosterhof, Eastwood Consulting


“Stencil Design, Materials & Processes for

Today’s Miniaturized Electronics Assembly”

Miniaturization of all electronics from tablets & smart phones to implantable medical devices continues to drive assembly complexity & creates major challenges in yield & reliability. Stencils play a key role in these assemblies as we approach several physical limitations in the printing processes, particularly those for applying solder paste. This panel explores the options currently in use as well as future directions for ensuring high yield, high quality & reliable electronics assemblies of all types. Topics of discussion include stencil materials, processes, coatings & design; solder & flux formulations; as well as interactions between components, boards & reflow processes.



Stencil Design, Materials & Processes for

Today’s Miniaturized Electronics Assembly

Rheology & Thixotropy – Bruce Barton, Alent

Specialized Printing Tools – Jim Price, ASM DEK

Stencil Metallurgy – Ben Scott, Datum Alloys

Specialty Stencils (3D) – Sue Holmes, Photo Stencil

Stencil Coatings – Tony Lentz, FCT Assembly

User Perspective – Ahne Oosterhof, Eastwood

Consulting

an Alent plc Company

ALPHA

Technical Services

Group

Bruce Barton


Highly Confidential and Privileged Information of Alpha an Alent plc Company

Detailed Cause and Effect

Machine

Process Parameters

Stroke

Length

Print Speed

Print

Pressure

Print Gap

Aperture

Size

Aperture

Geometry Aperture

Layout Material

Method of

Fabrication

Stencil

Area Ratio Thickness

OPERATION & METRICS EQUIPMENT &

TOOLING

Particle Size /

Distribution

Metallurgy Slump Environment

Viscosity

Paste

Residue

Rheology Solid

Content

Flux

Warpage

Pad Geometry

Cleanliness

Pad Metallurgy

Thickness Mask Issues Component

Mix

MATERIALS PERSONNEL &

ENVIRONMENT

Handling

Setup Board

Chemistry Activity Level

Residue

Aspect

Ratio

Squeegee

Material

Angle of Blade

Separation Speed

Frequency of

Cleaning

Auto./

Semi Auto.

Alignment

Accuracy

Repeatability Maintenance

Procedures Ease of

Operation

Serviceability

Cleaning

Hardness

Set-up Time

Stencil Cleaning Frequency

Procedures

Rate of Wear

GOOD

PRINT

Standard

Procedures

Paste Storage Stencil Storage Handling

Metrology

Repeatability Reproducibility

Procedures Training

Viscosity

Environment

Temperature Humidity

Support

Discipline

Chemistry

Process

Control

Defect Data

Collection

SPC Program Continuous

Improvement

Size & Shape of Edge

Length

Planarity

Pad Finish

Taper

Polish

Knead Parameters

Paste Bead

Diameter

Fine Feature Printing


Measuring Rheology

Because solder paste is subjected to a range of shear

forces, and the viscosity is different at each force,

rheology of solder paste is key. There are three

very common ways to measure solder paste

viscosity. Brookfield viscometry, Malcolm

Viscometry and Bohlin Rheology. Brookfield

viscometry is not commonly used because it is not

easy to vary the shear rate applied to the paste and

make multiple measurements. The viscosity shear

curve is very steep at low viscosity. Very small

changes in shear have very large changes in

viscosity.



Thixotropic nature of solder paste Viscosity vs. Malcolm RPM

0

500

1000

1500

2000

2500

3000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Lot Number

Vis

cosi

ty

5 RPM

10 RPM

15 RPM

20 RPM

30 RPM


Thixotropic - ADJECTIVE

Becoming more fluid when shaken or stirred and returning to a gel state when allowed to stand.

Forces on Solder Paste:

1. Printing – forces applied during squeegee motion across the stencil.

2. Squeegee Lift – adhesive forces between paste & squeegee.

3. Stencil Lift – balance of adhesive forces between stencil aperture side

walls & PWB pads.

4. Gravity/Standing – Resistance to slumping & spreading on the PWB.



Thixotropic - ADJECTIVE

Becoming more fluid when shaken or stirred and returning to a gel state when

allowed to stand.

The first type of shear force applied to the solder paste is the printing of the

board. A critical success factor is that the paste’s viscosity, or resistance to

movement, be minimized when the paste is being forced into the stencil

aperture. If a squeegee is moving at 100mm/second, and an aperture is

0.25 mm wide (typical for a 0.4mm pitch BGA), the solder paste has only

0.0025 seconds to flow into its designated aperture, usually .1mm deep.

The second type of shear force applied to solder paste is the lifting of the

squeegee off of the stencil after each print stroke. The bead of solder must

not stick to the squeegee, and remain on the stencil. Paste that has lost too

much solvent due to excess duration on the stencil and/or exposure to

elevated temperatures is prone to this failure mode. Improperly formulated

water soluble paste may suffer this consequence under very low humidity

conditions.


The third type of shear force applied to solder paste is the separation of

the stencil from the freshly printed PCB. The stencil must be

separated cleanly, leaving as much as the paste on the board as

possible. It is well known that if the area of the aperture walls is too

high relative to the area of the PWB pad that the paste was printed

on, some or all of the solder paste will remain in the aperture. This

results in too little solder paste to form the desired component to

board solder joint.

The fourth shear force seen by solder paste is the force of gravity after

the stencil has been removed. It is common to have .100mm high

deposits of solder paste printed in 0.400mm intervals. This requires

high viscosity when paste is subjected to the relatively low shear

force of gravity (and motion of the board). Without high viscosity at

low shear, the paste deposits would run together, causing bridging

and electrical failures.


today historical

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Area Ratio (Opening Area / Wall Area)

Tran

sfer

Eff

icie

ncy

01005’s 0.3CSP

Future

0201’s 0.4CSP

SMT Today Leading Edge

Heterogeneous Area Aperture Ratios (AAR)

What do You Need to Do?

The following things must happen when printing those world class applications:

Make certain the bottom of the stencil is as clean as is practically possible in a production

setting.

The board to the rail to the stencil must be as flat as is physically possible.

The board is flat to the table and supremely supported by the tooling.

Make sure the apertures are as fully loaded with solder paste as possible.

Make certain the PCB and the stencil separate at the same time over the entire printed

surface.

Confirm the squeegee pressure and support tooling are stable


Either Edge Clamping Snuggers or Over the Top Snuggers

What Features are Needed?

With standard PCB clamps, up to 50% of stencil release control is lost and

purely dependent on stencil rebound speed.

Closed Loop Squeegee Pressure with Tooling Deviation Monitor

Super co-planed PCB rails to the printing table to the stencil

+/- 10 microns rising table to rails to stencil support flatness

ProActiv squeegees, unique to DEK, patented

What Features are Needed? Con’t


Activation of the squeegee blade

- locally modifies the rheology of

the solder paste increasing fluidity

and…

- increases the downward

interaction at the blade tip

This combination improves the

packing density of particles into

apertures and enhances the

cohesive bond between solder

paste particles.

Heterogeneous Area Aperture Ratios (AAR), Cont’d

What About Other Needed Process Items?

DEK High Tension VectorGuard stainless steel stencil

No mesh mount to distort over time

4 mil thickness is typical for .6 AAR or lower AAR’s when 0201 / .4mm

CSP’s or smaller are being printed

Square apertures will increase the available volume of past by 23%

DEK Nano ProTek stencil treatment

Resists paste leakage when gasketing is compromised

Type 4 solder paste is typically used as type 3 solder particles are too

large


.5 AAR

.44 AAR

.38 AAR

.31 AAR

.25 AAR

Results

Stencil Design, Materials & Processes

for Today’s Miniaturized Electronics

Assembly

Presented by

Ben Scott, CEO

© Datum Alloys Ltd 2014


Stencils Today A lack of science

Substrate

• Chosen because of availability and wear resistance

Fabrication

• Laser cut

• Etching

• Additive

Mounting

• Trampoline screens evolved from silk screen printing

• Frameless systems driven by space saving

Supply Chain

• Lack of coordination between stencil houses, paste manufacturers, substrate

suppliers and point-of-use

The data says yes. © Datum Alloys Ltd 2014 / ISO 9001:2000

SMT Stencils Understand the variables at each process event

Substrate

• Squeegee side

• In the apertures (wall topography)

• Board side

Aperture Cutting

• Given we actually understand what we want from the aperture wall, what cutting

process would deliver this

• Do we actually need to apply secondary coatings?

Mounting

• Mounting impacts print performance

• Stencil must be held absolutely in X, Y and Z



Problems to Solve Some observations

Substrate

• Tolerances, CP values

• Delta in service environments

• Creep resistance

• Suitability to paste and flux

Manufacturing

• Precision

• Repeatability

• Multi level/ 3D

• Environment

Mounting

• Stencil moves, vibrates in Z

• X and Y control could be better


Stencil Check Summary

www.datumalloys.com © Datum Alloys Ltd 2014 / ISO 9001:2000

Apertures

• Concentricity

• Registration

• Wall topography

• Aspect ratio

Multi level / 3D

• Sharp edges

• Pockets

Stencil surface

• Desired Ra, Rz etc

• Do these differ between sides

• Contact angle

Semicon

• Over lay tolerances

• Stand-offs

• Creep resistance

• Thickness tolerances

• Aperture tolerances

• Clean sharp edges

All tolerances should be within a single

micron!


Thank you for your time If you have any questions, please contact

UK

Datum Alloys Ltd

Bridge Works

Kingsbridge

Devon TQ7 1ES

Call +44 (0) 1548 855 900

[email protected]

Skype uk.datum

Singapore

Datum Alloys Pte Ltd

5 Woodlands Terrace

Singapore

738430

Call +65 6593 0090

[email protected]

Skype sg.datum

USA

Datum Alloys Inc

407 Airport Road

Endicott

NY 13760

Call +1 607 239 6274

[email protected]

Skype na.datum

Presented by

Ben Scott, CEO

© Datum Alloys Ltd 2014

3 topics 1- Broadband printing (printing solder paste when both small pitch and normal pitch components are on the same substrate . 2- 3D Electroform Stencils for printing paste or flux when components are already attached to the substrate. 3- Area Ratio Calculator for determining proper aperture size, stencil thickness and stencil type for acceptable paste transfer

mailto:[email protected]




Good Joint Dry Joint

Re

flo

wS

ep

ara

tio

nP

rin

t

Large Component Small Component

Thick Stencil

Lean Joint Good Joint

Thin Stencil

Small ComponentLarge Component

Re

flo

wP

rin

tS

ep

ara

tio

n

Challenge of Broadband Printing large and small components on same substrate

Component Stencil Thickness

and typical

Aperture Size 2 mil 2.5 mil 3 mil 3.5 mil 4 mil 5 mil

50u 62u 75u 87u 100u 125u

01005

6 mil (150u) 0.75 0.60 0.50 0.43 0.38 0.30

7 mil (175u) 0.88 0.70 0.58 0.50 0.44 0.35

.3 mm CSP

6 mil (150u) 0.75 0.60 0.50 0.43 0.38 0.30

7 mil (175u) 0.88 0.70 0.58 0.50 0.44 0.35

8 mil (200u) 1.00 0.80 0.67 0.57 0.50 0.40

Green = OK Orange = Warning Red = Stop

Area Ratio Matrix


Stencil Solutions: •Step Stencils

•Thin area for small devices •Thick area for large devices

•Two Print Stencil •Thin stencil for 1st print (small devices) •Thick stencil for 2nd print (large devices)

•Improve stencil printing process •Improve paste release for Area Ratio’s <.5

View from the squeegee side showing the 3-D raised

pocket for relief of the IDC’s or Chip components.


Substrate

Stencil

Area Ratio =Aperture Open Area

Wall Surface Area

Aperture

Apertu

re

Apertu

re Stencil Area > .66 for All Stencils

Width in

Length

in

Radius

in

Thicknes

s Ratio .50 - .66 E-FAB or NicAlloy-XT or NiCut

mils mils mils in mils .47 - .50 E-FAB with Nano-Coat or NiCut

.42-.47 E-FAB w/ Nano-Coat

<.42 Redesign

6 6 2 2 0.792 Acceptable Design

5 6 2 3 0.477

Recommend E-FAB with Nano-Coat or

NiCut

7 7 2 3 0.618 Recommend E-FAB or NicAlloy-XT or NiCut

0 0 0 0 #DIV/0! #DIV/0!

0 0 0 0 #DIV/0! #DIV/0!

0 0 0 0 #DIV/0! #DIV/0!

0 0 0 0 #DIV/0! #DIV/0!

0 0 0 0 #DIV/0! #DIV/0!

0 0 0 0 #DIV/0! #DIV/0!

0 0 0 0 #DIV/0! #DIV/0!


Stencil Nano-Coatings

Wipe on

Coating B

Coating C

Self Assembled Monolayer

Spray coat and cure

Coating A

Coating D

Thermally Cured Polymer

Surface Function

After 20 prints with no underside cleaning

Uncoated stencil

Nano-coated stencil

Coatings A, B, C, D

Coating Bridging Profile

Shape

Uncoated 174 Deteriorates

Coating A 0 Consistent

Coating B 2 Consistent

Coating C 0 Consistent

Coating D 0 Consistent


Aperture Function – Transfer Efficiency*

50

67

50

42

72

30

40

50

60

70

80

90

U A B C D

Tra

nsfe

r E

ffic

ien

cy (

%)

Coating Type

Average Transfer Efficiency SAR 0.500 (0.4 mm BGA)

*SMTAI 2013, Can Nano-Coatings Really Improve Stencil Performance. T. Lentz

*SMTAI 2014, Performance Enhancing Nano-coatings: Changing the Rules of

Stencil Design. T. Lentz

Solder paste: No Clean SAC305 Type 4

Aperture Function – Transfer Efficiency*


Benefits & Negative Impacts

Benefits Nano-Coatings Tested

Underside cleaning improved All coatings – A, B, C, D

Bridging improved All coatings – A, B, C, D

Re-apply by the user Coatings B and C

Visible on the stencil Coatings A and D

Transfer efficiency increased Coatings A and D

Negative Impacts Nano-Coatings

Coating wears through abrasion Coatings B and C

Coating wear not visible Coatings B and C

Transfer efficiency decreased Coatings B and C


Stencil Design, Materials & Processes for

Today’s Miniaturized Electronics Assembly

Thank You!

Open Discussion

Bruce Barton, Alent

Jim Price, ASM DEK

Ben Scott, Datum Alloys

Sue Holmes, Photo Stencil

Tony Lentz, FCT Assembly

Ahne Oosterhof, Eastwood Consulting

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