Stencil Printing Presentation

8/11/2019 Stencil Printing Presentation

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PRIVILEGED AND CONFIDENTIAL MATERIALS

Stencil Printing Technology

Seminar

Tim LukeRegional Manager


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Agenda

Solder Paste Basics

Environment

Fine Feature Printing

Stencil

Board

Paste

Printer

Process Stability


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Standard Alpha Labels

Lead-Bearing

Lead-Free

Solder Paste Basics


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LOT # DECIPHERING

Alpha Metals Lot# 11130945

1 = 2011

11 = November

30 = 30th

day of November

9 = Mixer Size

45 = 45th

Sequential Production

Batch That Day

Solder Paste Basics


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Product Configuration Deciphering

Product ConfigurationProduct Configuration

SAC305 OM-338 88.5-3-M13

SAC 305 = Powder Alloy

OM-338 = Flux System

88.5 = Metal Percent, by weight

(50% metal by volume) 3 = Powder Size

M13 = Viscosity Range

Solder Paste Basics


6/45PRIVILEGED AND CONFIDENTIAL MATERIALS

STORAGE & HANDLING

Not Sure - Defer to manufactures

guidelines

Alpha Metals general guidelines

Refrigeration @ 33-45F

FIFO Inventory

Warm-up procedure

Be aware of heat and blowing air

Solder Paste Basics



Warm-up Procedure

Warm-up

Controlled Environment,

Room temp for 4hrs

Follow DocumentedProcedure

Do Not Open ColdContainer:

Warm-up paste beforeopening or dispensing toavoid moisture condensingonto paste.

Avoid HeatingJars/Cartridges to

Accelerate Warm-up

Accelerates mostreactions

Dries pasteprematurely

Reduces paste tackand stencil life

Solder Paste Basics



Operating Environment

Adequate environmental conditions are a

necessity for robust printing process

Solder paste condition is a function of its

environment, which includes;

Temperature

Humidity

Air Flow

Cleanliness

Solder Paste Basics



Environmental InfluencesTemperature: Minimizing process temperature window

will result in a more stable process.

Operating outside of the desired range can cause slumping or

excessive drying out of the paste.

Monitor Performance Outside of Window

Humidity: Minimizing process humidity window will resultin a more stable process

Less than 30% can result in the paste drying out, volatiles will

evaporate. Results in aperture clogging and skips orinconsistent deposit volumes.

Greater than 60% can cause the paste to absorb moisture from

the air causing out gassing during reflow, resulting in slump and

solder balls.

Solder Paste Basics



Environmental InfluencesAir Flow: Keep air flow in the print area to a minimum.

Increased air flow will drive off the volatiles in the paste resulting

in diminished print and reflow performance.

Keep doors and hoods closed.

Do not direct environmental controls at printer or printed boards.

Cleanliness: Keep the printer clean.

Solder paste buildup in the work nest or other moving areas will

decrease the effectiveness of the board support and the

repeatability of the machine.

Solder on board supports can contaminate future boards,possibly causing solder balls.

Dried solder mixed into the paste can cause aperture clogging

Cross contamination of cleaning solvents will effect solder paste

performance.

Solder Paste Basics



Objective

Consistently Deposit a Specific Mass of

Solder in a Specific Location




Key Inputs


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Stencils




Area Ratio Calculation

Force Balance

Area of aperture walls

Area of circuit side opening

=AR

For circular apertures,

D T

(D/2)2=AR

Where D = diameter

T = foil thickness

For square apertures,

4 D T

D2=AR

Where D = side of square

T = foil thickness




Area Ratio CalculationBoth formulae reduce algebraically to:

4 T

D=AR

Where D = Diameter of circle or

Side of square

T = foil thickness

Squares and circles of the same dimension

have the same area ratio

Squares have 4/ (27%)

more volume for the

same AR




Stencil Design


Reconcile Paste Requirements

Most Difficult to Print

Minimize Post Reflow Requirements

Maximize Area Ratio

Know Error Sources

Powder Type -

Packs

Flux Chemistry

Releases

Decrease Foil Thickness

Step and Stepless

Step Stencils

Keep Outs

Reverse Taper

Preforms

Tape and Reel

Stencil Scaling



10 mil circle in 4 mil foi l

AR = 0.63

TE = 49.4%Aperture Volume = 314 mil3

Deposit Volume = 155 mil3

10 mil circle in 5 mil foil

AR = 0.5

TE = 22.3%Aperture Volume = 393 mil3

Deposit Volume = 88 mil3

Reducing the stencil thickness by 20%

increases the deposit volume by 76%!


Stencil Design


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Boards

Mask Registration and Design

Minimize MCSB

Prevent Shorts

Registration Tell Tale

Over Etching

Increased Skips

Alignment

Bridges/Shorts

Ledged Ink -

Gasket

Mask Cure



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Fi F P i i


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Mask Location

Keep Out

Not Fully Cured Mask or Legend Inkcan Contribute

Rough Surface Better

Alignment and Placement Pressure

Reduce Foil Thickness

MCSB

Too Much

Solder Paste and/or in

the wrong location

Fi F t P i ti


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MCSB Mask Design


Fi F t P i ti


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FUNCTION OF PASTE INGREDIENTS

METAL

Melts & bonds to formconnection

FLUX SYSTEM

Wets surfaces

Cleans metal surface

Conducts heat

ACTIVATOR

Oxide reduction

ROSIN

Tack -

HT, Rheology

Activator

ADDITIVES

Tack -

LT, Release,Suspension, Smell,Detergent, Rheology

SOLVENT

Dissolve chemistry,Maintain Suspension


Fi F t P i ti


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PASTE Roll diameter = 1-2 cm (Refer to Manufacture guidelines)Keep paste off squeegee holders

Steady State

Small More Frequent Additions

Dime

1.8 cm

Penny

1.9 cm

Nickel

2.1 cm Quarter

2.4 cm



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Fi F t P i ti


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Printer

Blades

Attack Angle

Print Speed -

f(paste

and thru put)

Blade Pressure

f(print

speed and attack angle)

Stencil Release Rate

f(paste

and aperture)

Board Support

Dedicated vs. Pins

Board Clamping

Edge vs. Top


Fi F t P i ti


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Attach Angle

Combination of Static and Dynamic Angles

Low Angle May Improve Paste Packing

Most Important with Low Area Ratio

f(Blade

Compliancy, Blade Holder, PrintSpeed and Print Pressure)


Fi F t P i ti


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ThinStandard

64

62

60

58

7.255.44

82

64

62

60

58

Blade

Mean

Pressure (Kg)

Release (mm/sec)

Main Effects Plot for Volume TE of CVP-370 PasteData Means

0.15mm Apertures and 0.15mm Gap

Main Effects for 0.3mm Pitch/01005


Fi F t P i ti


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Blade

Pressure (Kg)

Release (mm/sec)

ThinStandard

7.255.447.255.44

82828282

120

100

80

60

40

20

0

Volume

60

100

Boxplot of Volume TE for 0.15mm Apertures with 0.15mm Gap

Fewer Outliers with Thinner Blade

More Volume TE with Thinner Blade

0.3mm Pitch, AR = 0.47


Fine Feat re Printing


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Distribution for 0.3mm Pitch, AR = 0.47

87.575.062.550.037.525.012.5

Median

Mean

65.5065.2565.0064.7564.5064.2564.00

1st Q uartile 59.231

Median 64.870

3rd Q uartile 70.452Maximum 90.834

64.091 65.068

64.337 65.334

8.294 8.986

A -Squared 1.22

P-V alue < 0.005

Mean 64.580

StDev 8.626

V ariance 74.412

Skewness -0.221408

Kurtosis 0.226652

N 1200

Minimum 32.214

A nderson-Darling Normality Test

95% C onfidence Interval for Mean

95% C onfidence Interval for Median

95% C onfidence Interval for StDev95 % C onfidence Intervals

Summary for Volume TE (%)Blade = Thin, Paste = CV_370, AR = 0.49342

87.575.062.550.037.525.012.5

Median

Mean

59.058.558.057.5

1st Q uartile 51.703

Median 58.348

3rd Q ua rtile 65.462Maximum 92.089

57.343 58.554

57.638 58.992

10.281 11.138

A -Squared 1.63

P-V alue < 0.005

Mean 57.949

StDev 10.693

V ariance 114.330

S kew ness -0.49932

Kurtosis 1.47222

N 1200

Minimum 2.872

A nderson-Darling Normality Test

95% C onfidence Interval for Mean

95% C onfidence Interval for Median

95% C onfidence Interval for StDev95% Confidence Intervals

Summary for Volume TE (%)Blade = Standard, Paste = CV_370, AR = 0.49342


Thinner Blade

Tighter Distribution Greater average Transfer Efficiency



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Board Support

Dedicated vs. Pins

Lowest Cost of Ownership

Component Density

Turn Around Time

Both Prone to Cleanliness Issues




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Board Clamping


Top Clamping

Secure, but loss of gasket at edge of board Edge Clamping

Good gasket, but issue with thin boards bowing

Calibration/Maintenance



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Printer

Automated Stencil Cleaning

Paste Rheology

Temp and Humidity

Print Speed and Pressure

Gasket

Board Design and Support

Stencil Aperture Design and Tension

Finest Pitch

Solvent

Steady State




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Determining Wipe Frequency

Wipe the Stencil

Print 2-3 boards

Jog vision system out to inspect

stencil apertures.

Repeat this until you see

squeeze out.

Subtract 1 -

2 prints from thenumber of prints to arrive at

your wipe frequency.




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Print to Fail Wipe Frequency




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Printer

Stencil Release Rate

Function of Solder Paste

Controlled separation of the board from thestencil after the print (slow snap-off)

Minimum of 2X Foil Thickness, More if

High Density

The board is pulled

from Stencil slowly untilthe paste deposit clears

the apertures Faster Separation Speed

2x stencil

Thickness

Board




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Real Time Release Videos

Fast (1 in/sec) vs.

Slow (0.1 in/sec)Separation

3.9 in/sec

8.5 mil Circle

0.53 AR

4 mil thick LC

OM338

Type 3 Powder

Slow: 2005_03_03_009

Fast: 2005_03_07_009



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Process Stability

Minimize Variation

Steady State

Dummy Boards

Auto Stencil Wipe

Inventory Control

MSL

FIFO

Lot Tractability


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QFNs

Pad Design

Aperture Design

Defects

Floating

Wetting

Voiding


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Questions?

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