An opening in the barrier layer of the screen through ... · An opening in the barrier layer of the screen through which the material to be applied can pass. ... How Does The Printing

Aperture

• An opening in the barrier layer of the screen through

which the material to be applied can pass

Screen Printing

• Screens consist of a mesh that is impregnated with an emulsion

• Mesh number: holes per square inch

• Apertures are formed by the exclusion of emulsion fromselected areas

• Lithographic techniques

• Wire diameter and mesh number determine paste type to beused

• Approximately 50% open area in aperture

Screen Printing

• Solder paste flows around mesh and is deposited on substrate

• Two steps in the screen printing process:• Flooding is the spreading of material over the area to be

printed without the application of pressure (loading of the mesh)

• Printing is executed in the reverse direction to force material through the screen and on to the substrate

‘Flood’

Basic Screen Process

‘Print’


Screen Printing

‘Finish’


15mm Dia.

Paste

Zero ‘Print Gap’

Basic Stencil Process

Paste

Rolling

Controlled ‘Print Speed’

Even ‘Squeegee Pressure’


Board is lowered under controlled

‘Separation Speed’


Stencil Printing

• Four phases of stencil printing

• On-contact method shown

Stencil Printing

• Stencils consist of a metal foil through

which holes are opened to allow the

passage of material

• Apertures are formed by the machining of

the foil

• Stainless steel, molybdenum, brass, nickel

• 100% open area in aperture

Stencils vs. Screens

Capable of smaller than stencil

size deposits

The Perfect Print

• Clean edges

• Flat on top

• Height = Stencil Thickness

• Good Alignment

• No Flux Bleed ...

• Clean edges

• Flat on top

• Height = Stencil

thickness

• Good alignment

• No flux bleed

PERFECT PRINT

Issue 2 - 06 November 2001

INITIALISATION

SET-UP

MAINTENANCE

PASTE CARTRIDGE LOW

PAPER EMPTY

SOLVENT EMPTY

OPERATIONAL

WAITING IN READY STATE

SYSTEM POWER OFF

HALTED BY ERROR

BEACON INDICATIONS

Note: Some machines fix stencil, move board instead.

Table at home position, substrate loaded into printer.

Company Confidential

Stencil Foil /

Image Area

Substrate

Squeegees

Print Material

Stencil

Tensioned

Mesh

Stencil

Frame

Adjustable

Width

Conveyer

Rail

Fixed

Conveyer

Rail

Rising Table

Heated ToolingRail to Table

Adjuster

Rail to Table

Adjuster

Table at home position, substrate loaded into printer.


Stencil Foil /

Image Area

Substrate

Squeegees

Print Material

Stencil

Tensioned

Mesh

Stencil

Frame

Adjustable

Width

Conveyer

Rail

Fixed

Conveyer

Rail

Rising Table

Heated ToolingRail to Table

Adjuster

Rail to Table

Adjuster

How Does The Printing Process Work?

Table raised to vision height, establishing tooling contact with substrate.


Camera

Substrate

Rising Table

Heated Tooling

Table raised to vision height, establishing tooling contact with substrate.


Camera

Substrate

Rising Table

Heated Tooling


After fiducial capture and stencil position is adjusted to achieve correct

alignment, table raises substrate to print height.


Substrate

Rising Table

Heated Tooling

After fiducial capture and stencil position is adjusted to achieve correct

alignment, table raises substrate to print height.


Substrate

Rising Table

Heated Tooling


Squeegee contacts print material, downward pressure applied.


Substrate

Rising Table

Heated Tooling

Squeegee contacts print material, downward pressure applied.


Substrate

Rising Table

Heated Tooling


Squeegee pushes print material along stencil.


Substrate

Rising Table

Heated Tooling

Squeegee pushes print material along stencil.


Substrate

Rising Table

Heated Tooling


Squeegee reaches end of stroke (print limit).


Substrate

Rising Table

Heated Tooling

Squeegee reaches end of stroke (print limit).


Substrate

Rising Table

Heated Tooling


Table lowered, initiating substrate separation from stencil.


Substrate

Rising Table

Heated Tooling

Table lowered, initiating substrate separation from stencil.


Substrate

Rising Table

Heated Tooling


Table moves to home position and substrate is transported out of printer.


Substrate

Rising Table

Heated Tooling

Table moves to home position and substrate is transported out of printer.


Substrate

Rising Table

Heated Tooling


What is Solder Paste ?

Alloy (solder powder)

Flux

Gelling Agents

100% 100%

~50%

Solder

by

volume

~90%

Solder

by

Weight

~50%

~10%

What does the Flux do ??

Flux ensures successful solder joint is formed between

component and pad

Suspends the powder

Provides correct rheology and tackiness

Cleans surface

Removes oxide from solder powder

Protects surface

Leaves safe residues...

What are gelling agents ??

• Makes the flux shear thinning, pseudoplastic (time independent) or thixotropic (time dependent)

Paste viscosity moves up and down this plot

depending on where it is in the print process.newtonic

Shear rate (s-1)

Viscosity

Temperature v. viscosity

The temperature - viscosity relationship for a typical no clean paste

100

110

120

130

140

150

15 20 25 30 35

Temperature (deg c)

Vis

co

sit

y (

Pa.s

)

Printing Rheology

• Ideal paste ispseudoplastic; most pastesare thixotropic to somedegree

• Viscosity will changethroughout the printingprocess• Allows for deposition and

retention of material

Printability

• Accuracy and repeatability of paste deposits

• Acceptance criteria include

• Deposited paste weight from print to print should not exceed

10%

• Deposit height should have a tolerance of 1 mil from deposit

to deposit within a print

• Uniform coverage without tailing or separation

Working Life

• Time between jar opening and shift of the

rheological properties of the material

outside the process window

• Working life can be checked by examining

print quality and checking tackiness

• Won‟t hold components in place prior to reflow

Slump Testing

• Standards for slump testing

• Print paste on test board of various pitch land

patterns

• Failure if deposits bridge after printing

READ THE LABEL

Harmful

Product Code: SN63RP11AGS90

FAST PRINT ROSIN SOLDER CREAM

Comp: SN63 Pb37

Flux: NC-SMQ51SC

Batch: 0270782

Contents:

500g

Use By:

21.05.99

Solder Paste - Read the Label!

Product Code: SN63 RP11 AGS 90

FAST PRINT ROSIN SOLDER CREAM

Alloy Type

63 % Tin / 37% Lead

(or 35% lead / 2% Silver)

Flux Medium Type

„Rosin Product‟

Particle Size

Type 3 (45 - 20 um)

Metal Content

90% (by weight)

Solder Powder

Type 2 Powder Type 3 Powder

Consistent alloy, particle size and particle shape.

Oxide free

53 - 38 um 45 - 20 um

IPC Powder Specification

• Type 3 is the typical size for current SMT

% By weight of sample

Type None Larger

than

Less than 1%

Larger than

80% minimum

between

10% maximum

less than

2 80 m 75 m 75 to 45 m 20 m

3 50 m 45 m 45 to 25 m 20 m

4 40 m 38 m 38 to 20 m 20 m

5

6

25 to 15 m

15 to 5 m

The smaller the particle size the better print definition but the

paste will be more expensive and a greater chance of reflow

problems!

Rule of thumb: Keep a minimum of 5 particles

per aperture width

Stencil

Board

PARTICLE SIZE

Good Working Practices

• Stir Paste Gently Before Use

• Regulate Paste Diameter at 15mm

• Perform Small Top-Ups Regularly

• Regulate Temperature and Humidity

• Do Not Leave Paste drying on Stencil ...

PASTE STORAGE

PASTE STORAGE

• First In First Out (FIFO)

Clearly label date in and date out

• Rotate jars and tubes regularly

• Store tubes upright

• Correct temperature

Maybe,

Temporary &

Overnight

Definitely for

3 - 6 months Never!

PASTE STORAGE

Considerations:

• Do you use squeegee paste deflectors to reduce

solder paste trails?

• Do you put solder paste trails back into working

paste?

• What do you do with the paste over a lunch break?

• How long will your solder paste remain workable?

• When to dispose of your paste?

• Do you have ProFlow?

REUSE AND DISPOSAL PROCEDURE

Considerations:

• Storage procedure

• Stock levels

• Availability

• FIFO

• Expiry date

• Stabilise temperature before use

PASTE STORAGE

USING THE PASTE

Cold

Storage

„Working Pot‟

Printer

Fresh Paste

Place in machine

After 8 Hours

End of Run / Extended Downtime

End of Day

Harmful

Disposal

Return only if paste quality

is good and it makes

economic sense

„Temporary Pot‟

• Good stencil

• Excellent results

• Few failures

• Predictable yields

GOOD/BAD STENCIL

• Poor stencil

• Lots of stencil wiping/cleaning

• Lots of bridging and open circuits

Remember, stencils are fragile......

• Store and handle with care

• Adequate storage

• Keep stencils clean

• Inspect for damage and stretch

• Repair fiducials when damaged

• Product coded to reduce mistakes

STENCIL MAINTENANCE

Manual:

• Use a lint free cloth

• Solvent soaked cloth in each hand, clean both

sides simultaneously

Dedicated Screen cleaner:

• Recommended

STENCIL CLEANING

Underscreen cleaner set-up (later)

• Better to have excellent low-tech stencil than

poor high-tech stencil.

• Correct stencil dimensions are vital in order

to achieve good prints.

• A great machine can do little to correct a

fundamentally flawed stencil.

• Ensure stencil is thoroughly clean

• Correct stowage

• Repair fiducials

BEST WORKING PRACTICES

Chemical Etching

• Image patterned on both sides simultaneously

• Parallel fabrication of

apertures

• Hourglass profile (~ ½ mil)

• Difficult to release solder

paste from apertures

• Paste deposit „hangs‟ on

girdle and does not

release well

Chemical Etching

• Over- and under-etching

• Etch is both vertical and horizontal

• Must allow for growth in both dimensions

Laser Cutting

• Fabrication by cutting holes in foil with

high-power laser

• Gerber data used to drive cutting tools

• Serial process- more expensive than etching

• Aperture size minimum 2 mil

• Tolerance .2-.3 mil

Laser Cutting

• Buildup of debris on surface

• Rough side walls

• Electropolishing used to polish side walls

• Selective chemical etch

• Don‟t want too smooth a stencil top

Laser cutting

Trapezoidal aperture

Step 3

Step 1

Step 2

Step 4

YAG Laser 1S.S Type 2G

Co2 LaserKaptonF

FormedNiE

YAG Laser 1NiD

YAG Laser 3S.S Type 1C

YAG Laser 2S.S Type 1B

YAG Laser 1S.S Type 1A

Fabrication

Method

Metal TypeStencil Type

YAG Laser 1S.S Type 2G

Co2 LaserKaptonF

FormedNiE

YAG Laser 1NiD

YAG Laser 3S.S Type 1C

YAG Laser 2S.S Type 1B

YAG Laser 1S.S Type 1A

Fabrication

Method

Metal TypeStencil Type

A B C

D E F

G

Electroforming

• Image built up to desired thickness in plating bath• Nickel built up around apertures created by pattern

• Very tight and smooth aperture tolerances• Aspect ratio 1.1-1.2 (width:thickness)

• 1-12 mil thickness

• May be too smooth in

some cases

Nickel/Brass

• Electroless nickel plated as the wear layer

over etched brass

• Easier fabrication with brass foil

• Lower surface tension with paste for longer

equipment life

• Tolerances comparable with etched

stencils

Stencil Selection

ChemicalEtching

LaserCut

Electro-formed

Hybrid

Cost Cost effective Cost effective Expensive Less Expensive

Accuracy OK Very Very Mixed

Min. Aperturesize

0.2mm(0.4mmPitch)

No Limit! No Limit! No Limit!

Aperture Shape

Material Stainless SteelBrass

Stainless Steel Nickel Alloy Stainless SteelBrass

?

Fiducials Good Good Weak Good

Aspect Ratio

• Ratio between aperture width and stencil thicknessshould not be smaller than 1.5 for good transfer

• Smaller aspect ratios will result in pad clogging

Aperture Profiles

• Tapered apertures preferred for improved release of

solder paste

• Natural consequence of electroforming and laser cutting

Tapered Apertures

• Higher smearing from wider bottom overwhelmed by

consistency from better release

• As pitch decreases, smearing becomes more of an issue

„O‟-ring gasket featureTaperedHourglass

Stencil Contrast

Fine-Pitch Considerations

• Micromodification is the reduction inaperture size to minimize bridging atfine pitch

• „Zipper‟ patterns also increase thepitch between neighboring deposits

• Poor registration or an excess ofsolder outside the lands can lead tothe formation of solder balls (andshorts)

Step-Down Stencils

• Areas of differing thickness used in same stencil to

deliver different paste quantities

• Thin area for fine-pitch devices

• Thick area for coarser pitch components

Stencil Design

Pitch

Length

Width

Design Stencil

Apertures to

allow a clearance

of at least 50 µm

between

Paste and edge

of Pad

Paste

Pad

Paste Transfer Efficiency Trend

Recommended Area Ratio > 0.66

Stencil Design

Release Equation 1

Release Equation 2

W

T

Paste Release Equation

W

Avg.Particle size

> 0.66

>1.5

> 5

W

T

L

L*W

2*(L+W)*T

IPC 7525 – Stencil Design Standard

Stencil thickness determined by finest pitch device.

Aperture opening usually slightly smaller than circuit

board pad.

If area ratio is violated, then aperture to be oversized.

Try to avoid making apertures 1:1 with pad.

Finest spacing between apertures should be more

than the thickness.

Laser cut stencils are most popular for SMT,

electropolishing is common surface treatment.

Fine pitch, small feature, or high aperture count

applications justify Electroforming.

Stencil Design Guidelines

Stencil Design

150µm

Solder paste type 3 Stencil thickness

component Aperture width µm 100µm 125µm

290

265

240

QFP 0.5

215

240

215

190

QFP 0.4

165

152

140

QFP 0.3

120

760BGA 1.5

500

380BGA 0.75

250

Solder paste type 4 Stencil thickness

component Aperture width µm 100µm 125µm 150µm

290

265

240

QFP 0.5

215

240

215

190

QFP 0.4

165

152

140

QFP 0.3

120

760BGA 1.5

500

380BGA 0.75

250

Stencil - Board alignment

• Cause:

• Poor Artwork

• Poor Quality Stencil Manufacture

• Poor board manufacturing

• Board Stretch

• Poor Fiducial Alignment

• Machine not calibrated

• Solution:

• Print Offsets

• Buy New Stencil

• Improve Fiducial Score

• Buy better boards

• Calibrate machine

Printability




10%




Paste Thickness

• Determines solder volume in joints

• Too thick will bridge

• Too thin will cause insufficients

• Governed by

• Mask thickness (or mesh number and emulsion

thickness for screens)

• Snap-off height

• Overall amount of solder also affected by metal

loading of paste

See anything wrong with these stencil designs?

Deposits are small, but

design is good as long

as there‟s enough alloy

for joint & repeatability.

TE is good, but

bridging will result.

5-mil thick laser cut stencil (no post processing)

Aperture geometry: circle, square

Aperture size: 6, 8, 10, 12, 14, 16 mils

Squeegees: 200mm long, 60 deg metal

Print Speed: 20 mm/sec

Print Pressure: 5 kg

Separation Speed: 0.5 mm/sec

Separation Distance: 2 mm

Print Gap: 0 mm

Substrate: 200mm Silicon Wafer

Print Stroke Direction: Forward Only

All photos shown after printing on known “clean” stencil.

Type 3 Solder Paste

Indium SMQ92J

25 - 45 micron particle

90% Metal Loading

Sn63 / Pb37 Alloy

9 mil aperture minimum

Type 4 Solder Paste

Indium SMQ92J

20 - 38 micron particle

89.5% Metal Loading

Sn63 / Pb37 Alloy

8 mil aperture minimum

Type 3 vs. Type 4 test

5-ball rule 5-ball rule

6-mil Circular Aperture 6-mil Square ApertureT

ype 3

Type 4

Stencil Thickness = 5 mils


ype 3

Type 4



ype 3

Type 4



ype 3

Type 4



ype 3

Type 4


Mesh density specification dictates opening and wire diameter

Mesh thickness determined by wire diameter and weave

Mesh Density = wire count / unit area

Best Result

Mesh Screen Printing Applications

Lower viscosity, smaller particle size materials used

Flux

Adhesive

Underfill

Die Attach

Ink

Frit

Thin uniform coatings

Fine detail lines / high resolution lines & fine features

Board considerations

• Pad surface HASL

Gold on nickel

Organic coating

• Resist Thinner than pads

Clear of fiducials?

• Etc. Size

Underside components

Obstructions, bar codes etc

Fiducials and holes

• Fibres

• these can block stencil apertures

• Old Solder

• this can cause solder balls

• Fingerprints

• these can corrode pads, tracks etc

handle board by edges, wear gloves

• Use CM20...

Ensure that production boards are free of ...

Board Issues

• Uneven Pad surface can cause

loss of gasket seal resulting in

bridging etc.

• If the resist is higher that the pad

the gasket seal will be lost

resulting in bridging etc. ...

The board design can impinge on the effectiveness and quality

of the print process. Here are some things to look out for ....

Pad with uneven

surface coating

Pad

Resist

Board Issues

• Underside components can restrict

the use of tooling pins - use tooling

plate or FPA

• Large underside components may

impede board transport - increase

under clearance ...



Board Issues

• Via holes can cause problems

with vacuum tooling

• Labels, bar code stickers etc.

increase board thickness leading

to tooling, print and pressure

problems ...



Tested ok

Board Issues

• Boards with tapered or

chamfered edges are difficult to

clamp and may be liable to

movement during print leading to

random offsets ...

Rail

Board Issues

• Uneven pad surface

• Solder resist too high

• Underside components restricts tooling

• Via holes with vacuum tooling

QUALITY ISSUES

• Warping and stretch

• Chamfered edges

• Fiducial quality

• Colour changes, surface finish

• Barcode labels

QUALITY ISSUES

||||| || |||| ||| ||||||| ||||||||| |||| |||||

• Board stop set-up

• Storage and handling

• Dirty boards:

• Fibres from production

• Old solder

• Finger prints

• Labels

PROCESS ISSUES

• Ensure adequate storage is available

• Make sure boards are clean before use

• When handling:

• Wear gloves

• Unpack carefully

• Differences between batches

• Colour changes

• Board stretch

STORAGE AND HANDLING

• Handle boards with care and always wear gloves

• Check batches carefully for:

• discolouration, warpage, fiducial quality, thickness of resist,

circuit legends, barcode labels, general board quality

• Ensure board cleanliness

• If boards are washed:

• identify them for inspection before reuse


Pitch

Length

Width

A good

design

Paste

Pad

ALIGNMENT

Possible causes:

• Poor quality stencil manufacture

• Poor board manufacturing

• Board stretch or warping

• Poor fiducial alignment

• Machine not calibrated

• Escalate if found

STENCIL - BOARD ALIGNMENT

STENCIL - BOARD ALIGNMENT

Possible solution:

• Improve fiducial score

• Print offsets

• Incorrect batch of boards

• Incorrect position of tooling

Squeegee Materials

• Hardness of material will affect transfer quality• Material will „scoop‟ out

of apertures

• Harder squeegees for

finer pitch printing

• Steel• Thin blade to conform to surface

• Contact angle varied to change applied force

• Will also cause stencil wear

Polyurethane squeegees

• Minimal friction with surface

• Geometry varied to optimize wiping

• Diamond for bi-directional, rectangular for

unidirectional

• Hardness affects scooping of deposits

Pressure

• The greater the angle, the greater the net force on

the squeegee

• Opposing force from fluid is minimized

Squeegee Pressure

• Low pressure can

• Cause skips and ragged edges on deposits

• High pressure can

• Smear deposits

• Damage fine webs or soft squeegees

• Scoop out deposits

Squeegee Effects

• Wear will affect print quality

• Edges that are not straight and sharp will result

in reduced print pressure

Transfer / Driving forces

are influenced by:-

•Squeegee angle

•Print speed

•Paste rheology

•Paste roll volume Driving force

Transfer force

Squeegee Principles

Reaction forceSqueegee force

Squeegee Principles

Paste

rollingStreamlines

Shear rate changes with

position of paste in roll

Lower shear rates

Higher shear rates

Effect of Excess Pressure

Print Direction

Aperture

Scooped

Pads

Board

stencil

„Scooping‟

Angle º

45º 60º

•Intrusive reflow

•High speed printing

•Printing thick stencils

•Printing Glue

•Printing mesh screens

•Standard SMT Printing

Increased

Force

Reduce

Force

Choose the closest length to the board size

Board

Squeegees

Paste

Dry Paste

Gap

LENGTH

• If your specified squeegee is too large………

Use stencil support blocks

LENGTH

Stencil Support Blocks

Note: an average squeegee may travel 5-6 miles in its

lifetime! ……….And that's across a rough surface and under

pressure!

Angle changes

EFFECT OF WEAR

• Used to prevent tramlines.

• Helps maintain paste in good condition.

• Maintains a constant transfer into through holes for

intrusive reflow in paste applications.

Paste Deflectors - set 0.25mm

above stencil surface

PASTE DEFLECTORS

• Polyurethane blades come in trailing edge

and diamond edge profiles.

• Pink (90 - 97 shore) Only for totally flat boards

• Fawn (85 -90 shore)

• Brown (80 - 85 shore)

• Blue (75 -80 shore) Excellent good coarse pitch,

• Robust, boards but must be straight

• Green polyurethane (70 - 75 shore)

• Only for screen printing

• Metal

• Excellent coarse and fine pitch

MATERIAL SELECTION

Polyurethane

Shore 65-85 (Dek Green)

Too Flexible for stencil printing, used only in mesh applications

Shore 95 (Dek Red)

Will need replacing or regrinding after 25000 prints.

Essential to control squeegee pressure to reduce scooping.

Causes little damage.

Commonly used in SMT.

Clamped and bonded squeegees

Polyurethane Squeegees

Metal

Shore +200

• Easy to damage

• Expensive

• High pressure may cause Bridging

+ Paste height higher than stencil thickness

+ Better print quality

+ Wider process window for pressure

+ High pressure will not cause Scooping

Metal squeegees

Metal squeegees

Overhang

15 mm - widely used, reduced

risk of stencil/blade damage

due to increased flexibility

6 mm - historically used but

no longer recommend due to

increased risk of stencil/blade

damage.

Composite

Shore 120 (Composite)

• Very Rigid

• Uneven boards cause damage to Stencil and

Blades

+ Used in applications where very large apertures

are used.

+ Wider process window for pressure

Property Metal Polyurethane

Fine pitch GoodExcellent

Large apertures Excellent Good

Pressure effect Does not cause

scooping

Needs control

Damage resistance Poor Good

Damage to stencil Poor Good

Life Until broken 25,000

Paste Height < Stencil Thickness> Stencil Thickness

Metal vs. Polyurethane

STORAGE

• Ensure blades and squeegees are stored

correctly

• Ensure there are adequate stocks

• Ensure blades are properly cleaned before

storing

• Check not damaged or worn

• Use correct length for board

• Ensure adequate spares are held

• Always ensure cleanliness

• Always calibrate squeegee heights before use


Metal blade edges can be sharpened. These

last as long as they don‟t get bent or chipped.

Polyurethane blades cannot be sharpened (easily),

however, with a clamped system you can reverse

the blade to use the opposite clean edge.

Squeegee blade guideline

TOOLING SOLUTION

Table

Rail Rail

Rails, Board and Stencil are flat and parallel

Perfect

Tooling

Block

• Problems

• Incorrectly positioned

• Paste deposits raising height of board

• Impacting underside components

• Missing under critical components

• Placed under rail

TABLE

RAIL RAIL

MAGNETIC TOOLING PILLARS

• UNDERSIDE COMPONENTS

GENERAL TOOLING PROBLEMS

Tooling Block

Under

Clearance

• Board lifted by tooling causing:

• Board damage

• Squeegee pressure problems

• Uneven paste height

• Dirty stencil

TABLE

RAIL RAIL



• Check• Cleanliness of table

• Cleanliness of pins

• Board clamps for damage

• Transport belt condition

• Board thickness

Table

Rail Rail

Bad

Tooling

Block

• Board warpage

• Stencil for damage

• Position of tooling pins

• Dowel pin location

• Underside components

Printability




10%




Print Gap

• The distance maintained between the

screen (or stencil) and the workpiece

• Also known as “snap-off”

• The stencil or screen lifts from the paste

deposits after the squeegee travels over

and fills the aperture

Gasketing

• The seal formed between the stencil or screen and

the substrate to be printed

• Good print definition and minimal bleed-out

Print Mode

• On-Contact• More accurate print

• Slow snap-off speed

• Off-Contact• Stencil kept 1-2 mil away from surface

• Local gasketing from squeegee pressure

• Used with screens to prevent smearing in flood mode

Print Speed

• Paste should roll for good print definition

• Downward component into apertures from roll provides even print

• Too fast a squeegee speed will

cause material to slide across

surface

• Material will lack the vertical

component and not fill apertures

properly, leading to inconsistencies in printing

• Balance throughput demands vs. aperture

Print Direction

• Less solder on pads parallel to squeegee orientation

than those perpendicular to travel

• Plan orientation of part

and stencil to

accommodate effect

Transfer Efficiency

• The ratio of the volume of paste actually

deposited versus the volume of the stencil

aperture

• Round apertures generally have a higher

transfer efficiency than rectangular apertures

Process Window

• Affected by solder volume and coplanarity

Start at 0.5 kg pressure per 1 inch of squeegee

Adjust print speed to suit cycle time requirement

Print gap = 0 (no snapoff)

Separation speed usually not critical parameter.

Process window for SMT can tolerate print

speeds between 10 and 100mm/sec for reputable

solder pastes.

Rules of thumb:

As speed pressure should go

as speed pressure should go

Increasing speed requires a corresponding increase in

pressure in order to maintain the same print results. This

leads to accelerated squeegee and stencil wear.

Print speed and Squeegee pressure are directly

related...

PRINT SPEED AND PRESSURE

• Separation speed

• The speed at which the board separates from the

stencil

• Adjust when deposits look conical or „Dog Eared‟

• This value effects print cycle time

• Separation distance

• The distance the board travels at the set separation

speed

• Keep to the minimum value required

• Faster separation speed, smaller distance required

SEPARATION SPEED AND DISTANCE

EXAMPLES BEYOND OUR DIRECT CONTROL

• TEMPERATURE (OPENING COVERS)

• HUMIDITY (OPENING COVERS)

• LIGHT (DAY TO NIGHT)

• HUMANS (BIGGEST PROBLEM)

• BOARD QUALITY

• “HEARTBEAT” IF LINE GOES DOWN

Constant temperature

Constant humidity

MACHINE ENVIRONMENT

Is the environment in control

• Temperature Control Unit

• Operator set

• ± 8º C from ambient

• Maintains to within ± 1º C

• Amber then Red beacon indications

• Audio alarm

•

TEMPERATURE CONTROL

COMMON PRINT PROCESS FAULTS

Standard paper roll under stencil cleaner

Wet, Vac, Dry modes

Controllable travel speed, number of passes,

vacuum level, cleaning interval.

Cleanroom grade paper available.

Alternative disposable porous foam cassette

cleaning system. Ultrasonic assisted systems

also offered. Topside cleaning uncommon.

Stencil bottomside contamination

Too many prints before bottomside cleaning.

Squeegee pressure too high.

Inadequate board support.

Poor stencil to board gasket.

Stencil bottomside contamination

…could be caused by circuit board topography.

i.e. traces, non uniform pad finish (HASL), solder

mask thickness / misregistration.

Result of printing on a perfect surface

Print #12

No stencil cleans between any prints.

• Squeegee pressure or

ProFlow pressure too low

• Separation speed too high

• USC set-up adequate

• Paste knead set-up

• Paste condition

STENCIL BLOCKAGE

Causes:

• Print/ProFlow pressure too high

• Alignment correct

• Print deposits greater than 1

• USC set-up adequate

• Paste knead too frequent

• Paste condition

STENCIL SMEAR

Causes:

• Print/ProFlow pressure

• Too many print deposits

• USC set-up


• Paste condition

• Stencil condition

BRIDGING

Causes:

• Print speed

• Print/ProFlow pressure

• Print deposits sufficient

• Separation speed

• Stencil blockage

• See next slide….

INSUFFICIENT PASTE

Causes:

• USC set-up


• Print speed too high

• ProFlow piston pressure toolow

• stencil wet after cleaning

• Wet or dirty boards

• No ProFlow stencil support

• Separation speed setincorrectly ...

INSUFFICIENT PASTE (CONTINUED)

Causes:

SCOOPING

• Squeegee pressure too high

or squeegee too soft

(polyurethane)

• Stencil apertures too large

• Tooling too high

Causes:

MISSING EDGE

• Squeegee/ProFlow

pressure too high

• ProFlow print speed too

low

• Tooling incorrect

Causes:

EXCESSIVE PASTE

• Squeegee ProFlow

pressure too high

• ProFlow print speed too

low

• Lack of tooling support

Causes:

• Fiducial set-up

• Correct offsets

• Alignment weighting

• Board to stencil mismatch

• Board stretch

• Machine calibration

ALIGNMENT

Causes:

• Clean edges

• Flat on top

• Height = Stencil Thickness

• Good Alignment

• No Flux Bleed ...

PERFECT PRINT

Goal:

Documents

An opening in the barrier layer of the screen through ... · An opening in the barrier layer of the screen through which the material to be applied can pass. ... How Does The Printing