Troubleshooting the Stencil Printing Process

Chrys Shea, Shea Engineering Services

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Topics

Printing– Solder paste properties– Key elements of the process

Inspecting– Phase Shift Interferometry– Typical Control Methods– SPI SPC

Overall system check– Will identify root cause 80% of the time

Investigating specific defect modes– For the other 20%

Solder Paste Properties

• Solder paste is not solid, it is not liquid, it is thixotropic:– Yields (moves) when pressure is applied to it

– Holds its shape when pressure is not applied

– Thins down and stiffens up (like peanut butter)

• Viscosity – how easily it flows under pressure

• Rheology – how its viscosity changes as the pressure changes

Key Elements of a Solder Paste Printing System

PWB

Stencil

Solder Paste

Squeegee

Mechanical Setup

- Support, Alignment, Contact, Gasketing

Separation

PWB

Positional Accuracy

Solder Mask: No taller than pad, not encroaching on pad (unless SMD)

Pads: Flat, Right Size, Free of Debris or Solder Mask

PWB: Minimal warp and bow

N O T T O S C A L E

Stencil

N O T T O S C A L E

Positional Accuracy

Apertures: Right Size & Shape, Smooth Walls, Uniform Thickness, Free of Debris or Solder Paste

Solder Paste

N O T T O S C A L E

Formulation, Rheology, Release Characteristics, Particle Size, Paste Temperature, Ambient Temperature & Humidity

Squeegee

N O T T O S C A L E

Angle, Speed, Pressure, Stiffness, Surface Energy

A Good Mechanical Setup is Essential

Aperture-Pad

Alignment

Board Support

Aperture-Pad

Gasketing

N O T T O S C A L E

Gasketing

Poor Alignment

Mask on pad

Pad smaller than aperture

HASL Dome

PWB Shrink Aperture larger than pad

Solder Mask on Pad

Paste on bottom of stencil

N O T T O S C A L E

Stencil SeparationIf printer has stepper motor on Z-axis, separation speed is very important to print quality:

• Steppers induce vibration in the system, which causes pastes to thin out at aperture walls• Some pastes benefit by the localized shear thinning and work best with slow separation speed• Some pastes thin too much with slow separation, producing poor print definition and stringy bridges, and work best with fast separation speed.

How does the engineer know? Simple experiment.

N O T T O S C A L E

Print Process

N O T T O S C A L E

1) Alignment, gasketing of apertures to Pads

2) Squeegee motion thins paste so it flows into apertures

3) Paste recovers; stiffens up 4) Stencil separates from PWB, paste deposits release from stencil onto pads

Results16-mil pitch QFP

Results20-mil pitch QFP

Preventing The Bad and The Ugly with Solder Paste Print Inspection

Phase Shift InterferometryShines a grated light on the subject

– Called a Moire pattern– Pattern appears to move as it crosses

topography– Like afternoon sunlight shining through blinds

Takes photos at known angles– Calculates height by changes in

pattern at different angles

– Extremely accurate and repeatable

– Used in all types of surface measurements

PSI and Solder Paste

“Chops” each deposit into tiny segments – as small as 10 µm

Calculates height volume of each segment

Compiles data for each deposit and returns:

– Area

– Height

– Volume

– Offsets in X and Y

– Solid Model

Solder Paste Inspection Criteria

Starting parameters

– Gets theoretical aperture volume from stencil Gerber file

– 50% to 150% of theoretical volume

– 50% offset in X or Y

Tightening the process

– Criteria can be set tighter or looser for each device on PWB (uBGA +/- 20%)

– Track effects of changes in process or in control parameters

– Use historical production yield data to optimize for individual processes

SPI SPCSolder Paste Inspection

Statistical Process Control

Start with standard SPC– X-bar, Sigma, Cp/Cpk, and Histograms– Optimize the basic process– Compare before and after adjustments

New Feature - Multiple Lines, Real-Time Line Monitoring– Yield and PPM data for multiple lines on one page– Grouping function allows monitoring of selected areas of

interest (eg. uBGA, 0201’s, 12-mil pitch, etc)

New Feature – Reporting and Exporting– Auto report setup function– Export to Excel worksheet

Comparing Before & After Process Adjustments

Multi-Line Monitoring

Multi-Line Reporting

Automatic Reporting & ExportingIn Excel (not Access) format!

Still Making Defects?

Check the easiest, most obvious

things first

Investing a few minutes in a system

once-over will find the problem

4 out of 5 times

Overall System Check

First: Knead the paste, wipe the stencil, print a board, observe

Does it roll over the surface of the stencil and release cleanly from squeegee blade?

• If no, replace it with fresh solder paste

• Check temperature and consistency

Is the right amount on stencil?

• Paste bead should be about 1.5 cm diameter (5/8”)

• Bead size affects fill pressure

Check the ToolingNext: Remove the stencil

Inspect the stencil

• Physical damage

• Paste/debris in apertures

• Worn out or dirty fiducials

• Rips or tears in mounting mesh

Inspect the board support

• Is dried paste interfering with PWB?

Shuttle a board into position

• Tap or press on top to verify support

• Check for movement in X & Y

• If vacuum, check for leaks

Check Alignment and SetupFinal: Reinstall the stencil

Check the alignment

• Watch the process, including the vision finding the fiducial

• Confirm alignment

• Check contact between stencil & board

Recheck Print Parameters

• Speed, Pressure

• Snap off Delay, Speed and Distance

Inspect the squeegees

• Damage or dings, angle

• Check pressure balance on blades

Didn’t find the Root Cause?

Time to dig a little deeper into the likely causes of typical print problems…

Solder Paste Print Defects

Solder Bridges

Poor Print Definition

- Peaks or “Dog Ears”

Insufficient Solder Volumes

- Poor Aperture Fill

- Poor Aperture Release

Poor Gasketing

Poor Alignment

Solder Bridges

Solder Bridges If you suspect… Then investigate:

Bad GasketingSee slides on possible reasons for bad gasketing.Check board support

Residual paste from previous print

Stencil cleaning parametersIncrease wipe frequency

Separation speed (too fast or too slow)

Increase or decrease separation speed - Different pastes have different optimums and its usually one or the other – no middle

Squeegee pressure too high

Decreasing the force. Most pastes work well with 1 – 1.25 lb/in force

Too much pasteCheck bead on stencil. ½ - ¾ inch is typical (the diameter of a dime or nickel)

Paste is too warmCompare working temperature and tech data sheet. If printer is getting hot inside, check exhaust fans

Poor Print DefinitionPeaks and “Dog Ears”

Poor Print Definition

If you suspect… Then investigate:

Bad GasketingSee possible reasons for bad gasketingCheck board support

Separation speed (too fast or too slow)

Increase or decrease separation speed - Different pastes have different optimums and its usually one or the other – no middle

Residual paste from previous print

Stencil cleaning parametersIncrease wipe frequency

Misalignment See section on alignment

Squeegee pressure too high or too low

Adjust force. Most pastes work well with 1 –1.25 lb/in.

Paste is too warm Check temperature and tech data sheet

InsufficientsPoor Aperture Fill

Bonus Question: what’s wrong with this aperture design?

Poor Aperture Fill

If you suspect… Then investigate:

Pause in printing raised paste viscosity

Knead 4 -1 0 strokes. Clean board used for kneading

Squeegee speed too high or too low

Check print speed

Squeegee pressure too low

Increase the force. Most pastes work well with 1 – 1.25 lb/in.

Not enough paste on stencil

Check bead on stencil. ½ - ¾ inch is typical

Paste is too warm Check temperature and tech data sheet

Paste sticking to squeegee blade

Check bead on stencil. ½ - ¾ inch is typical. Check paste temperature

Squeegee worn or damaged

Inspect blades and replace if necessary

InsufficientsPoor Aperture Release

Poor Aperture Release

If you suspect… Then investigate:

Pause in printing raised paste viscosity

Knead 4 -1 0 strokes. Clean board used for kneading

Residual paste building up in apertures

Check stencil cleaning parameters, increase frequency, clean after down time

Paste is too cold Check temperature and tech data sheet

Squeegee pressure too low

Increasing the force. Most pastes work well with 1 – 1.25 lb/in

Poor Gasketing

Poor Gasketing

If you suspect… Then investigate:

Board Support Check (clean or improve) PWB support

Bad alignment See section on alignment

Solder mask higher than pads

Check solder mask height and compare to specification

Stencil apertures larger than PWB pads

Measure and compare to specification

Hot Air Solder Level finish creates uneven printing surface

More planar, non-HASL finishes. Consult with PWB vendor on improving doming effect of HASL process.

Labels, inks, or other surface features prevent stencil from seating on PWB

Proximity of features to defects. Consider changing locations of those features or half-etching the bottom of the stencil to accommodate them.

Poor Alignment

Poor

If you suspect… Then investigate:

Board Support Check (clean or improve) PWB support

Printer alignment error

Check printer fiducial reading routine. Watch fiducial find on screen

Stencil mesh torn or tension too loose

Check for stencil movement at beginning of print stroke

PWB or stencil positional error

Corner-to-corner alignment of apertures and pads.

PWB shrink or stretchCorner-to-corner alignment. If alignment cannot be achieved, stencil can be scaled to compensate for PWB error.

Alignment

Summary - Troubleshooting

• Understand the key elements in the solder paste printing process

• Maintain control of the process

– It’s where most of the rework comes from

– It’s where the money is in SMT!

• When problems arise, first do the 5-minute overall system check

– 80% chance that you resolve the problem

• If specific defects continue to occur, follow logical troubleshooting guidelines

Thank You

Many thanks to:

• SMTA and Upper Midwest Chapter – Event hosts

• Jabil, San Jose, CA– GOS Laboratory engineers and technicians

– Hien Ly, Michael Lapitan, Mike Santos

• Christopher Associates– Koki

– Koh Young

Questions?

SHEA ENGINEERING SERVICES

Communicating Expertise

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Contact Chrys at:chrys@sheaengineering.com

609 239-2995

www.christopherweb.com714 979-7500