Troubleshooting the Stencil Printing Process

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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

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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

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Key Elements of a Solder Paste

Printing System

PWB

Stencil

Solder Paste

Squeegee

Mechanical Setup

- Support, Alignment, Contact, Gasketing

Separation

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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

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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

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Solder Paste

N O T T O S C A L E

Formulation, Rheology, Release

Characteristics, Particle Size, Paste

Temperature, Ambient Temperature

& Humidity

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Squeegee

N O T T O S C A L E

Angle, Speed, Pressure,

Stiffness, Surface Energy

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A Good Mechanical Setup is

Essential

Aperture-Pad

Alignment

Board Support

Aperture-Pad

Gasketing

N O T T O S C A L E

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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

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Stencil Separation

If 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.

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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

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Results

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Results

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Preventing The Bad and The Ugly

with Solder Paste Print Inspection

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Phase Shift Interferometry

Shines 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

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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

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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

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SPI SPC

Solder 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

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Comparing Before & After

Process Adjustments

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Automatic Reporting & Exporting

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Still Making Defects?

Check the easiest, most obvious

things first

Investing a few minutes in a system

once-over will find the problem

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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”)

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Check the Tooling

Next: 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

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Check Alignment and Setup

Final: 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

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Didn’t find the Root Cause?

Time to dig a little deeper into the likely

causes of typical print problems…

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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

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Solder Bridges

If you suspect…

Then investigate:

Bad Gasketing See 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 paste Check bead on stencil. ½ - ¾ inch is typical _{(the diameter of a dime or nickel)} Paste is too warm Compare working temperature and tech data sheet. If printer is getting hot inside, check

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Poor Print Definition

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Poor Print Definition

If you

suspect…

Then investigate:

Bad Gasketing See possible reasons for bad gasketing_{Check 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

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Insufficients

Poor Aperture Fill

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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

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Insufficients

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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

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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

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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 stretch Corner-to-corner alignment. If alignment cannot be achieved, stencil can be scaled to

compensate for PWB error.

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