Troubleshooting the Stencil Printing Process
Chrys Shea, Shea Engineering Services
Sponsored by:
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
Sponsored by:
Contact Chrys at:[email protected]
609 239-2995
www.christopherweb.com714 979-7500