Troubleshooting the Stencil Printing Process

Transcription

1 Troubleshooting the Stencil Printing Process Chrys Shea, Shea Engineering Services Sponsored by:

2 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%

3 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

4 Key Elements of a Solder Paste Printing System PWB Stencil Solder Paste Squeegee Mechanical Setup - Support, Alignment, Contact, Gasketing Separation

5 PWB Pads: Flat, Right Size, Free of Debris or Solder Mask Solder Mask: No taller than pad, not encroaching on pad (unless SMD) PWB: Minimal warp and bow Positional Accuracy N O T T O S C A L E

6 Stencil Apertures: Right Size & Shape, Smooth Walls, Uniform Thickness, Free of Debris or Solder Paste Positional Accuracy N O T T O S C A L E

7 Solder Paste Formulation, Rheology, Release Characteristics, Particle Size, Paste Temperature, Ambient Temperature & Humidity N O T T O S C A L E

8 Squeegee Angle, Speed, Pressure, Stiffness, Surface Energy N O T T O S C A L E

9 A Good Mechanical Setup is Essential Aperture-Pad Alignment Aperture-Pad Gasketing Board Support N O T T O S C A L E

10 Gasketing Poor Alignment PWB Shrink Aperture larger than pad Pad smaller than aperture Paste on bottom of stencil Solder Mask on Mask padon Pad HASL Dome N O T T O S C A L E

11 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. N O T T O S C A L E

12 Print Process 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 N O T T O S C A L E

13 Results 16-mil pitch QFP

14 Results 20-mil pitch QFP

15 Preventing The Bad and The Ugly with Solder Paste Print Inspection

16 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

17 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

18 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

19 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 Export to Excel worksheet

20 Comparing Before & After Process Adjustments

21 Multi-Line Monitoring

22 Multi-Line Reporting

23 Automatic Reporting & Exporting In Excel (not Access) format!

24 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

25 Overall System Check First: Knead the paste, wipe the stencil, print a board, observe Is the right amount on stencil? Paste bead should be about 1.5 cm diameter (5/8 ) Bead size affects fill pressure 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

26 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 If vacuum, check for leaks

27 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 Inspect the squeegees Damage or dings, angle Check pressure balance on blades Recheck Print Parameters Speed, Pressure Snap off Delay, Speed and Distance

28 Didn t find the Root Cause? Time to dig a little deeper into the likely causes of typical print problems

29 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

30 Solder Bridges

31 Solder Bridges If you suspect Bad Gasketing Residual paste from previous print Separation speed (too fast or too slow) Squeegee pressure too high Too much paste Paste is too warm Then investigate: See slides on possible reasons for bad gasketing. Check board support Stencil cleaning parameters Increase wipe frequency Increase or decrease separation speed - Different pastes have different optimums and its usually one or the other no middle Decreasing the force. Most pastes work well with lb/in force Check bead on stencil. ½ - ¾ inch is typical (the diameter of a dime or nickel) Compare working temperature and tech data sheet. If printer is getting hot inside, check exhaust fans

32 Poor Print Definition Peaks and Dog Ears

33 Poor Print Definition If you suspect Bad Gasketing Separation speed (too fast or too slow) Residual paste from previous print Misalignment Squeegee pressure too high or too low Paste is too warm Then investigate: See possible reasons for bad gasketing Check board support Increase or decrease separation speed - Different pastes have different optimums and its usually one or the other no middle Stencil cleaning parameters Increase wipe frequency See section on alignment Adjust force. Most pastes work well with lb/in. Check temperature and tech data sheet

34 Insufficients Poor Aperture Fill Bonus Question: what s wrong with this aperture design?

35 Poor Aperture Fill If you suspect Pause in printing raised paste viscosity Squeegee speed too high or too low Squeegee pressure too low Not enough paste on stencil Paste is too warm Paste sticking to squeegee blade Squeegee worn or damaged Then investigate: Knead strokes. Clean board used for kneading Check print speed Increase the force. Most pastes work well with lb/in. Check bead on stencil. ½ - ¾ inch is typical Check temperature and tech data sheet Check bead on stencil. ½ - ¾ inch is typical. Check paste temperature Inspect blades and replace if necessary

36 Insufficients Poor Aperture Release

37 Poor Aperture Release If you suspect Pause in printing raised paste viscosity Residual paste building up in apertures Paste is too cold Squeegee pressure too low Then investigate: Knead strokes. Clean board used for kneading Check stencil cleaning parameters, increase frequency, clean after down time Check temperature and tech data sheet Increasing the force. Most pastes work well with lb/in

38 Poor Gasketing

39 Poor Gasketing If you suspect Board Support Bad alignment Solder mask higher than pads Stencil apertures larger than PWB pads Hot Air Solder Level finish creates uneven printing surface Labels, inks, or other surface features prevent stencil from seating on PWB Then investigate: Check (clean or improve) PWB support See section on alignment Check solder mask height and compare to specification Measure and compare to specification More planar, non-hasl finishes. Consult with PWB vendor on improving doming effect of HASL process. Proximity of features to defects. Consider changing locations of those features or halfetching the bottom of the stencil to accommodate them.

40 Poor Alignment

41 Poor Alignment If you suspect Board Support Printer alignment error Stencil mesh torn or tension too loose PWB or stencil positional error PWB shrink or stretch Then investigate: Check (clean or improve) PWB support Check printer fiducial reading routine. Watch fiducial find on screen Check for stencil movement at beginning of print stroke Corner-to-corner alignment of apertures and pads. Corner-to-corner alignment. If alignment cannot be achieved, stencil can be scaled to compensate for PWB error.

42 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

43 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

44 Questions? Contact Chrys at: SHEA ENGINEERING SERVICES Communicating Expertise Sponsored by: