Troubleshooting the Stencil Printing Process

Transcription

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

2 Topics Prin-ng Solder paste proper4es Key elements of the process Inspec-ng Phase Shi: Interferometry Typical Control Methods SPI SPC and other applica4ons Overall system check Will iden4fy root cause 80% of the 4me Inves-ga-ng 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 s2ffens up (like peanut buoer) 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, Gaske4ng Separa-on

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 Posi-onal Accuracy NOT TO SCALE

6 Stencil Apertures: Right Size & Shape, Smooth Walls, Uniform Thickness, Free of Debris or Solder Paste Posi-onal Accuracy NOT TO SCALE

7 Solder Paste Formula-on, Rheology, Release Characteris-cs, Par-cle Size, Paste Temperature, Ambient Temperature & Humidity NOT TO SCALE

8 Squeegee Angle, Speed, Pressure, S-ffness, Surface Energy NOT TO SCALE

9 A Good Mechanical Setup is Essential Aperture- Pad Alignment Aperture- Pad Gaske-ng Board Support NOT TO SCALE

10 Gasketing Poor Alignment PWB Shrink Aperture larger than pad Pad smaller than aperture Paste on bowom of stencil Solder Mask on Mask pad on Pad HASL Dome NOT TO SCALE

11 Stencil Separation If printer has stepper motor on Z- axis, separa-on speed is very important to print quality: Steppers induce vibra4on 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 separa4on speed Some pastes thin too much with slow separa4on, producing poor print defini4on and stringy bridges, and work best with fast separa4on speed. How does the engineer know? Simple experiment. NOT TO SCALE

12 Print Process 1) Alignment, gaske-ng of apertures to Pads 2) Squeegee mo-on thins paste so it flows into apertures 3) Paste recovers; s-ffens up 4) Stencil separates from PWB, paste deposits release from stencil onto pads NOT TO SCALE

13 Results 16- mil pitch QFP

14 Phase Shift Interferometry Shines a grated light on the subject Called a Moire paoern PaOern appears to move as it crosses topography Like a:ernoon sunlight shining through blinds Takes photos at known angles Calculates height by changes in paoern at different angles Extremely accurate and repeatable Used in all types of surface measurements

15 PSI and Solder Paste Chops each deposit into -ny 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

16 Solder Paste Inspection Criteria Star-ng parameters Gets theore4cal aperture volume from stencil Gerber file 50% to 150% of theore4cal volume 50% offset in X or Y Tightening the process Criteria can be set 4ghter or looser for each device on PWB (ubga +/- 20%) Track effects of changes in process or in control parameters Use historical produc4on yield data to op4mize for individual processes

17 Process Control With Automatic Solder Paste Inspection Start with standard SPC X- bar, Sigma, Cp/Cpk, and Histograms Op4mize the basic process Compare before and a:er adjustments Mine Data to improve yields Create defect Paretos Boards with highest fallout, defect types, components with highest fallout rates Verify stencils before pujng them into produc-on Assure all apertures are in right posi4on and cut properly Avoid recurring defects on the line

18 Still Making Defects? Check the easiest, most obvious things first Inves4ng a few minutes in a system once- over will find the problem 4 out of 5 4mes

19 Overall System Check First: Knead the paste, wipe the stencil, print a board, observe Is the right amount on stencil? Does it roll over the surface of the stencil Paste bead should be about 1.5 and release cleanly from squeegee blade? cm diameter (5/8 ) If no, replace it with fresh solder paste Bead size affects fill pressure Check temperature and consistency

20 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 moun4ng mesh Inspect the board support Is dried paste interfering with PWB? ShuWle a board into posi-on Tap or press on top to verify support Check for movement in X & Y If vacuum, check for leaks

21 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

22 Didn t find the Root Cause? Time to dig a li:le deeper into the likely causes of typical print problems

23 Solder Paste Print Defects Solder Bridges Poor Print Defini-on - Peaks or Dog Ears Insufficient Solder Volumes - Poor Aperture Fill - Poor Aperture Release Poor Gaske-ng Poor Alignment

24 Solder Bridges

25 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

26 Poor Print Definition Peaks and Dog Ears

27 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

28 Insufficients Poor Aperture Fill Bonus Ques2on: what s wrong with this aperture design?

29 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 cold 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

30 Insufficients Poor Aperture Release

31 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

32 Poor Gasketing

33 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.

34 Poor Alignment

35 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.

36 Summary - Troubleshooting Understand the key elements in the solder paste prin-ng 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 con-nue to occur, follow logical troubleshoo-ng guidelines

37 Many thanks to: Thank You SMTA Chapter Event hosts Jabil, San Jose, CA GOS Laboratory engineers and technicians Hien Ly, Michael Lapitan, Mike Santos Christopher Associates Koki Solder Pastes Intek Plus Solder Paste Inspec4on

38 Questions? Contact Chrys at: Copies of this presenta4on can be downloaded at: Sponsored by: