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- 1. Aperture • An opening in the barrier layer of the screen through which the material to be applied can pass
- 2. Screen Printing • Screens consist of a mesh that is impregnated with an emulsion • Mesh number: holes per square inch • Apertures are formed by the exclusion of emulsion from selected areas • Lithographic techniques • Wire diameter and mesh number determine paste type to be used • Approximately 50% open area in aperture
- 3. Screen Printing • Solder paste flows around mesh and is deposited on substrate • Two steps in the screen printing process: • Flooding is the spreading of material over the area to be printed without the application of pressure (loading of the mesh) • Printing is executed in the reverse direction to force material through the screen and on to the substrate
- 8. 15mm Dia. Paste Zero ‘Print Gap’ Basic Stencil Process
- 9. Paste Rolling Controlled ‘Print Speed’ Even ‘Squeegee Pressure’ Basic Stencil Process
- 10. Board is lowered under controlled ‘Separation Speed’ Basic Stencil Process
- 11. Stencil Printing • Four phases of stencil printing • On-contact method shown
- 12. Stencil Printing • Stencils consist of a metal foil through which holes are opened to allow the passage of material • Apertures are formed by the machining of the foil • Stainless steel, molybdenum, brass, nickel • 100% open area in aperture
- 14. Capable of smaller than stencil size deposits
- 15. The Perfect Print • Clean edges • Flat on top • Height = Stencil Thickness • Good Alignment • No Flux Bleed ...
- 16. • Clean edges • Flat on top • Height = Stencil thickness • Good alignment • No flux bleed PERFECT PRINT
- 19. Issue 2 - 06 November 2001 INITIALISATION SET-UP MAINTENANCE PASTE CARTRIDGE LOW PAPER EMPTY SOLVENT EMPTY OPERATIONAL WAITING IN READY STATE SYSTEM POWER OFF HALTED BY ERROR BEACON INDICATIONS
- 26. Note: Some machines fix stencil, move board instead.
- 27. Table at home position, substrate loaded into printer. Company Confidential Stencil Foil / Image Area Substrate Squeegees Print Material Stencil Tensioned Mesh Stencil Frame Adjustable Width Conveyer Rail Fixed Conveyer Rail Rising Table Heated Tooling Rail to Table Adjuster Rail to Table Adjuster Table at home position, substrate loaded into printer. Company Confidential Stencil Foil / Image Area Substrate Squeegees Print Material Stencil Tensioned Mesh Stencil Frame Adjustable Width Conveyer Rail Fixed Conveyer Rail Rising Table Heated Tooling Rail to Table Adjuster Rail to Table Adjuster How Does The Printing Process Work?
- 28. Table raised to vision height, establishing tooling contact with substrate. Company Confidential Camera Substrate Rising Table Heated Tooling Table raised to vision height, establishing tooling contact with substrate. Company Confidential Camera Substrate Rising Table Heated Tooling How Does The Printing Process Work?
- 29. After fiducial capture and stencil position is adjusted to achieve correct alignment, table raises substrate to print height. Company Confidential Substrate Rising Table Heated Tooling After fiducial capture and stencil position is adjusted to achieve correct alignment, table raises substrate to print height. Company Confidential Substrate Rising Table Heated Tooling How Does The Printing Process Work?
- 30. Squeegee contacts print material, downward pressure applied. Company Confidential Substrate Rising Table Heated Tooling Squeegee contacts print material, downward pressure applied. Company Confidential Substrate Rising Table Heated Tooling How Does The Printing Process Work?
- 31. Squeegee pushes print material along stencil. Company Confidential Substrate Rising Table Heated Tooling Squeegee pushes print material along stencil. Company Confidential Substrate Rising Table Heated Tooling How Does The Printing Process Work?
- 32. Squeegee reaches end of stroke (print limit). Company Confidential Substrate Rising Table Heated Tooling Squeegee reaches end of stroke (print limit). Company Confidential Substrate Rising Table Heated Tooling How Does The Printing Process Work?
- 33. Table lowered, initiating substrate separation from stencil. Company Confidential Substrate Rising Table Heated Tooling Table lowered, initiating substrate separation from stencil. Company Confidential Substrate Rising Table Heated Tooling How Does The Printing Process Work?
- 34. Table moves to home position and substrate is transported out of printer. Company Confidential Substrate Rising Table Heated Tooling Table moves to home position and substrate is transported out of printer. Company Confidential Substrate Rising Table Heated Tooling How Does The Printing Process Work?
- 35. What is Solder Paste ? Alloy (solder powder) Flux Gelling Agents 100% 100% ~50% Solder by volume ~90% Solder by Weight ~50% ~10%
- 36. What does the Flux do ?? Flux ensures successful solder joint is formed between component and pad Suspends the powder Provides correct rheology and tackiness Cleans surface Removes oxide from solder powder Protects surface Leaves safe residues...
- 37. What are gelling agents ?? • Makes the flux shear thinning, pseudoplastic (time independent) or thixotropic (time dependent) Paste viscosity moves up and down this plot depending on where it is in the print process. newtonic Shear rate (s-1) Viscosity
- 38. Temperature v. viscosity The temperature - viscosity relationship for a typical no clean paste 100 110 120 130 140 150 15 20 25 30 35 Temperature (deg c) Viscosity (Pa.s)
- 39. Printing Rheology • Ideal paste is pseudoplastic; most pastes are thixotropic to some degree • Viscosity will change throughout the printing process • Allows for deposition and retention of material
- 40. Printability • Accuracy and repeatability of paste deposits • Acceptance criteria include • Deposited paste weight from print to print should not exceed 10% • Deposit height should have a tolerance of 1 mil from deposit to deposit within a print • Uniform coverage without tailing or separation
- 41. Working Life • Time between jar opening and shift of the rheological properties of the material outside the process window • Working life can be checked by examining print quality and checking tackiness • Won‟t hold components in place prior to reflow
- 42. Slump Testing • Standards for slump testing • Print paste on test board of various pitch land patterns • Failure if deposits bridge after printing
- 43. READ THE LABEL Harmful Product Code: SN63RP11AGS90 FAST PRINT ROSIN SOLDER CREAM Comp: SN63 Pb37 Flux: NC-SMQ51SC Batch: 0270782 Contents: 500g Use By: 21.05.99
- 44. Solder Paste - Read the Label! Product Code: SN63 RP11 AGS 90 FAST PRINT ROSIN SOLDER CREAM Alloy Type 63 % Tin / 37% Lead (or 35% lead / 2% Silver) Flux Medium Type „Rosin Product‟ Particle Size Type 3 (45 - 20 um) Metal Content 90% (by weight)
- 45. Solder Powder Type 2 Powder Type 3 Powder Consistent alloy, particle size and particle shape. Oxide free 53 - 38 um 45 - 20 um
- 46. IPC Powder Specification • Type 3 is the typical size for current SMT % By weight of sample Type None Larger than Less than 1% Larger than 80% minimum between 10% maximum less than 2 80 m 75 m 75 to 45 m 20 m 3 50 m 45 m 45 to 25 m 20 m 4 40 m 38 m 38 to 20 m 20 m 5 6 25 to 15 m 15 to 5 m
- 47. The smaller the particle size the better print definition but the paste will be more expensive and a greater chance of reflow problems! Rule of thumb: Keep a minimum of 5 particles per aperture width Stencil Board PARTICLE SIZE
- 48. Good Working Practices • Stir Paste Gently Before Use • Regulate Paste Diameter at 15mm • Perform Small Top-Ups Regularly • Regulate Temperature and Humidity • Do Not Leave Paste drying on Stencil ...
- 49. PASTE STORAGE
- 50. PASTE STORAGE • First In First Out (FIFO) Clearly label date in and date out • Rotate jars and tubes regularly • Store tubes upright • Correct temperature
- 51. Maybe, Temporary & Overnight Definitely for 3 - 6 months Never! PASTE STORAGE
- 52. Considerations: • Do you use squeegee paste deflectors to reduce solder paste trails? • Do you put solder paste trails back into working paste? • What do you do with the paste over a lunch break? • How long will your solder paste remain workable? • When to dispose of your paste? • Do you have ProFlow? REUSE AND DISPOSAL PROCEDURE
- 53. Considerations: • Storage procedure • Stock levels •Availability • FIFO • Expiry date • Stabilise temperature before use PASTE STORAGE
- 54. USING THE PASTE Cold Storage „Working Pot‟ Printer Fresh Paste Place in machine After 8 Hours End of Run / Extended Downtime End of Day Harmful Disposal Return only if paste quality is good and it makes economic sense „Temporary Pot‟
- 55. • Good stencil • Excellent results • Few failures • Predictable yields GOOD/BAD STENCIL • Poor stencil • Lots of stencil wiping/cleaning • Lots of bridging and open circuits
- 56. Remember, stencils are fragile...... • Store and handle with care • Adequate storage • Keep stencils clean • Inspect for damage and stretch • Repair fiducials when damaged • Product coded to reduce mistakes STENCIL MAINTENANCE
- 57. Manual: • Use a lint free cloth • Solvent soaked cloth in each hand, clean both sides simultaneously Dedicated Screen cleaner: • Recommended STENCIL CLEANING Underscreen cleaner set-up (later)
- 58. • Better to have excellent low-tech stencil than poor high-tech stencil. • Correct stencil dimensions are vital in order to achieve good prints. • A great machine can do little to correct a fundamentally flawed stencil. • Ensure stencil is thoroughly clean • Correct stowage • Repair fiducials BEST WORKING PRACTICES
- 59. Chemical Etching • Image patterned on both sides simultaneously • Parallel fabrication of apertures • Hourglass profile (~ ½ mil) • Difficult to release solder paste from apertures • Paste deposit „hangs‟ on girdle and does not release well
- 60. Chemical Etching • Over- and under-etching • Etch is both vertical and horizontal • Must allow for growth in both dimensions
- 61. Laser Cutting • Fabrication by cutting holes in foil with high-power laser • Gerber data used to drive cutting tools • Serial process- more expensive than etching • Aperture size minimum 2 mil • Tolerance .2-.3 mil
- 62. Laser Cutting • Buildup of debris on surface • Rough side walls • Electropolishing used to polish side walls • Selective chemical etch • Don‟t want too smooth a stencil top
- 63. Laser cutting
- 64. Trapezoidal aperture Step 3 Step 1 Step 2 Step 4
- 65. YAG Laser 1 S.S Type 2 G Co2 Laser Kapton F Formed Ni E YAG Laser 1 Ni D YAG Laser 3 S.S Type 1 C YAG Laser 2 S.S Type 1 B YAG Laser 1 S.S Type 1 A Fabrication Method Metal Type Stencil Type YAG Laser 1 S.S Type 2 G Co2 Laser Kapton F Formed Ni E YAG Laser 1 Ni D YAG Laser 3 S.S Type 1 C YAG Laser 2 S.S Type 1 B YAG Laser 1 S.S Type 1 A Fabrication Method Metal Type Stencil Type A B C D E F G
- 66. Electroforming • Image built up to desired thickness in plating bath • Nickel built up around apertures created by pattern • Very tight and smooth aperture tolerances • Aspect ratio 1.1-1.2 (width:thickness) • 1-12 mil thickness • May be too smooth in some cases
- 67. Nickel/Brass • Electroless nickel plated as the wear layer over etched brass • Easier fabrication with brass foil • Lower surface tension with paste for longer equipment life • Tolerances comparable with etched stencils
- 68. Stencil Selection Chemical Etching Laser Cut Electro- formed Hybrid Cost Cost effective Cost effective Expensive Less Expensive Accuracy OK Very Very Mixed Min. Aperture size 0.2mm (0.4mmPitch) No Limit! No Limit! No Limit! Aperture Shape Material Stainless Steel Brass Stainless Steel Nickel Alloy Stainless Steel Brass ? Fiducials Good Good Weak Good
- 69. Aspect Ratio • Ratio between aperture width and stencil thickness should not be smaller than 1.5 for good transfer • Smaller aspect ratios will result in pad clogging
- 70. Aperture Profiles • Tapered apertures preferred for improved release of solder paste • Natural consequence of electroforming and laser cutting
- 71. Tapered Apertures • Higher smearing from wider bottom overwhelmed by consistency from better release • As pitch decreases, smearing becomes more of an issue
- 73. Stencil Contrast
- 74. Fine-Pitch Considerations • Micromodification is the reduction in aperture size to minimize bridging at fine pitch • „Zipper‟ patterns also increase the pitch between neighboring deposits • Poor registration or an excess of solder outside the lands can lead to the formation of solder balls (and shorts)
- 75. Step-Down Stencils • Areas of differing thickness used in same stencil to deliver different paste quantities • Thin area for fine-pitch devices • Thick area for coarser pitch components
- 76. Stencil Design Pitch Length Width Design Stencil Apertures to allow a clearance of at least 50 µm between Paste and edge of Pad Paste Pad
- 79. Paste Transfer Efficiency Trend Recommended Area Ratio > 0.66
- 80. Stencil Design Release Equation 1 Release Equation 2 W T Paste Release Equation W Avg.Particle size > 0.66 >1.5 > 5 W T L L*W 2*(L+W)*T
- 81. IPC 7525 – Stencil Design Standard
- 82. Stencil thickness determined by finest pitch device. Aperture opening usually slightly smaller than circuit board pad. If area ratio is violated, then aperture to be oversized. Try to avoid making apertures 1:1 with pad. Finest spacing between apertures should be more than the thickness. Laser cut stencils are most popular for SMT, electropolishing is common surface treatment. Fine pitch, small feature, or high aperture count applications justify Electroforming. Stencil Design Guidelines
- 83. Stencil Design 150µm Solder paste type 3 Stencil thickness component Aperture width µm 100µm 125µm 290 265 240 QFP 0.5 215 240 215 190 QFP 0.4 165 152 140 QFP 0.3 120 760 BGA 1.5 500 380 BGA 0.75 250 Solder paste type 4 Stencil thickness component Aperture width µm 100µm 125µm 150µm 290 265 240 QFP 0.5 215 240 215 190 QFP 0.4 165 152 140 QFP 0.3 120 760 BGA 1.5 500 380 BGA 0.75 250
- 84. Stencil - Board alignment • Cause: • Poor Artwork • Poor Quality Stencil Manufacture • Poor board manufacturing • Board Stretch • Poor Fiducial Alignment • Machine not calibrated • Solution: • Print Offsets • Buy New Stencil • Improve Fiducial Score • Buy better boards • Calibrate machine
- 85. Printability • Accuracy and repeatability of paste deposits • Acceptance criteria include • Deposited paste weight from print to print should not exceed 10% • Deposit height should have a tolerance of 1 mil from deposit to deposit within a print • Uniform coverage without tailing or separation
- 86. Paste Thickness • Determines solder volume in joints • Too thick will bridge • Too thin will cause insufficients • Governed by • Mask thickness (or mesh number and emulsion thickness for screens) • Snap-off height • Overall amount of solder also affected by metal loading of paste
- 87. See anything wrong with these stencil designs? Deposits are small, but design is good as long as there‟s enough alloy for joint & repeatability. TE is good, but bridging will result.
- 88. 5-mil thick laser cut stencil (no post processing) Aperture geometry: circle, square Aperture size: 6, 8, 10, 12, 14, 16 mils Squeegees: 200mm long, 60 deg metal Print Speed: 20 mm/sec Print Pressure: 5 kg Separation Speed: 0.5 mm/sec Separation Distance: 2 mm Print Gap: 0 mm Substrate: 200mm Silicon Wafer Print Stroke Direction: Forward Only All photos shown after printing on known “clean” stencil. Type 3 Solder Paste Indium SMQ92J 25 - 45 micron particle 90% Metal Loading Sn63 / Pb37 Alloy 9 mil aperture minimum Type 4 Solder Paste Indium SMQ92J 20 - 38 micron particle 89.5% Metal Loading Sn63 / Pb37 Alloy 8 mil aperture minimum Type 3 vs. Type 4 test 5-ball rule 5-ball rule
- 89. 6-mil Circular Aperture 6-mil Square Aperture Type 3 Type 4 Stencil Thickness = 5 mils
- 90. 8-mil Circular Aperture 8-mil Square Aperture Type 3 Type 4 Stencil Thickness = 5 mils
- 91. 10-mil Circular Aperture 10-mil Square Aperture Type 3 Type 4 Stencil Thickness = 5 mils
- 92. 12-mil Circular Aperture 12-mil Square Aperture Type 3 Type 4 Stencil Thickness = 5 mils
- 93. 14-mil Circular Aperture 14-mil Square Aperture Type 3 Type 4 Stencil Thickness = 5 mils
- 96. Mesh density specification dictates opening and wire diameter Mesh thickness determined by wire diameter and weave
- 101. Mesh Density = wire count / unit area
- 104. Best Result
- 107. Mesh Screen Printing Applications Lower viscosity, smaller particle size materials used Flux Adhesive Underfill Die Attach Ink Frit Thin uniform coatings Fine detail lines / high resolution lines & fine features
- 108. Board considerations • Pad surface HASL Gold on nickel Organic coating • Resist Thinner than pads Clear of fiducials? • Etc. Size Underside components Obstructions, bar codes etc Fiducials and holes
- 109. • Fibres • these can block stencil apertures • Old Solder • this can cause solder balls • Fingerprints • these can corrode pads, tracks etc handle board by edges, wear gloves • Use CM20... Ensure that production boards are free of ... Board Issues
- 110. • Uneven Pad surface can cause loss of gasket seal resulting in bridging etc. • If the resist is higher that the pad the gasket seal will be lost resulting in bridging etc. ... The board design can impinge on the effectiveness and quality of the print process. Here are some things to look out for .... Pad with uneven surface coating Pad Resist Board Issues
- 111. • Underside components can restrict the use of tooling pins - use tooling plate or FPA • Large underside components may impede board transport - increase under clearance ... The board design can impinge on the effectiveness and quality of the print process. Here are some things to look out for .... Board Issues
- 112. • Via holes can cause problems with vacuum tooling • Labels, bar code stickers etc. increase board thickness leading to tooling, print and pressure problems ... The board design can impinge on the effectiveness and quality of the print process. Here are some things to look out for .... Tested ok Board Issues
- 113. • Boards with tapered or chamfered edges are difficult to clamp and may be liable to movement during print leading to random offsets ... Rail Board Issues
- 114. • Uneven pad surface • Solder resist too high • Underside components restricts tooling • Via holes with vacuum tooling QUALITY ISSUES
- 115. • Warping and stretch • Chamfered edges • Fiducial quality • Colour changes, surface finish • Barcode labels QUALITY ISSUES ||||| || |||| ||| ||||||| ||||||||| |||| |||||
- 116. • Board stop set-up • Storage and handling • Dirty boards: • Fibres from production • Old solder • Finger prints • Labels PROCESS ISSUES
- 117. • Ensure adequate storage is available • Make sure boards are clean before use • When handling: • Wear gloves • Unpack carefully • Differences between batches • Colour changes • Board stretch STORAGE AND HANDLING
- 118. • Handle boards with care and always wear gloves • Check batches carefully for: • discolouration, warpage, fiducial quality, thickness of resist, circuit legends, barcode labels, general board quality • Ensure board cleanliness • If boards are washed: • identify them for inspection before reuse BEST WORKING PRACTICES
- 120. Possible causes: • Poor quality stencil manufacture • Poor board manufacturing • Board stretch or warping • Poor fiducial alignment • Machine not calibrated • Escalate if found STENCIL - BOARD ALIGNMENT
- 121. STENCIL - BOARD ALIGNMENT Possible solution: • Improve fiducial score • Print offsets • Incorrect batch of boards • Incorrect position of tooling
- 122. Squeegee Materials • Hardness of material will affect transfer quality • Material will „scoop‟ out of apertures • Harder squeegees for finer pitch printing • Steel • Thin blade to conform to surface • Contact angle varied to change applied force • Will also cause stencil wear
- 123. Polyurethane squeegees • Minimal friction with surface • Geometry varied to optimize wiping • Diamond for bi-directional, rectangular for unidirectional • Hardness affects scooping of deposits
- 124. Pressure • The greater the angle, the greater the net force on the squeegee • Opposing force from fluid is minimized
- 125. Squeegee Pressure • Low pressure can • Cause skips and ragged edges on deposits • High pressure can • Smear deposits • Damage fine webs or soft squeegees • Scoop out deposits
- 126. Squeegee Effects • Wear will affect print quality • Edges that are not straight and sharp will result in reduced print pressure
- 127. Transfer / Driving forces are influenced by:- •Squeegee angle •Print speed •Paste rheology •Paste roll volume Driving force Transfer force Squeegee Principles Reaction force Squeegee force
- 128. Squeegee Principles Paste rolling Streamlines Shear rate changes with position of paste in roll Lower shear rates Higher shear rates
- 129. Effect of Excess Pressure Print Direction Aperture Scooped Pads Board stencil „Scooping‟
- 130. Angle º 45º 60º •Intrusive reflow •High speed printing •Printing thick stencils •Printing Glue •Printing mesh screens •Standard SMT Printing Increased Force Reduce Force
- 131. Choose the closest length to the board size Board Squeegees Paste Dry Paste Gap LENGTH
- 132. • If your specified squeegee is too large……… Use stencil support blocks LENGTH Stencil Support Blocks
- 133. Note: an average squeegee may travel 5-6 miles in its lifetime! ……….And that's across a rough surface and under pressure! Angle changes EFFECT OF WEAR
- 134. • Used to prevent tramlines. • Helps maintain paste in good condition. • Maintains a constant transfer into through holes for intrusive reflow in paste applications. Paste Deflectors - set 0.25mm above stencil surface PASTE DEFLECTORS
- 135. • Polyurethane blades come in trailing edge and diamond edge profiles. • Pink (90 - 97 shore) Only for totally flat boards • Fawn (85 -90 shore) • Brown (80 - 85 shore) • Blue (75 -80 shore) Excellent good coarse pitch, • Robust, boards but must be straight • Green polyurethane (70 - 75 shore) • Only for screen printing • Metal • Excellent coarse and fine pitch MATERIAL SELECTION
- 136. Polyurethane Shore 65-85 (Dek Green) Too Flexible for stencil printing, used only in mesh applications Shore 95 (Dek Red) Will need replacing or regrinding after 25000 prints. Essential to control squeegee pressure to reduce scooping. Causes little damage. Commonly used in SMT.
- 137. Clamped and bonded squeegees
- 139. Metal Shore +200 • Easy to damage • Expensive • High pressure may cause Bridging + Paste height higher than stencil thickness + Better print quality + Wider process window for pressure + High pressure will not cause Scooping
- 140. Metal squeegees
- 141. Metal squeegees
- 142. Overhang 15 mm - widely used, reduced risk of stencil/blade damage due to increased flexibility 6 mm - historically used but no longer recommend due to increased risk of stencil/blade damage.
- 143. Composite Shore 120 (Composite) • Very Rigid • Uneven boards cause damage to Stencil and Blades + Used in applications where very large apertures are used. + Wider process window for pressure
- 144. Property Metal Polyurethane Fine pitch Good Excellent Large apertures Excellent Good Pressure effect Does not cause scooping Needs control Damage resistance Poor Good Damage to stencil Poor Good Life Until broken 25,000 Paste Height < Stencil Thickness > Stencil Thickness Metal vs. Polyurethane
- 145. STORAGE • Ensure blades and squeegees are stored correctly • Ensure there are adequate stocks • Ensure blades are properly cleaned before storing
- 146. • Check not damaged or worn • Use correct length for board • Ensure adequate spares are held • Always ensure cleanliness • Always calibrate squeegee heights before use BEST WORKING PRACTICES
- 147. Metal blade edges can be sharpened. These last as long as they don‟t get bent or chipped. Polyurethane blades cannot be sharpened (easily), however, with a clamped system you can reverse the blade to use the opposite clean edge. Squeegee blade guideline
- 148. TOOLING SOLUTION Table Rail Rail Rails, Board and Stencil are flat and parallel Perfect Tooling Block
- 149. • Problems • Incorrectly positioned • Paste deposits raising height of board • Impacting underside components • Missing under critical components • Placed under rail TABLE RAIL RAIL MAGNETIC TOOLING PILLARS
- 150. • UNDERSIDE COMPONENTS GENERAL TOOLING PROBLEMS Tooling Block Under Clearance
- 151. • Board lifted by tooling causing: • Board damage • Squeegee pressure problems • Uneven paste height • Dirty stencil TABLE RAIL RAIL GENERAL TOOLING PROBLEMS
- 152. GENERAL TOOLING PROBLEMS • Check • Cleanliness of table • Cleanliness of pins • Board clamps for damage • Transport belt condition • Board thickness Table Rail Rail Bad Tooling Block • Board warpage • Stencil for damage • Position of tooling pins • Dowel pin location • Underside components
- 153. Printability • Accuracy and repeatability of paste deposits • Acceptance criteria include • Deposited paste weight from print to print should not exceed 10% • Deposit height should have a tolerance of 1 mil from deposit to deposit within a print • Uniform coverage without tailing or separation
- 154. Print Gap • The distance maintained between the screen (or stencil) and the workpiece • Also known as “snap-off” • The stencil or screen lifts from the paste deposits after the squeegee travels over and fills the aperture
- 155. Gasketing • The seal formed between the stencil or screen and the substrate to be printed • Good print definition and minimal bleed-out
- 156. Print Mode • On-Contact • More accurate print • Slow snap-off speed • Off-Contact • Stencil kept 1-2 mil away from surface • Local gasketing from squeegee pressure • Used with screens to prevent smearing in flood mode
- 157. Print Speed • Paste should roll for good print definition • Downward component into apertures from roll provides even print • Too fast a squeegee speed will cause material to slide across surface • Material will lack the vertical component and not fill apertures properly, leading to inconsistencies in printing • Balance throughput demands vs. aperture
- 158. Print Direction • Less solder on pads parallel to squeegee orientation than those perpendicular to travel • Plan orientation of part and stencil to accommodate effect
- 159. Transfer Efficiency • The ratio of the volume of paste actually deposited versus the volume of the stencil aperture • Round apertures generally have a higher transfer efficiency than rectangular apertures
- 160. Process Window • Affected by solder volume and coplanarity
- 161. Start at 0.5 kg pressure per 1 inch of squeegee Adjust print speed to suit cycle time requirement Print gap = 0 (no snapoff) Separation speed usually not critical parameter. Process window for SMT can tolerate print speeds between 10 and 100mm/sec for reputable solder pastes. Rules of thumb:
- 162. As speed pressure should go as speed pressure should go Increasing speed requires a corresponding increase in pressure in order to maintain the same print results. This leads to accelerated squeegee and stencil wear. Print speed and Squeegee pressure are directly related... PRINT SPEED AND PRESSURE
- 163. • Separation speed • The speed at which the board separates from the stencil •Adjust when deposits look conical or „Dog Eared‟ • This value effects print cycle time • Separation distance • The distance the board travels at the set separation speed • Keep to the minimum value required • Faster separation speed, smaller distance required SEPARATION SPEED AND DISTANCE
- 164. EXAMPLES BEYOND OUR DIRECT CONTROL • TEMPERATURE (OPENING COVERS) • HUMIDITY (OPENING COVERS) • LIGHT (DAY TO NIGHT) • HUMANS (BIGGEST PROBLEM) • BOARD QUALITY • “HEARTBEAT” IF LINE GOES DOWN
- 165. Constant temperature Constant humidity MACHINE ENVIRONMENT Is the environment in control
- 166. • Temperature Control Unit • Operator set • ± 8º C from ambient • Maintains to within ± 1º C • Amber then Red beacon indications • Audio alarm • TEMPERATURE CONTROL
- 167. COMMON PRINT PROCESS FAULTS
- 169. Standard paper roll under stencil cleaner Wet, Vac, Dry modes Controllable travel speed, number of passes, vacuum level, cleaning interval. Cleanroom grade paper available.
- 170. Alternative disposable porous foam cassette cleaning system. Ultrasonic assisted systems also offered. Topside cleaning uncommon.
- 171. Stencil bottomside contamination Too many prints before bottomside cleaning. Squeegee pressure too high. Inadequate board support. Poor stencil to board gasket.
- 172. Stencil bottomside contamination …could be caused by circuit board topography. i.e. traces, non uniform pad finish (HASL), solder mask thickness / misregistration.
- 173. Result of printing on a perfect surface Print #12 No stencil cleans between any prints.
- 174. • Squeegee pressure or ProFlow pressure too low • Separation speed too high • USC set-up adequate • Paste knead set-up • Paste condition STENCIL BLOCKAGE Causes:
- 175. • Print/ProFlow pressure too high • Alignment correct • Print deposits greater than 1 • USC set-up adequate • Paste knead too frequent • Paste condition STENCIL SMEAR Causes:
- 176. • Print/ProFlow pressure • Too many print deposits • USC set-up • Paste knead set-up • Paste condition • Stencil condition BRIDGING Causes:
- 177. • Print speed • Print/ProFlow pressure • Print deposits sufficient • Separation speed • Stencil blockage • See next slide…. INSUFFICIENT PASTE Causes:
- 178. • USC set-up • Paste knead set-up • Print speed too high • ProFlow piston pressure too low • stencil wet after cleaning • Wet or dirty boards • No ProFlow stencil support • Separation speed set incorrectly ... INSUFFICIENT PASTE (CONTINUED) Causes:
- 179. SCOOPING • Squeegee pressure too high or squeegee too soft (polyurethane) • Stencil apertures too large • Tooling too high Causes:
- 180. MISSING EDGE • Squeegee/ProFlow pressure too high • ProFlow print speed too low • Tooling incorrect Causes:
- 181. EXCESSIVE PASTE • Squeegee ProFlow pressure too high • ProFlow print speed too low • Lack of tooling support Causes:
- 182. • Fiducial set-up • Correct offsets • Alignment weighting • Board to stencil mismatch • Board stretch • Machine calibration ALIGNMENT Causes:
- 183. • Clean edges • Flat on top • Height = Stencil Thickness • Good Alignment • No Flux Bleed ... PERFECT PRINT Goal: