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Comprehensive Guide to Surface Mount Adhesives in Electronics Manufacturing

Understand the chemistry, advantages, and limitations of epoxy, acrylic, and conductive adhesives. Learn application methods like syringe dispense, stencil printing, and the benefits of isotropic and anisotropic adhesives.

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Comprehensive Guide to Surface Mount Adhesives in Electronics Manufacturing

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  1. ELECTRONICS MANUFACTURING PROCESSES Assembly Raw Materials R. Thompson March 2, 2000

  2. Surface Mount Adhesives R. Thompson March 2, 2000

  3. THE ASSEMBLY PROCESS Dispense Adhesive Solder Paste Printing Component Placement Component Insertion Adhesive Cure Reflow Print Adhesive Hand Soldering / Rework PCB Cleaning (when desired) Encapsulation (when desired) Component Insertion Wave Soldering TEST R. Thompson March 2, 2000

  4. Surface Mount for Wave Soldering R. Thompson March 2, 2000

  5. Use: adhering chip packages and discrete devices to printed circuit boards prior to wave or reflow soldering Chemistry Epoxy Acrylic Application Automated syringe dispense Stencil printing Pin transfer Critical requirements Rheology to control dot profile, slump, and dispense speed Adhesion before and after cure Dielectric properties Water absorption Surface Mount Adhesives R. Thompson March 2, 2000

  6. Advantages Low shrinkage High “Green Strength” Good adhesion to a wide variety of substrates Excellent dielectric Good chemical resistance Limitations Require refrigeration Short “Pot Life” Epoxy Adhesives R. Thompson March 2, 2000

  7. Advantages Very high-speed dispense Low modulus Fast fixturing time No refrigeration Limitations High-temperature cure UV-assisted cure Limited solvent resistance Acrylic Adhesives R. Thompson March 2, 2000

  8. Syringe Dispense • Pressure-Time • Positive Displacement • Screw (Augur) • Piston • Effective Rate – 50k dot/hour • Adjustable – Meets Variable Production Needs • Adhesive Not Exposed to Environment Prior to Application R. Thompson March 2, 2000

  9. Syringe Dispense R. Thompson March 2, 2000

  10. Syringe Dispense R. Thompson March 2, 2000

  11. Stencil Printing R. Thompson March 2, 2000

  12. Stencil Printing • Current Chip-shooters Place Components at 80k Chips/Hour • Current syringe-dispense rates are < 50k dots/hour • A Single Board Can Be Stencil-printed in < 12 Seconds R. Thompson March 2, 2000

  13. Stencil Printing R. Thompson March 2, 2000

  14. Advantages Fast cycle time All adhesive dots placed simultaneously All dots are the same height Limitations Typically limited to small-standoff components Stencil cleanliness essential Adhesive open to environment Stencil Printing R. Thompson March 2, 2000

  15. Design Criteria R. Thompson March 2, 2000

  16. Electrically Conductive Adhesives R. Thompson March 2, 2000

  17. Function • Make an electrical connection where solder is not feasible R. Thompson March 2, 2000

  18. Advantages of Conductive Adhesives • Reduced Joint Stress • Reduced thermal mismatch • Adhesive able to “absorb” mechanical joint stress • Process Costs (not material costs) • Bond Non-Solderable Substrates • “Low Temperature” Cure • Reduced stress on temperature-sensitive components • Environmental Aspects • Lead Free R. Thompson March 2, 2000

  19. Electrically Conductive Adhesives • Electrically Conductive Fillers • 2 Main Adhesive Types • I. Isotropic • II. Anisotropic R. Thompson March 2, 2000

  20. Electrically Conductive Filler Types • Silver • Most commonly used • Moderate cost • Best combination of thermal and electrical conductivity • Gold • High Cost • Good thermal and electrical conductivity R. Thompson March 2, 2000

  21. Electrically Conductive Filler Types • Nickel • Low Cost • Possible corrosion after aging • Difficult to get high electrical conductivity at low viscosity • Copper • Will corrode even if coated with silver • Carbon Black • Low cost, but low electrical conductivity R. Thompson March 2, 2000

  22. Isotropic Adhesives • Conductive In All Material Axes • Solder “Replacement” R. Thompson March 2, 2000

  23. Advantages Low-temperature cure Wide compatibility Simplified assembly process Fine-pitch capable Limitations Needs improved thermomechanical fatigue resistance Higher thermal & electrical resistance than solder Not suitable for high-power applications Isotropic Adhesives R. Thompson March 2, 2000

  24. Anisotropic Adhesives • “Z-axis” Conduction • Heat and Pressure Cure • UV Fixture, Then Heat • Films - Ordered Array R. Thompson March 2, 2000

  25. Anisotropic Adhesives R. Thompson March 2, 2000

  26. Anisotropic Adhesives R. Thompson March 2, 2000

  27. Anisotropic Adhesives • The primary limitation is that the resistance for each I/O will vary based on the dispersion of the particles • This can be overcome with ordered anisotropic conductive adhesive films (ACAF) R. Thompson March 2, 2000

  28. Anisotropic Films R. Thompson March 2, 2000

  29. Material Types • Epoxies • Silicones R. Thompson March 2, 2000

  30. Epoxy Adhesives • Benefits • Good adhesion to a wide variety of substrates • High cohesive strength • Low shrinkage • Good thermal and chemical resistance • High Tg possible R. Thompson March 2, 2000

  31. Silicone Adhesives • Benefits • Low Tg (< -60°C) • Consistent performance over wide temp range • Excellent high-temperature performance • Low modulus • Good resistance to polar solvents R. Thompson March 2, 2000

  32. Soldering Flux R. Thompson March 2, 2000

  33. Dip • Used for Low to High Volume • Lowest Cost for High Volume • Complete Encapsulation • Masking is Difficult R. Thompson March 2, 2000

  34. Fluxes Types of Fluxes Flux Components J-STD-004 Classifications Packaging Application Methods Evaluation R. Thompson March 2, 2000

  35. Types of Fluxes Rosin/Resin Abietic and Pimaric acid Relatively non-corrosive Hard to pin test Water-soluble Very corrosive Low Solids (“No-clean”) Dicarboxylic acid activators Easier to pin test IPA-based and water-based (VOC-free) R. Thompson March 2, 2000

  36. Flux Components Solvent Alcohol, water, etc. Activator Amine hydrohalides, organic acids, bases Reacts with metal oxides Carrier Rosin/resin, polyols, high boiling solvents Coats metal surface, removes oxides & prevents reoxidation, allows heat transfer Additives Surfactants, thixotropes, biocide, anti-corrosion etc. R. Thompson March 2, 2000

  37. J-STD-004 Classifications Military specifications have been replaced by Joint Industry Standards QQ-S-571 and MIL-F-14256 replace by J-STD-004, J-STD-005 and J-STD-006 MIL-STD-2000A replaced by J-STD-001B New flux classifications: ROsin, REsin, ORganic and INorganic Low, Medium and High activity levels 0 (halide-free) and 1 (halide) R. Thompson March 2, 2000

  38. Packaging Liquid Flux Wave fluxing Spray fluxing Foam fluxing Hand soldering (rework) Paste Flux (without Solder) Solder bumped chips, BGA packages Paste Flux (with Solder) Reflow Paste-in-hole Flux-core Wire Solders R. Thompson March 2, 2000

  39. Spray Fluxing • Ultrasonic • Pulse R. Thompson March 2, 2000

  40. Potting & Conformal Coating • Encapsulants • Chemistry • Cure Mechanisms • Application Methods R. Thompson March 2, 2000

  41. THE ASSEMBLY PROCESS Dispense Adhesive Solder Paste Printing Component Placement Component Insertion Adhesive Cure Reflow Print Adhesive Hand Soldering / Rework PCB Cleaning (when desired) Encapsulation (when desired) Component Insertion Wave Soldering TEST R. Thompson March 2, 2000

  42. Encapsulants • Conformal Coating • Applied in a thin layer (5 to 10 mils) • Minimal stress relief and vibration damping • Potting • Applied in a thick layer (over 125 mils) • Used for stress relief and vibration damping R. Thompson March 2, 2000

  43. Critical Requirements • Electrically Insulating • Good dielectric • Thermally Conductive • “Better than air” • Low-to-Moderate CTE and Low Modulus • Distribute solder joint stress • Necessary for fine-pitch • Good Adhesion To Each Interface Material R. Thompson March 2, 2000

  44. What They Do • Dielectric • Environmental (Moisture) Barrier • Encapsulant • Conductor Protectant • Solder Joint Stress Relief • Vibration Damping R. Thompson March 2, 2000

  45. Dielectric • Function • Prevents Arcing in High Humidity • Immobilizes Particulates • Prevents Electrical Shorts • Retards Corrosion R. Thompson March 2, 2000

  46. Function Breathable membrane Prevents liquid contact Prevents moisture condensation at board surface Provides solvent resistance Moisture Barrier R. Thompson March 2, 2000

  47. Function Immobilized device Immobilizes debris Supports Components Provides solder joints stress distribution Encapsulant R. Thompson March 2, 2000

  48. Function Insulative coating Shields bare conductors Reduces high-voltage arcing Protects Conductors R. Thompson March 2, 2000

  49. Chemistry • Acrylics • Epoxy • Urethanes • Silicones • Parylene (Paraxylylene) R. Thompson March 2, 2000

  50. Benefits Good adhesion to many substrates Good cohesive strength Wide variety of modulus Tolerant of hydrocarbon contamination Limitations 125°C upper temperature limit Generally high modulus materials Free-radical cure can be oxygen-inhibited Limited Solvent resistance Typically solvent-based Acrylic Coatings R. Thompson March 2, 2000

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