A Practical Guide to Surface Mount Technology (SMT) Covering Solder Paste, Reflow, Components, and Process Control
1. Standard SMT workshop temperature is 25°C ±3°C (77°F ±5.4°F). 2. Required tools for solder paste printing include: solder paste, stencil, squeegee, stencil wipes, lint-free cloth, cleaning solvent, and a paste spatula. 3. Common solder paste alloy: Sn/Ag/Cu 305 (96.5/3.0/0.5), melting point: 217°C (422.6°F). 4. Solder paste contains two main components: solder powder and flux. 5. The flux removes oxides, breaks surface tension, and prevents re-oxidation during soldering. 6. Typical volume ratio of solder powder to flux: 1:1; weight ratio: 9:1. 7. Always follow FIFO (first-in, first-out) when using solder paste. Two required steps before use: bring to room temperature and stir thoroughly. 8. For moisture-sensitive devices (MSDs), the humidity indicator card (HIC) must show blue to proceed with assembly. 9. Common stencil manufacturing methods: etching, laser cutting, electroforming, and nano-coating. Among them, laser-cut stencils are reworkable. 10. SMT stands for Surface Mount Technology – attaching electronic components directly onto the surface of PCBs.
11. ESD stands for Electrostatic Discharge, a critical factor in electronic manufacturing. 12. SMT machine programs generally consist of five data sets: PCB data, fiducial mark data, feeder data, nozzle data, and part data. 13. The relative humidity inside dry cabinets for MSDs should be maintained below 10% RH. 14. Common passive components: resistors, capacitors, inductors, diodes. Common active components: transistors, ICs. 15. SMT stencils are typically made from stainless steel, with common thicknesses being 0.12 mm or 0.15 mm. 16. Try to balance cycle times between high-speed and multi-purpose placement machines to optimize line efficiency. 17. Sources of static charge include friction, separation, induction, and conduction. ESD risks: device failure, contamination. Elimination methods: ionization, grounding, and shielding. 18. Package sizing: - Imperial 0603 = 0.06" × 0.03" - Metric 3216 = 3.2 mm × 1.6 mm 19. Example of component markings: - Resistor “ERB-05604-J81”: “4” = 4 circuits, 56Ω - Capacitor “ECA-0105Y-M31”: C = 106 pF = 0.1 nF 20. Document types: - ECN = Engineering Change Notice - SWR = Special Work Request These must be signed off by relevant departments and issued by document control to be valid.
21. The 5S system stands for: Sort, Set in order, Shine, Standardize, Sustain. 22. Vacuum-sealed PCB packaging helps protect against dust and moisture. 23. The “Three No” Quality Policy: - No receiving of defects - No making of defects - No passing on of defects 24. Quality assurance steps: - Understand customer needs - Translate needs into standards - Train operators - Audit and improve continuously - Ensure customer satisfaction 25. In fishbone (Ishikawa) analysis, 4M1H refers to: - Man, Machine, Material, Method, and Environment (H = “Huánjìng” in Chinese)
26. Solder paste composition includes: - Metal powder (85–92% by weight) - Solvent, flux, anti-slump agent, activators 27. Before use, refrigerated solder paste must return to room temperature to prevent defects such as solder balling after reflow. 28. Common SMT feeder modes: Preparation, Priority Exchange, Exchange, and Quick Exchange. 29. PCB positioning methods in SMT lines include: vacuum, mechanical holes, edge clamping, and side edge guides. 30. Resistor marking: - “272” = 2.7 kΩ - “485” = 4.8 MΩ
31. BGA component markings typically include: manufacturer name, part number, specifications, and date/lot code. 32. True definition of quality: “Getting it right the first time.” QC types: IQC, IPQC, FQC, OQC. 33. The fishbone diagram in quality control emphasizes cause-and-effect analysis. 34. CPK (Process Capability Index) represents the real-time performance capability of a process. 35. Flux starts evaporating and activates chemically in the preheat zone during reflow. 36. Ideal reflow profiles: cooling curve should mirror heating curve for balanced thermal stress. 37. Typical reflow soldering profile: Ramp → Soak → Reflow → Cool. 38. Most commonly used PCB substrate material is FR-4 (flame-retardant fiberglass). 39. Maximum allowed PCB warpage is 0.75% of the diagonal length. 40. Uneven heating of components during reflow may cause tombstoning, misalignment, or cold solder joints.
41. Standard BGA ball diameter used in motherboards: 0.76 mm. 42. ABS system refers to absolute coordinate systems; high-speed placers can place resistors, capacitors, ICs, transistors. 43. Ceramic chip capacitor “ECA-0105Y-K31” has a tolerance of ±10%. 44. Panasonic Panasert SMT machines typically operate at 3-phase, 200V ±10VAC. 45. SMT tape reel sizes are generally 13-inch and 7-inch diameter. 46. Stencil apertures are usually 4μm smaller than PCB pad to reduce solder balling. 47. BIOS = Basic Input/Output System, critical for booting and hardware control. 48. If an IC moisture indicator card shows humidity >30%, the IC is considered moisture-absorbed and compromised. 49. Correct solder paste ratio: - Weight: 90% solder / 10% flux - Volume: 50% solder / 50% flux 50. SMT originated in the mid-1960s, developed by military and aerospace industries.
51. Paper tape reels with 8mm width usually have a component pitch of 4mm. Note: “20mm” is not a valid carrier tape width. 52. In early 1970s, HCC referred to a new SMD type: Hermetically Sealed Leadless Chip Carriers. 53. Component code “272” = 2.7 kΩ resistor; “100nF” capacitor is equivalent to 0.1μF. 54. Common pick-and-place machines include: - In-line, - Rotary turret, - Gantry-type systems. 55. Flux types based on rosin include: R, RA, RSA, and RMA. 56. The most commonly used material for SMT components is ceramic. 57. SMT repair tools: soldering iron, hot air rework station, desoldering pump, tweezers. 58. Ideal solder pot temperature during manual soldering: 245°C (473°F). 59. Basic SMT production flow: Board loader → Solder paste printer → High-speed placer → Multi-purpose placer → Reflow oven → Unloader. 60. Most PCBs used in current computers are made of fiberglass-reinforced epoxy (FR-4).
61. Resistor arrays (networks) used in SMT are non-polarized. 62. Most commercial solder pastes maintain optimal tackiness for ~4 hours after application. 63. Standard air pressure for SMT equipment: 5 kg/cm² (≈ 71 psi). Placement rule: small components first, larger ones later. 64. For PCBs with PTH on the top side and SMT on the bottom, use dual-wave soldering with turbulence flow. 65. Common SMT inspection methods include: - Visual inspection - X-ray inspection - Automated Optical Inspection (AOI)
66. Heat transfer during soldering iron repairs involves conduction + convection. 67. Typical BGA solder ball composition: Sn90 / Pb10. 68. Common stencil fabrication methods: laser cutting, electroforming, chemical etching. 69. Reflow oven temperature should be set based on actual measurements via thermocouples. 70. Post-reflow, components are expected to be securely soldered onto the PCB.
71. The evolution of quality management: TQC → TQA → TQM (Total Quality Control → Assurance → Management) 72. ICT (In-Circuit Testing) uses bed-of-nails fixtures and static testing for components. 73. Static electricity has low current and is greatly affected by humidity. 74. Advantages of solder alloys: - Lower melting point - Good fluidity at low temperatures - Meets mechanical strength requirements 75. If reflow oven components or process settings are changed, temperature profiling must be remeasured.
76. Siemens 80F/S system uses electronic drive control. 77. Solder paste thickness testers use laser measurement to inspect: - Paste height - Thickness - Printed width 78. Common SMT feeding methods: - Vibratory feeders - Tray feeders - Tape-and-reel feeders 79. Mechanical structures used in SMT machines include: - Cam mechanisms - Linkage mechanisms - Screw drives - Sliding mechanisms 80. If visual inspection is inconclusive, refer to: - BOM file - Supplier confirmation - Reference board (Golden Sample)
81. For packaging marked “12w8P,” adjust pitch counter to 8 mm feed per advance. 82. Types of reflow ovens include: - Hot air reflow - Nitrogen reflow - Laser reflow - Infrared reflow 83. Sample production methods for SMT prototyping: - Inline automation - Manual printing + machine placement - Manual printing + manual placement 84. Common fiducial mark shapes: - Circle, cross, square, diamond, triangle, swastika 85. Improper reflow profiling can cause microcracks, especially in preheat or cooling zones.
86. Main causes of solder balls during reflow: - Poor PCB pad design - Oversized stencil apertures - Over-penetrated component placement - Excess paste collapse or low viscosity - Excessive ramp rate in reflow profile 87. Common reasons for solder bridging due to bad stencil printing: a. Low metal content → paste collapse b. Oversized apertures → too much solder c. Poor stencil quality → poor paste release d. Residual solder under stencil → reduce squeegee pressure and apply proper vacuum + solvent cleaning 88. Purpose of each reflow profile zone: - Preheat zone: solvent evaporation - Soak zone: flux activation, oxidation removal - Reflow zone: solder melting - Cooling zone: joint solidification and metallurgical bonding