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
