Through hole insertion technology THT

1. Basic concepts and principles

1. What is Through Hole Technology (THT)?

THT is a technology that inserts electronic component leads into through-holes of PCBs and achieves electrical connection and fixation via soldering. It is one of the traditional mainstream electronic assembly processes.
2. What are the main differences between THT and Surface Mount Technology (SMT)?

- THT component leads require through-hole soldering, while SMT components are directly mounted on the PCB surface.

- THT has lower production efficiency and larger space occupation; SMT offers higher efficiency and density (IPC data: SMT density is 3-5x that of THT).

- THT suits high-power/large-size components; SMT suits miniaturized/high-density components.
3. What is the basic process flow of THT?

Component preparation → Insertion → Soldering (wave soldering for general use, manual/dip soldering for special components) → Lead trimming → Inspection
4. What types of electronic components are suitable for THT?

- High-power components (e.g., power transistors, transformers, large electrolytic capacitors)

- Components requiring high mechanical strength (e.g., connectors)

- Components with large lead spacing and low precision requirements
5. What is the minimum via diameter for PCB in THT process?

Standard process: 0.8mm; Special process: 0.4mm (with significantly increased cost and difficulty)

2. Component and Pin Handling

6. What are the standard shapes of THT component leads?

- Straight leads (suitable for automatic insertion machines)

- Bent leads (e.g., J-type, L-type, enhancing post-soldering mechanical stability)
7. What are the solderability requirements for component leads?

Per IPC-J-STD-001: At 235℃±5℃, immersed in solder pot for 2-3 seconds, solder coverage ≥95%
8. How to improve solderability of component leads?

- Metal coating (e.g., Sn, Ag plating)

- Flux application before soldering to remove oxide layers and enhance wettability
9. What is the appropriate lead length for THT components?

Lead protrusion from PCB soldering surface: 2-3mm (too short causes poor soldering; too long risks short circuits and requires extra trimming)
10. How do different lead materials affect soldering?

- Copper: Excellent solderability, most common

- Iron: Prone to oxidation, poor solderability, requires special treatment

- Alloys (e.g., copper alloys): Balances strength and solderability

3. Printed Circuit Board (PCB) related

11. What are the requirements for PCB materials in THT process?

Commonly uses FR-4 substrate with Tg ≥130℃, featuring good electrical insulation, mechanical strength, and heat resistance
12. What are the tolerance standards for PCB vias?

- Via diameter tolerance: ±0.05mm

- Hole position tolerance: ±0.1mm
13. How to prevent PCB deformation during THT soldering?

- Select appropriate substrate thickness (standard 1.6mm)

- Optimize PCB layout (avoid concentrated large copper areas)

- Use process edges and support fixtures
14. How does via wall roughness affect soldering?

Ra value should be controlled at 1.6-3.2μm: Excessive roughness causes poor solder filling; too smooth reduces wettability
15. What are the special requirements for multi-layer PCBs in THT?

- Requires precise interlayer alignment (usually processed by CNC drilling)

- Attention to interlayer insulation during soldering to avoid short circuits

4. Welding Process - Wave Soldering

16. What is the basic principle of wave soldering?

A solder pump jets molten solder into a wave, and the PCB/leads are soldered when passing through the wave. Double-wave soldering reduces bridging and cold joints.
17. What is the typical solder temperature for wave soldering?

- Sn63Pb37 (leaded): 245℃-255℃

- Sn96.5Ag3.0Cu0.5 (lead-free): 255℃-265℃
18. What is the typical soldering time for wave soldering?

Lead-wave contact time: 2-4 seconds (too short causes poor soldering; too long risks component damage or PCB deformation)
19. How to adjust the wave height in wave soldering?

Wave height: 1/2-2/3 of PCB thickness (adjusted via solder pump flow and pressure; too high causes bridging, too low leads to insufficient soldering)
20. What are the flux application methods in wave soldering and their characteristics?

- Spray: Low consumption, uniform, suitable for high
-precision soldering

- Foam: High concentration, suitable for thick
-film circuits

- Wave: High efficiency, high waste

5. Welding Process - Manual Welding and Others

21. How to control temperature and time for manual soldering of THT components?

Soldering iron temperature: 300℃-350℃; per joint: 2-3 seconds (avoid high-temperature damage)
22. What are the characteristics and applications of dip soldering?

Characteristics: High efficiency but prone to bridging; Applications: Mass production of simple PCBs with large lead spacing
23. What are the advantages of selective soldering?

Targeted soldering for specific areas, avoiding solder impact on other regions. Suitable for mixed THT/SMT boards to reduce wave soldering effects on SMT components.
24. Can reflow soldering be used for THT components?

Reflow soldering is primarily for SMT. It can be used for special THT cases (e.g., small-size/few-lead components with special fixtures), but not mainstream.
25. What are the solder requirements for different soldering processes?

- Wave soldering: High fluidity, oxidation resistance

- Manual soldering: Moderate melting point, good wettability

- Dip soldering: Strong coverage

- Common forms: Wave/dip soldering uses solder bars; manual soldering uses solder wire

6. Solder and Flux

26. What are the common compositions of THT solders?

Common solders include tin-lead (Sn63Pb37, eutectic point 183℃) and lead-free (e.g., Sn96.5Ag3.0Cu0.5, melting point 217℃). Lead-free solders are increasingly adopted for environmental reasons.
27. What are the differences in soldering performance between lead-free and leaded solders?

Lead-free solders have a 30-40℃ higher melting point and poorer wettability, requiring higher soldering temperatures and process precision, but comply with RoHS standards.
28. What is the main role of flux?

Removes oxides from soldering surfaces, reduces solder surface tension, enhances wettability, and prevents re-oxidation during soldering.
29. How are flux activity levels classified?

Classified per IPC-J-STD-001 as R (non-active), RMA (moderate activity), and RA (active). Higher activity improves oxide removal but may increase residue corrosion risk.
30. What are the effects of flux residues on circuits?

May cause short circuits, leakage, or corrosion of components/PCBs by highly active residues, often requiring post-soldering cleaning.

7. Equipment and Tooling

31. What are the main types of automatic insertion machines?

Main types: horizontal (suitable for axial components like resistors) and vertical (suitable for radial components like electrolytic capacitors).
32. What is the typical insertion accuracy of automatic insertion machines?

Modern machines achieve ±0.1mm accuracy, suitable for most THT components.
33. What does daily maintenance of wave soldering machines include?

Regular cleaning of solder pots, solder replacement, temperature/wave height calibration, flux system cleaning, and mechanical lubrication.
34. What are the key points for selecting and using manual soldering tools?

Choose 30W-60W soldering irons with various tip shapes; tin the tip before use to prevent oxidation.
35. What types of fixtures are commonly used in THT processes?

Includes PCB positioning fixtures, component holders, and soldering supports to ensure positional accuracy and stability.

8. Quality control and defect analysis

36. What are common defects in THT soldering?

Cold joints, bridging, insufficient solder, solder balls, etc. Cold joints refer to poor electrical connections; bridging causes short circuits between pads.
37. What are the causes and solutions for cold joints?

Causes: Oxidized leads, insufficient solder, improper temperature/time, low flux activity. Solutions: Clean leads, adjust parameters, select appropriate flux.
38. What are the causes and preventive measures for bridging?

Causes: Excessive solder, high wave height, small lead spacing, uneven flux. Prevention: Optimize solder volume, wave height, PCB design, and flux application.
39. How to inspect THT soldering quality?

Visual inspection (AOI), X-ray (AXI), in-circuit test (ICT). Visual checks for surface defects; X-ray for internal cold joints.
40. What are the quality standards for THT soldering?

Per IPC-A-610, solder joints should be smooth, crack-free, with ≥95% lead coverage and reliable electrical connections.

9. Environmental Protection and Safety

41. What are the environmental requirements for THT processes?

Comply with RoHS directives to restrict hazardous substances; purify exhaust gases; properly recycle waste solder and flux.
42. What special requirements do lead-free soldering operators have?

Wear heat-resistant gloves and goggles; undergo regular health checks to prevent burns and harmful gas inhalation.
43. How to dispose of waste solder and flux in THT processes?

Waste solder should be recycled by qualified companies; waste flux (hazardous chemical) must be sealed and handled by professional hazardous waste companies.
44. What are the ventilation requirements for soldering workshops?

Maintain good ventilation to ensure harmful gas concentrations are below occupational exposure limits, with ≥12 air changes per hour.
45. What are the safety precautions for chemicals used in THT processes?

Flux and other chemicals are flammable; keep away from fire sources. For corrosive chemicals, rinse immediately after contact and store/use as per instructions.

10. Process optimization and cost control

46. How to improve THT process efficiency?

Optimize insertion sequence, use high-speed automatic machines, rationalize process flow, and improve equipment stability.
47. What are the main cost components of THT processes?

Components, PCB, solder/flux, equipment depreciation/maintenance, labor, and inspection costs.
48. How to reduce THT production costs?

Bulk purchasing to reduce component costs, optimize PCB design to minimize waste, improve yield to reduce rework, and rationalize resource allocation.
49. How does THT cost compare to SMT?

THT has lower equipment investment but higher labor/material costs. SMT has higher equipment costs but higher efficiency/density, leading to lower long-term unit costs.
50. How to improve THT soldering yield through process optimization?

Precisely control temperature, time, and flux volume; strengthen incoming inspection; regular equipment maintenance; operator training.

11. Special applications and mixed processes

51. What are the advantages of mixed THT/SMT assembly?

Combines THT's suitability for high-power components with SMT's high-density miniaturization, meeting complex circuit design needs.
52. What is the process flow for mixed THT/SMT assembly?

SMT placement first, then THT insertion and soldering (wave or selective soldering). Ensure SMT components can withstand THT soldering temperatures.
53. How to prevent SMT component detachment during wave soldering in mixed processes?

Use high-temperature adhesive to fix SMT components, adjust wave soldering parameters to reduce impact, and employ double-wave for optimized wave shape.
54. What special requirements does THT have for high-reliability applications?

Higher-quality components, PCBs, and solders; strict soldering process control; additional inspection steps; possible use of special processes like vacuum soldering.
55. What are the quality control key points for THT in military applications?

Strictly follow military standards (e.g., GJB), 100% raw material inspection, traceable soldering processes, and advanced testing (X-ray/ultrasonic).

12. Emerging Technologies and Future Development

56. What are the future development trends of THT?

Moving toward automation and intelligence, integrating new materials and processes to meet higher-power/higher-density assembly needs.
57. What is the impact of 3D printing on THT?

Enables rapid fabrication of special PCBs and fixtures, reducing development time and low-volume production costs.
58. What are the application prospects of AI in THT?

Applications in soldering parameter optimization, automatic defect detection, and equipment failure prediction to enhance intelligence.
59. How do new solder materials drive THT development?

New solders (e.g., low-temperature, high thermal conductivity) meet special application needs and expand THT's scope.
60. What challenges and opportunities do green requirements bring to THT?

Challenges: Need for eco-friendly materials/processes. Opportunities: Drive technological upgrading and new market demands.

13. Process parameter adjustment

61. How to adjust wave soldering parameters for frequent cold joints?

Cold joints often result from insufficient temperature/time. Increase solder temperature by 5-10℃ and contact time by 0.5-1s, optimizing gradually.
62. How does flux concentration affect soldering quality? How to adjust?

Low concentration causes cold joints; high concentration leads to excess residue. Monitor specific gravity: add concentrate if low, diluent if high, to maintain process range.
63. How does insertion speed affect quality in automatic machines?

Excessive speed causes misalignment or lead deformation; slow speed reduces efficiency. Determine optimal speed via trial production to balance quality and efficiency.
64. What is the role of preheating temperature in THT soldering? How to set it?

Preheating removes moisture, reduces thermal stress, and improves wettability. Set at 100-150℃, adjusted for PCB material, thickness, and component heat resistance.
65. How does ambient temperature affect THT soldering?

For every 10℃ change in ambient temperature, adjust soldering temperature by 3-5℃. Low temperatures reduce solder fluidity; high temperatures accelerate flux evaporation. Preheat PCBs below 15℃.
66. How to prevent PCB pad detachment in THT?

Limit soldering time to 2-4s, avoid repeated soldering; use PCBs with Tg≥130℃; desolder with low-temperature hot air guns (≤350℃) and flux paste, reducing pad detachment by >80%.
67. How to evaluate THT solder joint fatigue resistance?

Thermal cycle testing (-40℃~125℃, 500 cycles) to observe crack propagation; SEM for microstructure analysis. Fatigue life should meet 3x design safety factor.
68. What is the role of nitrogen protection in THT soldering and its applications?

14. Process details and problem-solving

69. How to handle black residues on THT solder joints?

Black residues are often flux carbonization. Use low-activity flux, increase preheat temperature by 10-15℃, or ultrasonically clean with isopropyl alcohol for severe cases.
70. What are the roles of dual waves in THT wave soldering?

Turbulent wave (A-wave) cleans leads and improves solder penetration; smooth wave (B-wave) shapes joints and eliminates bridging. Height ratio recommended at 1:1.2-1:1.5.
71. How does conveyor speed fluctuation affect THT soldering quality?

Speed fluctuations >±5% cause inconsistent soldering time, leading to cold or over-soldering. Use closed-loop servo control to limit fluctuations within ±1%.
72. How to measure electrical contact resistance of THT solder joints?

Use a micro-ohmmeter with 1A constant current to measure voltage drop. Contact resistance should be ≤5mΩ (≤1mΩ for high-current joints).
73. What are the special requirements for THT in EV charging piles?

Require IP67 protection, resistance to 85℃/85%RH humidity aging for 1000 hours; high-current terminals must have ≤0.5mΩ contact resistance and ≥50N tensile strength.

15. Process parameters and equipment maintenance

74. What is the relationship between flux solid content and THT soldering?

Solid content typically 10%-25%. High content increases residue; low content reduces oxide removal. 15%-20% is recommended for lead-free soldering.
75. How to resolve component tilting after THT insertion?

Check nozzle pressure (0.4-0.6MPa), adjust feeder position accuracy (≤0.1mm error), and use pre-cured adhesive for heavy components.
76. How to design PCB solder mask opening size for THT?

Openings should be 0.1-0.2mm larger than pads on each side to prevent mask contamination and improve yield by ~15%.
77. What are the key daily checks for THT equipment?

Check solder pot temperature (±3℃), flux concentration (±5% fluctuation), wave height (1.5-2.5mm), conveyor tension (20-30N), and ground resistance (≤4Ω).
78. How to identify THT solder defects via AOI?

Set parameters for joint shape, color, wetting angle (≤30°), and use machine learning for ≥95% defect detection rate.

16. Special Applications and Reliability

79. What are the special requirements for transformer lead soldering in THT?

Use cross-soldering for multi-leads, extend soldering time to 4-6s; apply silver-containing solder (e.g., Sn96.5Ag3.0Cu0.5) with thermal resistance ≤3℃/W.
80. How does altitude affect THT soldering?

For every 1000m altitude increase, atmospheric pressure drops 10%, solder boiling point drops 5-8℃. Reduce soldering temperature and increase flux activity.
81. How to handle odd-form component insertion in THT?

Use customized fixtures or robot vision guidance for ±0.02mm accuracy; pre-form irregular leads for precise fit.
82. Why do THT solder joints turn white and how to fix it?

White spots are due to impurities (e.g., zinc) or overheating (>270℃). Replace solder and maintain temperature at 230-260℃.
83. What does annual calibration of THT equipment include?

Calibrate temperature profile (±5℃), wave height (±0.1mm), flux volume (±10%), and timing (±0.1s) for core parameters.

17. Quality Control and Industry Standards

84. How to calculate THT first-pass yield?

First-pass yield = (Total units - First-fail units)/Total units × 100%. Excellent level ≥98%.
85. What are the certification requirements for THT in railway signaling equipment?

Require EN 50121 EMC and EN 50155 environmental reliability certification. Solder joints must pass 5-50Hz/2g vibration test for 3 hours.
86. How does PCB preheat slope affect THT soldering quality?

Preheat slope recommended at 1-3℃/s. Excessive slope causes thermal stress; insufficient slope reduces flux activation. Proper slope reduces defects by 20%.
87. How to evaluate solderability of THT solders?

Use wetting balance test: Sn-Pb solder should reach peak wetting force ≤2s at 235℃, or wetting angle ≤25° for optimal performance.
88. How to prevent connector solder joint loosening in THT?

Apply threadlocker (e.g., Loctite 243) for 10-15N・m anti-loosening torque, or use mechanical latches for vibration resistance.

18. Process optimization and emerging technologies

89. How does soldering frequency affect THT joint reliability?

Repeated soldering >3 times increases intermetallic layer thickness by 50% and reduces fatigue life by 60%. Limit to ≤2 soldering attempts per joint.
90. How to calculate solder dross generation rate in THT?

Dross rate = (Total solder used - Effective solder used)/Total solder used × 100%. Normal control ≤3%-5%.
91. What are the cleanliness requirements for THT in military products?

Must meet MIL-P-28809 standards with surface ionic contamination ≤1.0μg/cm², tested via ion chromatography to prevent electrochemical migration.
92. How to optimize cooling rate in THT wave soldering?

Cooling rate recommended at 3-5℃/s. Too fast causes cracking; too slow reduces efficiency. Use forced air+water cooling for precision control.
93. Where can THT combine with emerging soldering technologies (e.g., laser soldering)?

In high-density mixed boards, use laser soldering for SMT and wave soldering for high-power THT components; or laser rework for THT defect repair.

Through hole insertion technology THT

1. Basic concepts and principles

1. What is Through Hole Technology (THT)?

2. What are the main differences between THT and Surface Mount Technology (SMT)?

3. What is the basic process flow of THT?

4. What types of electronic components are suitable for THT?

5. What is the minimum via diameter for PCB in THT process?

2. Component and Pin Handling

6. What are the standard shapes of THT component leads?

7. What are the solderability requirements for component leads?

8. How to improve solderability of component leads?

9. What is the appropriate lead length for THT components?

10. How do different lead materials affect soldering?

3. Printed Circuit Board (PCB) related

11. What are the requirements for PCB materials in THT process?

12. What are the tolerance standards for PCB vias?

13. How to prevent PCB deformation during THT soldering?

14. How does via wall roughness affect soldering?

15. What are the special requirements for multi-layer PCBs in THT?

4. Welding Process - Wave Soldering

16. What is the basic principle of wave soldering?

17. What is the typical solder temperature for wave soldering?

18. What is the typical soldering time for wave soldering?

19. How to adjust the wave height in wave soldering?

20. What are the flux application methods in wave soldering and their characteristics?

5. Welding Process - Manual Welding and Others

21. How to control temperature and time for manual soldering of THT components?

22. What are the characteristics and applications of dip soldering?

23. What are the advantages of selective soldering?

24. Can reflow soldering be used for THT components?

25. What are the solder requirements for different soldering processes?

6. Solder and Flux

26. What are the common compositions of THT solders?

27. What are the differences in soldering performance between lead-free and leaded solders?

28. What is the main role of flux?

29. How are flux activity levels classified?

30. What are the effects of flux residues on circuits?

7. Equipment and Tooling

31. What are the main types of automatic insertion machines?

32. What is the typical insertion accuracy of automatic insertion machines?

33. What does daily maintenance of wave soldering machines include?

34. What are the key points for selecting and using manual soldering tools?

35. What types of fixtures are commonly used in THT processes?

8. Quality control and defect analysis

36. What are common defects in THT soldering?

37. What are the causes and solutions for cold joints?

38. What are the causes and preventive measures for bridging?

39. How to inspect THT soldering quality?

40. What are the quality standards for THT soldering?

9. Environmental Protection and Safety

41. What are the environmental requirements for THT processes?

42. What special requirements do lead-free soldering operators have?

43. How to dispose of waste solder and flux in THT processes?

44. What are the ventilation requirements for soldering workshops?

45. What are the safety precautions for chemicals used in THT processes?

10. Process optimization and cost control

46. How to improve THT process efficiency?

47. What are the main cost components of THT processes?

48. How to reduce THT production costs?

49. How does THT cost compare to SMT?

50. How to improve THT soldering yield through process optimization?

11. Special applications and mixed processes

51. What are the advantages of mixed THT/SMT assembly?

52. What is the process flow for mixed THT/SMT assembly?

53. How to prevent SMT component detachment during wave soldering in mixed processes?

54. What special requirements does THT have for high-reliability applications?

55. What are the quality control key points for THT in military applications?

12. Emerging Technologies and Future Development

56. What are the future development trends of THT?

57. What is the impact of 3D printing on THT?

58. What are the application prospects of AI in THT?

59. How do new solder materials drive THT development?

60. What challenges and opportunities do green requirements bring to THT?

13. Process parameter adjustment

61. How to adjust wave soldering parameters for frequent cold joints?

62. How does flux concentration affect soldering quality? How to adjust?

63. How does insertion speed affect quality in automatic machines?

64. What is the role of preheating temperature in THT soldering? How to set it?

65. How does ambient temperature affect THT soldering?

66. How to prevent PCB pad detachment in THT?