The Benefits of Mixed PCB Assembly: Combining SMT and THT Technologies

Mixed PCB assembly—integrating Surface Mount Technology (SMT) and Through-Hole Technology (THT)—has become a cornerstone of modern electronics manufacturing. By leveraging the precision of SMT for compact components and the durability of THT for high-power or stress-resistant parts, this hybrid approach delivers a rare balance of performance, flexibility, and cost efficiency. From automotive control systems to medical devices, mixed assembly meets the diverse demands of today’s most challenging applications.

This guide explores why engineers and manufacturers choose mixed PCB assembly, its key benefits over single-technology approaches, real-world applications, and best practices for design and production. Whether you’re building a consumer gadget or a rugged industrial system, understanding mixed assembly is critical to optimizing your PCB’s performance and reliability.

Key Takeaways
1.Mixed PCB assembly combines SMT’s density and speed with THT’s strength and power-handling, reducing field failure rates by 30–40% in harsh environments.
2.It enables design flexibility, supporting both tiny 01005 SMT components and large THT connectors in a single board, with 50% more component variety than single-technology assemblies.
3.Cost savings of 15–25% are achieved by automating high-volume SMT steps while using THT only where necessary (e.g., high-power components).
4.Industries like automotive, medical, and industrial electronics rely on mixed assembly for its ability to balance precision, durability, and versatility.

What Is Mixed PCB Assembly?
Mixed PCB assembly is a manufacturing approach that merges two core technologies:

  a.Surface Mount Technology (SMT): Components are mounted directly onto the PCB’s surface, using solder paste and reflow ovens for attachment.
  b.Through-Hole Technology (THT): Components have leads inserted into drilled holes, with solder applied via wave soldering or manual soldering.

This combination addresses the limitations of each technology alone: SMT excels at miniaturization and speed but struggles with high-power or mechanically stressed parts; THT offers ruggedness and power handling but lacks density. Together, they create PCBs that are both compact and robust.

SMT vs. THT: Core Differences

How Mixed Assembly Works
Mixed assembly integrates these technologies in a single workflow:

1.SMT First: Automated machines place surface-mount components (resistors, ICs, small capacitors) onto the PCB.
2.Reflow Soldering: The board passes through a reflow oven to melt solder paste, securing SMT components.
3.THT Integration: Through-hole components (connectors, large inductors) are inserted into pre-drilled holes.
4.Wave Soldering or Manual Soldering: THT leads are soldered—either via a wave solder machine (high volume) or hand-soldering (low volume/sensitive parts).
5.Inspection: Combined AOI (for SMT) and X-ray (for hidden THT joints) ensures quality.

Key Benefits of Mixed PCB Assembly
Mixed assembly outperforms single-technology approaches in critical areas, making it the go-to choice for complex electronics.
1. Enhanced Reliability and Durability
In applications with vibration, temperature swings, or mechanical stress, mixed assembly shines:

  a.THT’s Role: Through-hole leads create a mechanical anchor, resisting vibration (20G+) and thermal cycling (-40°C to 125°C). This is critical for automotive underhood PCBs or industrial machinery.
  b.SMT’s Role: Precise SMT soldering reduces joint fatigue in low-stress areas, with 99.9% of SMT joints surviving 10,000+ thermal cycles.

Example: A car’s engine control unit (ECU) uses SMT for sensors and microcontrollers (low stress) and THT for power connectors (high vibration), reducing failure rates by 35% compared to all-SMT designs.

2. Design Flexibility
Mixed assembly unlocks designs that would be impossible with SMT or THT alone:

  a.Density + Ruggedness: Fit 0.4mm pitch BGAs (SMT) alongside large D-sub connectors (THT) in the same board—ideal for compact yet versatile devices like medical monitors.
  b.Component Variety: Access a broader range of parts, from tiny RF chips (SMT) to high-voltage transformers (THT), without design compromises.

Data Point: Mixed assembly supports 50% more component types than all-SMT or all-THT designs, according to IPC industry studies.

3. Optimized Performance
By matching technology to component function, mixed assembly boosts overall PCB performance:

  a.Signal Integrity: SMT minimizes trace lengths, reducing signal loss in high-speed paths (10Gbps+). For example, SMT-mounted 5G transceivers achieve 30% lower insertion loss than THT equivalents.
  b.Power Handling: THT components (e.g., terminal blocks) manage 10A+ currents without overheating, critical for power supplies and motor controllers.

Testing: A mixed assembly PCB in a 48V industrial power supply showed 20% higher efficiency than an all-SMT design, thanks to THT’s superior power dissipation.

4. Cost Efficiency
Mixed assembly balances automation and manual labor to reduce costs:

  a.SMT Automation: High-volume SMT placement (50,000 parts/hour) cuts labor costs for small components.
  b.Targeted THT: Using THT only for critical parts (e.g., connectors) avoids the expense of hand-soldering all components.

Cost Breakdown: For a 1,000-unit run, mixed assembly costs 15–25% less than all-THT (due to SMT automation) and 10% less than all-SMT (by avoiding expensive SMT-compatible high-power parts).

5. Versatility Across Industries
Mixed assembly adapts to diverse application needs, from consumer gadgets to aerospace systems:

  a.Consumer Electronics: SMT for miniaturization (e.g., smartphone ICs) + THT for charging ports (high 插拔 stress).
  b.Medical Devices: SMT for precision sensors + THT for power connectors (sterility and durability).
  c.Aerospace: SMT for lightweight avionics + THT for ruggedized connectors (vibration resistance).

Applications of Mixed PCB Assembly
Mixed assembly solves unique challenges across key industries, proving its versatility.
1. Automotive Electronics
Cars demand PCBs that handle vibration, temperature extremes, and both low-signal sensors and high-power systems:

  a.SMT: Used for ECU microcontrollers, radar sensors, and LED drivers (compact, low weight).
  b.THT: Used for battery terminals, fuse holders, and OBD-II connectors (high current, frequent plugging).

Result: Mixed assembly ECUs in electric vehicles (EVs) reduce warranty claims by 40% compared to all-SMT designs, per automotive industry data.

2. Medical Devices
Medical PCBs require precision, sterility, and reliability:

  a.SMT: Powers tiny sensors in pacemakers and EEG monitors (low power, high density).
  b.THT: Secures connectors for patient cables and power inputs (mechanical strength, easy cleaning).

Compliance: Mixed assembly meets ISO 13485 and FDA standards, with THT’s rugged joints ensuring long-term reliability in implantables and diagnostic tools.

3. Industrial Machinery
Factory equipment needs PCBs that survive dust, moisture, and heavy use:

 a.SMT: Controls PLCs and sensor arrays (fast signal processing).
 b.THT: Handles motor drivers, power relays, and Ethernet connectors (high current, vibration resistance).

Example: A mixed assembly PCB in a robotic arm reduced downtime by 25% by combining SMT’s signal speed with THT’s resistance to mechanical stress.

4. Consumer Electronics
From smartphones to home appliances, mixed assembly balances size and durability:

  a.SMT: Enables slim designs with 01005 passives and 5G modems.
  b.THT: Adds sturdy USB-C ports and power jacks (withstand daily use).

Market Impact: 70% of modern smartphones use mixed assembly, per industry reports, to balance miniaturization and port durability.

Design Best Practices for Mixed PCB Assembly
To maximize the benefits of mixed assembly, follow these design guidelines:
1. Component Placement
  a.Segregate Zones: Keep SMT components in low-stress areas (away from connectors) and THT parts in high-stress zones (edges, ports).
  b.Avoid Overcrowding: Leave 2–3mm between THT holes and SMT pads to prevent solder bridging during wave soldering.
  c.Align for Automation: Place SMT components in grids compatible with pick-and-place machines; orient THT parts for easy insertion.

2. Layout Considerations
  a.Thermal Management: Use THT heat sinks and vias near high-power SMT ICs to dissipate heat.
  b.Signal Routing: Route high-speed SMT traces away from THT power paths to reduce EMI.
  c.Hole Sizing: THT holes should be 0.1–0.2mm larger than component leads to ensure proper soldering.

3. DFM (Design for Manufacturability)
  a.SMT Stencil Design: Use laser-cut stencils with 1:1 pad-to-aperture ratios for consistent solder paste application.
  b.THT Hole Placement: Space THT holes ≥2mm apart to avoid PCB weakening.
  c.Test Points: Include both SMT (for AOI) and THT (for manual probing) test points to simplify inspection.

Overcoming Challenges in Mixed Assembly
Mixed assembly has unique hurdles, but careful planning mitigates them:
1. Thermal Compatibility
Challenge: SMT components (e.g., plastic ICs) may melt during THT wave soldering (250°C+).
Solution: Use high-temperature SMT components (rated for 260°C+) or shield sensitive parts with heat-resistant tape during wave soldering.

2. Assembly Complexity
Challenge: Coordinating SMT and THT steps can slow production.
Solution: Use automated workflows with integrated SMT placement and THT insertion machines, reducing changeover time by 50%.

3. Quality Control
Challenge: Inspecting both SMT and THT joints requires different tools.
Solution: Combine AOI (for SMT surface joints) and X-ray (for hidden THT barrel solder) to catch 99.5% of defects.

FAQs
Q: Is mixed assembly more expensive than single-technology assembly?
A: Initially, yes—by 10–15%—but it reduces long-term costs via lower failure rates and better performance. For high-volume production, the savings often offset the upfront expense.

Q: Can mixed assembly handle high-frequency designs (5G, RF)?
A: Absolutely. SMT’s short traces minimize signal loss in 5G/RF paths, while THT connectors provide robust RF shielding where needed.

Q: What’s the minimum order quantity for mixed assembly?
A: Most manufacturers accept small runs (10–50 units) for prototypes, with high-volume automation kicking in for 1,000+ units.

Q: How do I choose between SMT and THT for a specific component?
A: Use SMT for small, low-power, or high-density parts (ICs, resistors). Use THT for large, high-power, or frequently plugged components (connectors, relays).

Q: Does mixed assembly work with flexible PCBs?
A: Yes—flexible mixed PCBs use SMT for bendable areas and THT for rigid sections (e.g., foldable phone hinges with SMT sensors and THT charging ports).

Conclusion
Mixed PCB assembly bridges the gap between SMT’s precision and THT’s ruggedness, offering a versatile solution for today’s electronics. By combining the right technology for each component, manufacturers achieve designs that are compact, reliable, and cost-effective—critical in industries from automotive to medical.

With careful design (DFM practices, strategic component placement) and quality control (AOI + X-ray inspection), mixed assembly delivers PCBs that outperform single-technology approaches in durability, flexibility, and performance. As electronics grow more complex, mixed assembly will remain a key driver of innovation, enabling the next generation of devices to be both smaller and stronger than ever before.