THT stands for Through-Hole Technology when referring to PCBs. It represents the mounting and interconnecting of electronic components using leads passed through holes drilled in the circuit board.
THT contrast with SMT (Surface Mount Technology) where components are directly soldered onto surface pads without leads passing through the board. This article provides a comprehensive overview of THT components, PCB design considerations, manufacturing processes, assembly, pros/cons, and comparisons to SMT.
Overview of Through-Hole Components
THT components have electrically conductive leads or pins that insert through corresponding holes in the PCB during assembly. The leads are then soldered on the opposite side to form mechanical and electrical connections.
Common examples of through-hole components:
- Axial lead resistors, capacitors, inductors
- Radial lead resistors, capacitors, crystals
- DIP ICs and sockets
- Pin headers and female connectors
- Transformers, chokes, and coils
- Terminal blocks and screw terminals
- TO-220 and TO-247 style power packages
- Single row pin connectors
- Wire-to-board connectors
THT components represent mature, well-established packaging that continues to be used across many applications due to advantages.
Purposes of PCB Through-Holes
In addition to mounting holes, holes are drilled in PCBs for:
Interconnections – Through-holes with annular ring pads allow component leads to pass through and connect between layers.
Vias – Holes with plated barrels electrically connect between layers of multi-layer boards.
Heatsinking – Clearance holes without pads can dissipate heat from metal component tabs.
Mechanical Fixing – Unplated holes allow securing components mechanically using screws.
Access – Holes can allow physical access to trim component values.
Tooling – Provide locating references for production using dowel pins.
So “THT” refers to drilling holes specifically to allow insertion and soldering of leaded components.
Advantages of Using THT Components
THT components offer several advantages including:
Simpler Soldering – Through-hole pins with plated barrels give excellent capillary solder wicking.
Higher Currents – Thicker leads enable higher current carrying capacity.
No Planarity Issue – Lead flexibility reduces impact of board warpage.
Easier Repair/Replacement – Components can be desoldered and replaced individually.
Improved Shock/Vibration Tolerance – Leaded connections withstand mechanical stresses better.
Human Assembly – Through-hole parts are easier for manual assembly vs SMT.
No Reflow Oven Needed – THT can be manually soldered or wave soldered.
Lower Cost – Very economical common components are available.
No Solder Paste – Through-hole assembly avoids solder paste-related defects.
For many applications, THT components are preferred due to these factors.
Considerations for THT PCB Design
Designers should account for the following THT characteristics:
Hole Size – Match drill diameter to lead width with sufficient annular ring.
Plating – Full plating of through-holes is needed for soldering.
Pad Size – Provide sufficient pad for solder contact and flux cleaning.
Spacing – Account for component lead pitch when placing pads.
Via-in-Pad – Avoid placing vias inside thermal pads due to solder voiding.
Mask Defined vs. Non-Mask Defined – Determine if solder mask or pad defines joint area.
Thermal Relief – Use spokes or isolation gaps on large copper pads for thermal contraction relief.
Plane Cutouts – Allow clearance cutouts for pins through power or ground planes.
Mixed Technology – Plan layout when using both through-hole and SMT devices.
Careful consideration of these factors ensures a THT-capable PCB design.
PCB Through-Hole Manufacturing Process Technologies
Several fabrication technologies are used for creating through-holes and plating in PCBs:
Drilling – Holes are mechanically drilled through the complete PCB stackup using carbide or diamond coated drill bits.
Deburring – Drill bit burrs are removed using abrasive deburring tools.
Desmear – Chemical desmear removes resin smear from hole walls exposed during drilling.
Plating – Electrolytic copper plating deposits a conductive barrel lining on drilled holes.
PTH Filling – Through-holes are filled with conductive material to plug holes.
Hole Wall Preparation – Micro-etching roughens hole walls for enhanced plating adhesion.
Plating Checks – Testing ensures plating quality and verifies hole connectivity.
Advanced PCB fabs use automated drilling, cleaning, and plating lines to achieve high hole quality, reliability, and throughput.
THT Assembly Processes
THT components are assembled to PCBs using:
Manual Insertion – Operators manually insert component leads through holes. Lower volumes, complex boards.
Wave soldering – Bottom-side solder waves simultaneously solder all THT leads. High volume.
Hand Soldering – Individual leads hand soldered using iron for rework or touch-up.
Press-fit – Leads pressed into plated through-holes to form gas-tight interference cold connections.
Selective Wave Solder – Bottom-side waves only solder select holes for mixed SMT and THT assemblies.
Laser Soldering – Laser energy heats holes for soldering.
Gluing – Adhesives can mechanically fasten THT components prior to soldering.
THT insertion can be automated but remains primarily manual assembly unlike SMT.
Through-Hole Component Reliability Considerations
Reliability factors for through-hole components include:
Solder joint integrity – Sufficient fillet size, wetting, pin hole avoidance.
Plating thickness – Plating laps onto pad ensure continuity if barrel thins during soldering.
Annular ring – Adequate clearance to hole prevents pad lifting.
Vibration resistance – Passing leads through holes provide good mechanical anchoring.
Thermal cycling – Differences in CTE between component materials induces stresses.
Mechanical shock – Thicker leads resist flexing damage better than SMT joints.
Solder voiding – Occurs if leads shift prior to reflow orthermodynamic stability issues.
Process compatibility – Lead alloy versus PCB plating metallurgy must be compatible.
Strict process controls around plating quality, hole drilling accuracy, and soldering is key.
Mixed SMT and THT PCB Assembly
For boards requiring both surface mount and through-hole components:
- Place SMT first using pick-and-place followed by reflow soldering.
- Insert THT components next into plated through-holes.
- Use selective wave soldering to only solder the THT leads.
- Additional manual soldering can also be used for rework.
This order prevents reflow heat damage to THT parts. Certain components may also be placed as press-fit pins to avoid soldering entirely. Careful process design is needed for combined SMT and THT boards.
Comparison of THT vs SMT
Comparison between through-hole and surface mount technologies:
|Component sizes||Tend to be larger||Smaller footprint|
|Thermal dissipation||Leads aid heat conduction||Depends on thermal pad|
|Design complexity||Simpler||Tighter component spacing|
|Assembly method||Manual insertion or wave solder||Automated pick-and-place|
|Assembly cost||Lower||Much higher|
|Repair||Easier to desolder/replace||Very difficult|
|Mechanical robustness||Greater lead strength||Brittle joints, no compliance|
|Routing channels||More space required||Allows higher density routing|
- THT refers to leaded through-hole components soldered on PCBs.
- Holes provide interconnection, mounting, heat transfer and access functions.
- THT maintains advantages like ease of soldering and repair that sustain use.
- THT assembly requires specialized hole drilling, plating, and soldering processes.
- Reliability considerations are primarily around solder joint integrity and thermal stresses.
- THT can be combined with SMT for assemblies requiring both technologies.
Understanding the meaning of THT and how it impacts PCB design and manufacturing ensures designers select the most appropriate component technology for their application needs.
Frequently Asked Questions
What are some common examples of THT components?
Axial and radial resistors/capacitors, DIP ICs, pin headers, terminal blocks, wire-to-board connectors, power transistors in TO packages, transformers, and inductors are among the most prevalent THT component types.
What drill size should be used for an IC lead diameter of 0.64 mm?
For a 0.64mm diameter component lead, the drill size should be around 0.61mm allowing a 0.015mm annular ring on either side. This allows sufficient space for soldering without excessive clearance gaps.
What are advantages of using press-fit pins instead of soldered THT?
Press-fit pins avoid the need for soldering by tightly fitting into plated holes for electrical contact. Advantages include no solder high temperatures, ability to insert pins late in assembly, and easy removal and replacement.
What causes tombstoning when soldering THT components?
Uneven heating of component leads causes one lead to solder first, tilting the part vertically or “tombstoning”. Solutions include lead forming, adhesive securing, matching lead lengths, and tweaking process parameters.
Why are thermal relief connections used on THT pads?
Thermal relief features like spokes or isolation gaps prevent solder pad lifting or cracking from the thermal expansion mismatch between the copper pad and PCB substrate material.