What is the difference between TH, THR and SMT?

TH (through-hole) is wave- or hand-soldered. THR (Through-Hole Reflow, also called pin-in-paste) uses a through-hole pin with paste printed on the pad and reflowed like an SMT part. SMT is surface-mount, with no hole.

Which gives the strongest mechanical connection?

Through-hole is strongest because the pin is soldered inside the barrel. THR is close, since the pin still goes through the hole. SMT is weakest because retention depends on the pad adhesion.

Can a PA66 terminal survive reflow?

Most PA66 housings are rated for wave or hand soldering only. For reflow profiles above about 245 degrees C peak, specify LCP or PA9T housings.

What is the best solder profile for THR?

Follow the paste and terminal datasheets. Typical lead-free THR profiles use a ramp of 1 to 3 degrees C per second, a soak around 150 to 200 degrees C, and a peak of 235 to 245 degrees C for 30 to 60 seconds above liquidus.

Is selective soldering an option for mixed TH and SMT?

Yes. Selective soldering solders TH or THR pins on a board that has already been reflow-soldered on the SMT side. It is common when TH pins must be added to a predominantly SMT assembly.

Terminal Blocks

PCB Terminal Block Soldering: TH, THR and SMT Guide

Q: Why does one pin show poor hole fill?

Large copper planes, insufficient paste or flux, pin-to-hole mismatch, oxidation and thermal imbalance are common causes. Correct the process window rather than simply increasing heat everywhere.

August 10, 2026 3 min read

Tags:

#PCB assembly #PCB terminal block #reflow soldering #SMT terminal #terminal block #THR pin-in-paste #through-hole soldering #wave soldering

PCB terminal blocks combine an electrical connector with a mechanically loaded board interface. The solder process must create a reliable current path while keeping the housing seated, the pins aligned and the joint strong enough for wire insertion and service loads. This guide compares through-hole, pin-in-paste/THR and surface-mount methods and highlights the process data that should come from the exact component datasheet.

Process Quick Reference

Temperatures and dwell times are representative starting points only. The terminal supplier’s soldering profile and the solder-alloy specification take priority.

Mounting	Soldering Method	Max Temperature	Notes
Through-hole (TH)	Wave solder or hand-solder	260°C (3s per pin)	Standard for PCB terminals
Through-hole reflow (THR)	SMT reflow in same process	235–245°C peak (SAC305)	Requires THR-compatible terminal
SMT	Reflow oven	235–245°C peak	PA9T or LCP housing required
Press-fit	No solder — mechanical interference	N/A	Requires plated through-holes, special pin design

1. Through-Hole PCB Terminal: The Standard

Most PCB terminal blocks use through-hole mounting with square pins (0.6–1.0mm) soldered into plated holes.

Solder Process Options

Process	Equipment	Best For
Wave soldering	Wave solder machine	High-volume production, single-sided PCB
Hand soldering	Soldering iron (60W+)	Prototypes, low volume, repair
Selective soldering	Robotic soldering	Mixed TH/SMT boards, bottom-side only

Hand-Soldering Procedure

Insert terminal block into PCB — ensure it sits flush
Tack-solder one end pin to anchor the terminal
Solder the remaining pins using the component’s specified tip temperature and dwell time; a common manual starting point is about 350°C for 2–3 seconds per pin
Inspect wetting, fillet shape and hole fill against the applicable workmanship standard; lead-free joints are not always mirror-shiny
No solder bridging between adjacent pins (critical for fine pitches like 3.5mm)

Common Issues

Issue	Cause	Solution
Terminal lifted off PCB	Too much solder in hole before insertion	Insert terminal first, then solder
Poor wetting or disturbed joint	Oxidation, insufficient heat, contamination or movement during cooling	Correct the process cause, then rework with compatible flux and controlled heat
Housing melted	Iron too hot or dwelled >5 seconds	350°C max, 2–3 seconds per pin, let cool between pins
Bridging (3.5mm pitch)	Excess solder, large tip	Use fine conical tip (0.8mm), less solder

2. Through-Hole Reflow (THR): SMT + TH in One Pass

THR terminals are designed to be soldered in the same reflow oven as SMT components. Paste is printed into plated through-holes, the THR terminal pins are inserted, and the entire board goes through reflow.

Requirements

Parameter	Requirement
Terminal housing	Reflow-qualified high-temperature material such as an appropriate LCP or polyamide grade; verify the exact peak profile. Do not assume material family alone proves compatibility.
Pin design	Tapered tip for easy insertion into paste-filled holes
Paste volume	Calculate for the required barrel fill, pin displacement and acceptable top/bottom fillets; validate by cross-section or process inspection
Stencil	Step stencil with thicker paste deposit at THR holes

Advantages Over Wave Soldering

One pass for all components (SMT + TH)
Eliminates separate wave solder step
Greater freedom for mixed-technology board layouts, subject to component keep-outs and reflow access

3. SMT PCB Terminal Blocks

SMT terminals eliminate through-holes — all pads are on the board surface. Saves PCB layers (no via needed for each pin) and allows components on both sides.

Parameter	SMT PCB Terminal
Pitch	3.5, 5.0, 5.08mm
Housing	PA9T or LCP (reflow-safe)
Coplanarity	<0.10mm across all pads
Mechanical strength	Lower than TH (no through-hole anchor)

SMT can simplify double-sided assembly and eliminate plated-through mounting holes, but the board pads and mechanical support must withstand wiring and service loads. Through-hole remains common in industrial equipment because the pins provide additional mechanical anchoring. For large pluggable connectors or high mating forces, consider board locks, flanges or chassis support.

4. Design-for-Soldering Checklist

Use the recommended hole, pad and stencil dimensions for the exact part number.
Confirm housing material and packaging are qualified for the intended process.
Include keep-outs for solder pallets, selective nozzles, reflow shadowing and inspection access.
Control connector seating and coplanarity before soldering.
Account for copper planes that draw heat away from individual pins.
Inspect hole fill, wetting and bridging to the applicable workmanship class.
Perform pull, torque or mating-force validation when the connector sees significant mechanical load.
Recheck electrical temperature rise after final solder-process validation.

Common Questions

Can every through-hole terminal block go through reflow?

No. Pin-in-paste/THR requires a housing, pin design and packaging qualified for the reflow profile. A standard wave-solder component may deform or shift.

Is a dull lead-free solder joint defective?

Not necessarily. Judge the joint by wetting, contour, fill, cracks and the applicable inspection criteria—not brightness alone.

Why does one pin show poor hole fill?