Efficiency isn’t just a metric in PCBA manufacturing industry—it’s your bottom line. If you are a Product Designer, a purchasing engineer, or the head of an industrial automation firm, you’ve likely faced the “mixed assembly” headache. Your board is 95% surface-mount technology (SMT), but those few heavy-duty connectors or power modules require through-hole mounting for mechanical strength.
Traditionally, this meant two separate processes: high-speed SMT followed by slow, costly wave soldering or manual labor. But what if you could process through-hole components in your standard SMT reflow oven?
Welcome to the world of Through-Hole Reflow (THR)—also known as Pin-in-Paste. At Fumax Technology, we’ve mastered this bridge between tradition and innovation. Today, we’re diving deep into why THR is a must-have for your next industrial PCBA project and how we optimize it for zero-defect reliability.
Figure 1Through-Hole Reflow
The drive toward miniaturization and Electronic Product Development excellence has pushed many to abandon through-hole components. However, for industrial PCBA, automotive PCBA, and telecom PCBA, through-hole components remain essential because they offer superior mechanical bonding strength.
The “necessity” of THR boils down to three major advantages:
1.Cost Reduction: By eliminating the wave soldering stage, you save on equipment maintenance, energy, and floor space.
2.Increased Efficiency: Everything happens in one pass. Your PCBA assembly lead times drop significantly.
3.High Reliability: THR reduces the thermal shock that boards experience during multiple heating cycles (reflow then wave), preserving the integrity of your Multi-layer PCB.
The process is deceptively simple: we apply solder paste into the through-holes during the standard SMT assembly service, insert the through-hole pins into that paste, and run the entire assembly through the reflow oven. However, the “magic” lies in the precision of the execution.
In PCB Layout and Circuit Design, the hole-to-lead ratio is the most vital variable. If the hole is too large, the capillary action fails, and the solder “falls through.” If it’s too small, the pin pushes all the paste out during insertion.
Our DFM (Design for Manufacturability) experts at Fumax use advanced Altium Designer simulations to calculate the exact volume of solder needed to form a perfect fillet. We typically look for a hole diameter approximately 0.2mm to 0.3mm larger than the pin diameter to ensure optimal paste retention.
Figure 2diameter
The biggest challenge in THR is “volume.” A through-hole joint requires much more solder than a standard SMT pad. To achieve a 100% vertical fill, we often employ:
Overprinting: Printing paste beyond the pad area onto the solder mask (which pulls back into the hole during melting).
Step Stencils: Using a thicker stencil for the through-hole areas to deposit extra volume.
Failure to calculate this correctly leads to insufficient solder—a nightmare for industrial PCBA that must withstand high vibration.
Figure 3SMT workshop
Not every through-hole component can handle THR. As part of our Electronic Hardware Design consulting, we guide our clients in selecting components that meet two criteria:
High-Temperature Tolerance: The component body must withstand $260°C$ for at least 10 seconds without warping or melting.
Clearance: The component must sit slightly off the board (stand-offs) to allow the solder paste to outgas and prevent “solder balls” from forming under the part.
For small and medium-sized enterprises (SMEs), managing the transition from Prototyping to High Volume PCB Assembly is fraught with risk. You need a partner who understands Industrial Design for Electronics and doesn’t treat your order like just another number.
At Fumax, we don’t just provide SMT assembly; we provide a full-spectrum ODM PCBA solution. Our engineers look at your Mechanical Design and PCB Design holistically. If we see an opportunity to switch your assembly to THR to save you $15\%$ on production costs, we tell you. That’s the Fumax difference.
AOI & X-Ray Testing: To ensure the internal solder fill of through-holes meets IPC Class 2 or Class 3 standards.
Custom Stencil Design: Tailored step-stencils for complex custom PCBA.
Signal Integrity Analysis: Ensuring that the THR process doesn’t interfere with high-speed signals on your Multi-layer PCB.
Many of our clients come to us for Reverse Engineering Services. They have an old, reliable industrial controller that they need to modernize. We take that legacy through-hole design and apply modern PCB Layout Service and THR techniques to make it more manufacturable and cost-effective for the 2026 market.
Beyond the board, our Box Build Assembly team handles the final integration. Whether it’s an IoT Hardware Design for a smart factory or a robust motor controller for a medical device, we provide a Turnkey PCB Assembly experience that lets you focus on your business while we handle the hardware.
If your product requires the durability of through-hole pins but you are tired of the overhead of dual-process manufacturing, Through-Hole Reflow is your answer. It is a sophisticated technique that requires a PCB Manufacturer with high technical acumen and rigorous quality control.
Are you ready to optimize your production?
Whether you need Small Batch PCBA for a pilot run or are ready for High Volume PCB Assembly, Fumax Technology is here to help. Our team is standing by to provide a detailed DFM review and show you how THR can revolutionize your product’s reliability and cost.
Contact Fumax Technology today for a quote on your next PCBA project!
