Pin-in-Paste Soldering: Achieving Reliable Connections for Mixed-Technology PCBs

Introduction

In modern PCB assembly, combining through-hole and surface-mount components on the same board demands efficient, reliable soldering. Pin-in-paste (PIP) soldering meets this need by enabling both component types to be soldered in a single reflow step, eliminating the separate wave soldering process. This method reduces production time, minimizes thermal stress, and supports automation. To achieve consistent, high-quality joints, manufacturers must carefully control material selection, stencil design, placement accuracy, and reflow profiles.

What Is Pin-in-Paste Soldering?

Pin-in-paste soldering is a process where solder paste is printed onto PCB pads and into through-holes, component leads are inserted, and the entire assembly goes through reflow. The paste melts and forms the solder joint around the lead. This technique works best for smaller through-hole components and allows mixed-technology boards to be assembled in one production line. The key advantage is a single thermal cycle, which reduces thermal shock and speeds up throughput.

Advantages of Pin-in-Paste Over Wave Soldering

PIP offers several benefits over traditional wave soldering:

– Single-step process: Combines reflow for SMDs and through-hole components. – Up to 30% faster production: Eliminates the second wave soldering step. – Reduced thermal stress: Components experience only one heating cycle. – Better automation compatibility: Fits seamlessly into SMT lines with pick-and-place and reflow ovens. – Lower defect rates: With proper control, PIP can achieve fewer voids and bridges than wave soldering.

However, PIP is not ideal for large or heat-sensitive through-hole components; those are better served by selective or wave soldering.

Design Guidelines for Pin-in-Paste

Proper PCB and component design is critical for PIP success. Follow IPC-2221 recommendations and these guidelines:

– Solder mask expansion: Enlarge the solder mask opening around through-holes to improve paste flow and prevent solder wicking. – Pad size: Use IPC-recommended pad diameters to ensure strong fillet formation. – Hole size: Keep finished hole diameters between 0.2 mm and 0.25 mm. Larger holes may need additional paste volume to fill completely. – Aspect ratio: Maintain a board thickness to hole diameter ratio ≤ 8:1 to facilitate paste filling. – Component lead protrusion: Limit lead extension below the board to 0.5–1.5 mm to avoid paste displacement.

Stencil design also plays a vital role. Aperture size should be 110–130% of the hole diameter, with rounded or trapezoidal shapes for better paste release. Use multi-level stencils to deposit more paste over through-holes while keeping SMT pads at standard thickness. Electropolished surfaces improve paste transfer consistency.

Solder Paste Selection and Printing

Select Type 3 or Type 4 solder paste (particle sizes 25–45 µm and 20–38 µm respectively) for optimal flow into small holes. The paste must have good tack strength to hold components during transport. Typical paste volume per hole is 0.5–0.8 mm³, depending on hole diameter and board thickness.

Printing tips: – Clean the stencil every 5–10 prints to prevent clogging. – Store stencils flat in a dry, clean environment. – Regularly inspect stencil apertures for wear. – Adjust squeegee pressure, speed, and separation speed to suit the stencil and paste. – Use a vacuum fixture to support the board during printing.

Component Placement

Accurate component placement ensures leads center in the holes, keeping paste in place. Misalignment can cause bridging or insufficient solder. Use automated pick-and-place with vision alignment for best results. For manual placement, ensure leads are straight and inserted fully. Keep the leads short to avoid excessive paste disturbance.

Reflow Process Optimization

The reflow profile must match the paste specifications and board complexity. A typical profile includes:

– Preheat: 1–3°C/s ramp to 150°C to activate flux and drive off volatiles. – Soak: 60–90 seconds at 150–180°C to ensure uniform heating. – Reflow: Peak temperature 235–250°C with time above liquidus (183°C for Sn63/Pb37) of 45–75 seconds. – Cooling: 2–4°C/s ramp down to solidify joints.

Use a thermal profiler to verify temperatures at multiple points on the board. Adjust for heavy copper layers or large thermal masses. A slow preheat helps avoid void formation.

Inspection and Quality Control

After reflow, inspect joints for defects such as bridges, voids, insufficient solder, or cold joints. Use:

– Visual inspection: Microscope for fillet quality per IPC-A-610. – Automated optical inspection (AOI): For high-volume production. – X-ray inspection: For hidden joints (e.g., under BGAs) to detect voids and solder balls. – Electrical testing: Continuity and functional tests.

Document findings to track process improvements.

Common Defects and Troubleshooting

| Defect | Cause | Solution | |——–|——-|———-| | Solder bridging | Excess paste, misalignment | Reduce paste volume, improve alignment, adjust reflow profile | | Solder balling | Moisture in paste, dirty board | Use fresh paste, clean PCB before printing | | Insufficient solder in hole | Too little paste, large hole | Increase aperture size, use thicker stencil, reduce hole diameter | | Tombstoning | Uneven heating, misalignment | Optimize profile, ensure symmetric pad design | | Voids | Flux outgassing, fast preheat | Slow preheat rate, use paste with lower voiding characteristics |

Regularly maintain equipment, check stencil tension, and monitor paste condition to minimize defects.

Conclusion

Pin-in-paste soldering is a powerful technique for mixed-technology PCBs, offering faster production, lower thermal stress, and compatibility with automated lines. Success depends on rigorous design rules, careful material selection, precise printing and placement, and well-controlled reflow profiles. By understanding and controlling each step, manufacturers can achieve reliable, defect-free joints that meet IPC Class 2 or Class 3 requirements.

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About LT CIRCUIT

At LT CIRCUIT, we specialize in manufacturing high-precision PCBs, including HDI, multilayer, rigid-flex, ceramic, and heavy copper boards. Our in-house processes—stack-up lamination, laser drilling, and direct customer communication—ensure consistent quality beyond IPC-3 standards. We maintain stock of Rogers, high TG FR4, and other high-frequency materials to accelerate production. With experience serving major companies like Firstronic, Virtex, SIGNIFY, and Osram, we understand the demands of high-reliability assembly. Whether you need prototypes or pilot volume production, our factory can deliver fast, even with 12-hour turnaround. Let us help you achieve robust pin-in-paste soldering results. Contact us today to discuss your next project.