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Next‑Gen Mixed‑Assembly Soldering for Complex PCBs

Wed, 16 Jul 2025 01:08:55 GMT PCBASAIL

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In traditional electronics‐assembly processes, wave soldering has been the go‑to method for soldering printed circuit boards (PCBs) that carry plated‑through hole (PTH) components. But wave soldering isn’t without its flaws: you can’t use it on boards that have high‑density, fine‑pitch surface‑mount devices (SMDs); it’s prone to solder bridging and misses; it requires flux spraying; and the board itself takes a hefty thermal shock, often warping or distorting. Nowadays, circuit assemblies are getting denser and denser, and you inevitably end up with fine‑pitch SMDs right next to your PTH parts. Wave soldering just can’t keep up. The usual workaround is to reflow only the SMDs first, then manually touch‑up the remaining PTH joints—but that leads to inconsistent joint quality.

To tackle these challenges, several hybrid soldering techniques have emerged: selective soldering, through‑hole reflow (THR), and masked wave soldering, just to name a few. These methods shield the delicate SMDs while you solder the PTH parts, slashing both the number of production steps and the total cycle time. What follows is a rundown of these hybrid processes.

1. Overview of Hybrid Soldering Technologies

1.1 Selective Soldering

You can really appreciate selective soldering if you compare it side‑by‑side with wave soldering. The biggest difference is this: with wave soldering, you dunk the entire underside of the PCB into molten solder, whereas in selective soldering, only specific zones ever touch the solder wave. Since a PCB is such a lousy conductor, the nearby components and pads don’t get enough heat to reflow unintentionally.

Just like wave soldering, you still need to pre‑apply flux. But instead of spraying the whole board, you only coat the individual areas you plan to solder. Note that selective soldering isn’t made for SMDs—it’s purely for PTH joints.

There are two main flavors of selective soldering:

Drag‑Solder Process
Dip‑Solder Process

(1) Drag‑Solder Process

In drag soldering, a single small nozzle creates a mini solder wave (see Figure 1). Drag soldering is great when you have just a handful of joints or a tight cluster of pins to solder. The PCB is moved past the solder nozzle at specific speeds and angles to nail each joint. To keep things stable, that nozzle’s diameter is under 6 mm. Once you set the solder flow direction, you can orient and optimize the nozzle however you need. A robot arm can approach the solder wave at angles anywhere between 0° and 12°, but most folks find a 10° tilt gives the best results.

Because both the solder wave and board are moving in drag soldering, heat transfer is more efficient than in dip soldering. But since each joint depends on that tiny solder wave, you need higher solder temperatures—typically around 275–300 °C—and a drag speed of about 10–25 mm/s. People also flood the solder zone with nitrogen to prevent oxidation, which in turn reduces bridging defects and boosts process stability and reliability.

These machines are built for precision and flexibility. The modular design means you can customize them to your unique production needs—and upgrade later as volumes grow. One robot arm can handle flux spraying, pre‑heat, and soldering, all in one setup, thanks to a synchronized process flow that shrinks cycle times. The arm’s precise positioning (±0.05 mm) ensures each board is treated exactly the same way. And because it can move in five axes (X, Y, Z, plus U and θ), you can hit joints from any angle for top‑notch quality. There’s even a titanium probe that regularly measures the solder wave height—and feeds back to the pump speed—to keep everything rock‑solid.

But drag soldering has its downside: it’s time‑intensive. Between fluxing, pre‑heating, and soldering, each joint is handled one at a time. More joints means more time—so drag soldering can’t match wave soldering’s sheer throughput. That’s changing, though, with multi‑nozzle heads. A dual‑nozzle drag setup can double your output, and you can even add a second flux nozzle to match.

(2) Dip‑Solder Process

Dip soldering uses multiple solder nozzles—one for each PTH joint—so it trades off some of drag soldering’s flexibility for higher throughput that rivals wave soldering. It’s also less expensive to set up. Depending on board size, you can do single or multiple boards in parallel, with each board getting fluxed, pre‑heated, and soldered all at once.

Because each board’s joint layout is unique, you need a custom nozzle array for each board. These nozzles are as large as possible to ensure process stability without disturbing nearby parts. That design work is crucial—and tricky—because a stable process often hinges on having just the right nozzle dimensions.

Dip soldering can handle joint sizes from 0.7 to 10 mm, including short leads and small pads, with low bridging risk—just make sure there's at least 5 mm clearance between nozzles, components, and pad edges. Typical process settings:

Solder temp: 275–300 °C
Dip speed: 20–25 mm/s
Dwell time: 1–3 s
Lift‑off speed: 2 mm/s
Pump rate: scaled to number of nozzles

1.2 Through‑Hole Reflow (THR)

In simple terms, Through‑Hole Reflow (THR) uses reflow soldering to assemble both PTH and odd‑shaped components in one go. As miniaturization, added features, and higher component density become critical, most single‑ and double‑sided boards are SMD‑heavy. Yet PTH parts—especially edge connectors—still have the edge in strength, reliability, and compatibility.

Traditionally, you’d fit SMDs via stencil and reflow, then wave solder or hand‑solder the PTH parts. But with THR, you stencil both SMD and PTH pads in one shot, place all parts, then reflow everything together . For boards with mostly SMDs and only a few PTH parts—think motherboards with connectors, discrete semiconductors, switches, jacks—you can skip wave soldering altogether.

1.2.1 Solder‑Paste Application for THR

THR typically needs about 30× more solder paste for each PTH joint versus an SMD pad. There are two main ways to deposit that extra paste:

Stencil Printing
Use a stepped stencil: thicker areas line up with the PTH pads, giving you the volume you need without over‑applying on the SMD pads.
Automated Dispensing
A jetting system can deposit precise volumes right into each PTH hole. Use a nozzle slightly larger than the hole diameter so paste hugs the barrel and even squeezes out the bottom for good coverage. If the nozzle’s too small, paste squirts back out and you lose material.

By using THR, you not only protect your SMDs, you also dramatically improve through‑hole joint quality—often making up for the higher equipment cost.

1.2.2 Component Placement for THR

High‑profile or oddly shaped PTH parts—like tall connectors or heavy switches—demand special handling. Your pick‑and‑place needs:

Custom nozzles with strong vacuum grip
Adjustable fixtures for odd shapes
Support and clamping systems for the board
High placement force capability
Advanced vision for top‑side alignment

Modern machines—like Advantis AX72 or Polaris—handle taped, tube, and waffle‑pack PTH parts right on the same head they use for SMDs. They’re fast, accurate, and reliable. Manual insertion is a backup for big pin‑count connectors—low cost but slower and less precise.

1.2.3 THR Reflow Profile

Reflow profiles for THR must deliver enough heat to every pin—all the way up those tall, heavy connectors. IR reflow ovens are the norm, since hot‑air convection can cause temperature swings. Split‑zone top/bottom heating helps manage the delta T across the board. For a heavy 25‑pin D‑SUB, you might raise the bottom heater temp and lower the top to get the board body hot enough without overheating the connector shell. You also stretch the time above liquidus so flux has time to boil off from the hole. Cross‑section analysis is key to validate your profile. Always monitor peak temperatures, ramp rates, time above liquidus, and cooling rate to avoid voids, thermal shock, and excessive gradients.

1.3 Masked Wave Soldering

Since wave soldering can’t handle fine‑pitch SMDs, masked (or shielded) wave soldering steps in: you slip a custom mask—or “shadow” fixture—over the board, covering the SMDs so only the PTH leads are exposed.

1.3.1 Advantages of Masked Wave

Double‑sided mixed assemblies: solder both sides’ PTH parts in one pass, cutting out manual touch‑up.
Less flux prep: you don’t need to mask‑coat the entire board, saving time and cost.
Throughput: matches traditional wave speeds.

1.3.2 Mask Materials

Must be ESD‑safe: aluminum, synthetic stone (domestic or imported), or fiberboard. Avoid black stone—it can foul the wave sensors.

Thickness: pick 5–8 mm based on the tallest backside component.

1.3.3 Fixture Design Guidelines

Overall size: fixture length and width = PCB length/width + 60 mm for the carrier frame. Max width is 350 mm. If your PCB is under 140 mm wide, you can fit two boards side‑by‑side.
Keep 8 mm clearance from edges; add 10 × 10 mm bakelite strips on the other sides to stiffen the fixture.
Reinforce rails with screws spaced under 150 mm.
Hold‑down clips: mount around the perimeter, spaced under 100 mm. Ensure they don’t rub components or block DIP leads, but firmly lock the PCB.
Corner chamfers: R5 radius on all four corners.
Anti‑lift for parts: some components get lifted by the wave. Countermeasures include iron‑block clamps, clip‑on clamps, or dedicated anti‑lift jigs.

2. Conclusion

As PCB assemblies get more complex, wave soldering alone can’t cut it—especially when you have fine‑pitch, high‑density SMDs on your solder side. Manual PTH soldering breeds inconsistent quality. The answer is hybrid soldering: selective soldering, THR, or masked wave. Which you choose depends on your product mix and volumes:

Low‑volume, many variants: drag‑solder selective soldering—no custom fixtures, but higher capital investment.
High‑volume, single variant: masked wave soldering—fixtures pay off, and throughput equals traditional waves.

Both are easy to control and are already widespread. THR is a bit tougher to nail down but offers the best joint quality, process simplification, and future growth potential.

With assembly densities set to keep climbing, these hybrid technologies will only matter more—and give PCB designers fresh process options.