Latest / Hardware Startups with Fexingo: Devices, Robotics, and Manufacturing Tech Companies / How Hardware Startups Use Selective Soldering for Mixed-Technology Boards
Transcript
- Lucas: If you've ever looked at a circuit board from a startup's first production run, you've probably seen a mix of tiny surface-mount components and a handful of bigger through-hole parts — connectors, relays, maybe a big electrolytic capacitor. Luna: Right. And the standard approach is either hand-solder those through-hole parts, which is slow and inconsistent, or run the whole board through a wave soldering machine, which can damage sensitive surface-mount parts. Lucas: Exactly. That tension is where selective soldering comes in. It's a process that uses a programmable solder nozzle to target only the through-hole joints on a board that's already been surface-mounted and reflowed. You get the reliability of machine soldering without subjecting the rest of the board to a wave of molten solder. Luna: I love that we're talking about this because for a lot of hardware founders I talk to, through-hole is almost an afterthought. They focus all their design energy on the surface-mount side. Lucas: And that's a mistake, because through-hole assembly — especially at low volumes — can be the bottleneck that kills your production schedule and eats your margin. Let's make this concrete. I was looking at a medical device startup that makes a wearable patient monitor. Their board has about 200 surface-mount components and eight through-hole parts — a couple of connectors, a power jack, an audio jack, and a battery holder. At their first production run of 500 units, they were having an assembler hand-solder those eight parts. It took about 12 minutes per board. Luna: Twelve minutes just for eight joints? That seems high. Lucas: It is, but it's realistic when you have to handle the board carefully, position it, solder, inspect, and touch up. At 500 units, that's 100 hours of labor. At, say, $50 an hour for a skilled assembler, that's $5,000 in direct labor — about $10 per board just for those eight joints. And that's not counting the rework rate. Hand-soldering consistently yields a few percent defects — cold joints, bridges, insufficient fillet. They were seeing about 4 percent rework. Luna: So they're paying someone to solder, then paying someone else to fix the soldering. That's a drag. Lucas: Exactly. So they looked at selective soldering. A benchtop selective soldering machine — something like a Pillarhouse or an ERSA — runs about $50,000 to $80,000 for a single-nozzle system. That's a big number for a startup. But they ran the numbers. At 500 units per run, four runs a year, that's 2,000 boards annually. They projected they'd save about $18,000 a year in labor and rework. Payback in under three years. Plus they got consistent joint quality and could scale to higher volumes without hiring more assemblers. Luna: And that's assuming they own the machine. What about contract manufacturers that offer selective soldering as a service? Lucas: That's actually the smarter move for most early-stage startups. Many mid-tier CMs have selective soldering lines and will quote a per-board price. For that medical device board, they got a quote of $2.50 per board for the selective soldering step — a quarter of the hand-soldering cost. And because it's machine-done, the defect rate dropped below 0.5 percent. So they never bought the machine. They just designed their board to be compatible with selective soldering and sent it to a CM who had the capability. Luna: That's the key, right? Designing for selective soldering. What does that look like in practice? Lucas: A few things. First, you need to keep all your through-hole components on the same side of the board — typically the top side, because the selective soldering nozzle comes from below. If you have parts on both sides, you need a more complex pallet or a second pass. Second, you need adequate clearance around each through-hole pad — usually at least 3 millimeters from the nearest surface-mount component — so the nozzle can get in without touching anything. Third, your through-hole holes should be standard sizes, nothing exotic. And fourth, you need to ensure your board can be held in a pallet that masks the surface-mount side from any solder splash. Most selective soldering systems use a titanium or aluminum pallet that clamps the board and has cutouts only for the through-hole joints. Luna: So in a way, you're designing the board around the soldering process, not the other way around. Lucas: Exactly. And that's a mindset shift for a lot of hardware founders. They want to optimize for electrical performance, component cost, or board size, but they forget that assembly cost — especially for through-hole — can dominate at low volumes. Let me give you another example. A robotics startup was building a motor controller board with ten through-hole terminal blocks for power and signal wiring. They had designed the board with those terminal blocks scattered around the edges. That made selective soldering difficult because the nozzle would have to reposition a lot, increasing cycle time. By regrouping all the terminal blocks into two clusters, they cut the selective soldering cycle time from 90 seconds per board to 35 seconds. At their volume of 10,000 boards a year, that saved them about $15,000 in machine time. Luna: That's a great example of a small layout change having a big cost impact. What about the kind of through-hole parts that are hardest to selective solder? Lucas: Anything with large thermal mass — like a heavy transformer or a thick connector with big pins. Those need more heat, which means a longer dwell time or a higher temperature, which can damage the board or the component. Also, components that sit very low to the board — like some tactile switches — can be hard to get the nozzle under. And oddly, components with very short leads, like some LED holders, because the solder joint is right at the board surface and the nozzle might not wet properly. In those cases, you sometimes need a custom nozzle design — a 'custom' that can cost a few hundred dollars per nozzle, but if you're running thousands of boards, it's worth it. Luna: So selective soldering isn't a silver bullet. It's a tool in the toolbox. But for startups with mixed-technology boards, it seems like a no-brainer to at least design for it. Lucas: Absolutely. And the best part is that the design rules for selective soldering are largely compatible with wave soldering too. So if you ever scale to volumes where wave soldering makes sense, your board is already ready. It's a low-risk, high-upside design decision. One more thing — flux. Selective soldering uses a flux that's typically no-clean, which means you don't have to wash the board afterward. That saves a process step and avoids potential contamination issues. But you do need to make sure the flux is compatible with any conformal coating you plan to apply later. Some no-clean fluxes leave a residue that can interfere with adhesion. Luna: Good point. So if you're doing selective soldering and then conformal coating, you need to coordinate materials upfront. That's another thing that can trip up a startup. Lucas: Right. And that's really the theme of this whole episode: the details matter. Through-hole soldering might seem like a commodity step, but it's often where the hidden costs live. For a hardware startup trying to get from prototype to production, understanding selective soldering — when to use it, how to design for it, and how to buy it as a service — can be the difference between a product that's profitable at 1,000 units and one that only works at 100,000. Luna: That's a great place to land this. Lucas, thanks for walking us through that.