Latest / Hardware Startups with Fexingo: Devices, Robotics, and Manufacturing Tech Companies / How Hardware Startups Use Selective Soldering for Mixed Tech
Transcript
- Lucas: You look at a modern printed circuit board and it's almost entirely surface-mount components — tiny resistors, microcontrollers, all placed by a pick and place machine and reflowed in an oven. Luna: Right, that's the standard for anything high-volume. But what happens when you need a big connector or a high-power relay that only comes in a through-hole package? Lucas: Exactly the problem. And a lot of hardware startups hit this wall. They design a board that's 95 percent surface-mount, but then they need one or two through-hole parts — a terminal block, a large electrolytic capacitor, a power inductor — and suddenly the whole assembly process gets complicated. Luna: Because you can't just run that board through a wave solder machine without damaging all the delicate SMT parts already reflowed on the other side. Lucas: Right. Wave soldering submerges the entire underside of the board in molten solder. If you've already placed surface-mount components, the wave will wash them off or cause thermal shock. So what do you do? Hand solder those few through-hole joints? Luna: And hand soldering is fine for one-off prototypes, but if you're building 500 or a thousand units, it's slow and inconsistent. Every joint looks a little different. Lucas: That's where selective soldering comes in. It's essentially a programmable solder fountain — a narrow nozzle that moves precisely to each through-hole pin, applies a small wave of molten solder only to that specific spot, then moves on. The rest of the board never touches solder. Luna: So it's like wave soldering, but with a scalpel instead of a fire hose. Lucas: Exactly. And the machines have gotten affordable enough that a hardware startup can actually buy one for their own lab. The entry-level benchtop models from companies like SEHO or Pillarhouse start around twelve to fifteen thousand dollars. That's not nothing, but compare it to the cost of hand-soldering a thousand boards at, say, three minutes of labor per board. Luna: At fifty dollars an hour for a skilled assembler, that's two fifty per board just in labor. On a thousand boards, you've spent two hundred and fifty thousand dollars. The machine pays for itself. Lucas: And that's just labor. You also get consistency — every joint has the same amount of solder, the same thermal profile, the same mechanical strength. Luna: Let's talk about a real case. I was talking to a startup last month that makes industrial vibration sensors. Their board has a 24-volt power connector — big, through-hole, needs to handle some current — and then the rest is all tiny SMT components for the MEMS accelerometer and the microcontroller. Lucas: Classic mixed-technology board. What were they doing before? Luna: Hand soldering the connector. And they were seeing intermittent failures — cold joints, cracked solder after thermal cycling. They were reworking about 8 percent of boards after final test. Lucas: Ouch. That's a huge quality hit. And when you're selling to industrial customers who might put your sensor on a conveyor belt in a factory, a field failure is catastrophic. Luna: Exactly. So they bought a benchtop selective soldering machine. They spent about fourteen thousand dollars. They also had to buy a custom nozzle for that specific connector — another couple hundred. But after the first batch of 200 boards, their rework rate dropped to under 1 percent. Lucas: And that doesn't just save money on rework. It saves time, it saves reputation. They can now ship boards with confidence. Luna: One thing I found interesting — they had to tune the thermal profile pretty carefully. The connector's ground plane was sinking heat fast, so they needed a higher preheat temperature and a longer dwell time on that pin than the machine's default recipe. Lucas: Yeah, the thermal profile is everything in selective soldering. If your preheat is too low, the solder won't wet properly and you get a cold joint. Too high and you risk damaging adjacent surface-mount components or causing tombstoning — where a tiny resistor lifts off one pad because the solder on that side melts faster. Luna: And since you're only heating one area at a time, the rest of the board stays relatively cool. That's actually an advantage over wave soldering, where the whole board gets dunked. Lucas: Right. With selective soldering, you can keep the heat localized. The machine typically has a nitrogen atmosphere around the nozzle to prevent oxidation, which gives you shinier, stronger joints. Luna: So when does selective soldering not make sense? I mean, if you only have one through-hole component and you're building fifty boards, is it worth buying a machine? Lucas: Probably not. At that volume, hand soldering or outsourcing to a contract manufacturer that offers selective soldering as a service is smarter. Many CM's now have selective soldering lines and will take runs as small as a hundred boards. You just send them your board, they program the nozzle path, and you get back consistent joints. Luna: And if your through-hole count is really high — say, more than twenty or thirty pins per board — wave soldering might still be cheaper per joint, provided you can protect the SMT side with a selective solder mask or a temporary coating. Lucas: That's an important trade-off. The sweet spot for selective soldering is usually boards with between two and twenty through-hole pins, where the rest is all surface mount. That covers a huge range of hardware startups — IoT devices with a power jack and a connector, sensor nodes with a terminal block, motor controllers with high-current screw terminals. Luna: And the machine programming is getting easier. Modern selective soldering machines let you import the board's CAD file, select the pins you want to solder, and the machine generates the path automatically. You can even simulate the thermal profile before running a board. Lucas: That's a huge leap from five years ago, when you had to manually teach each point with a joystick. So the barrier to entry is lower than ever. Luna: It's one of those tools that, once you know about it, you start seeing opportunities everywhere. I've talked to startups doing LED lighting arrays, medical devices, even a small drone manufacturer using selective soldering for their battery connectors. Lucas: And it's not just startups. Even established manufacturers use selective soldering for low-volume, high-mix production. But for a hardware startup, it can be the difference between a reliable product and a field failure disaster. Luna: Speaking of keeping the show running reliably — if you're getting value from these episodes, a couple of dollars a month at buy me a coffee dot com slash fexingo genuinely keeps us ad-free and focused on exactly this kind of practical manufacturing detail. Lucas: Yeah, it's a small thing that makes a big difference for us. And we appreciate every single supporter. Luna: So back to selective soldering — one more nuance. Some boards have components on both sides, and you might need to solder a through-hole part on the top side after reflowing bottom-side SMT parts. Selective soldering can handle that because the nozzle can reach into tight spots. Lucas: Right, and if you need to solder a connector that's near a large heat-sensitive component like a battery holder or a crystal oscillator, you can program the nozzle to move quickly past that area and only dwell on the pins. Luna: There's also a variant called laser soldering, which uses a focused laser beam instead of a solder wave. That's even more precise but also more expensive — machines start around fifty thousand dollars. Lucas: For most hardware startups, selective soldering hits the sweet spot of cost and capability. And if you're not ready to buy, contract manufacturers are increasingly offering it as a standard service. I know a CM in the Midwest that runs selective soldering for batches as small as fifty boards at a time. Luna: So the takeaway for any hardware startup designing a mixed-technology board: don't default to hand soldering just because you have a few through-hole parts. Evaluate selective soldering, either in-house or through a CM. It'll save you time, improve reliability, and scale with your volume. Lucas: And if you're in the design phase right now, consider whether you can even avoid through-hole parts altogether. Some connectors come in surface-mount versions now. But if you can't avoid them, selective soldering is a proven solution that's more accessible than you might think. Luna: Good advice. Next episode we'll talk about something every hardware startup eventually faces: how to choose between potting and conformal coating for protecting boards in wet environments. Lucas: Looking forward to that one. Until then, keep building.