Latest / Hardware Startups with Fexingo: Devices, Robotics, and Manufacturing Tech Companies / How Hardware Startups Use Selective Soldering for Through-Hole Components
Transcript
- Lucas: If you're building a hardware product and your prototype board has a mix of tiny surface-mount chips and a few big through-hole connectors, you've hit the classic assembly dilemma. Luna: Right — reflow solder works great for the small parts, but it won't handle those big connectors. And wave soldering is overkill unless you're running thousands of boards. Lucas: Exactly. That's where selective soldering comes in. It's basically a programmable solder fountain that touches only the specific leads you want soldered, while leaving the rest of the board untouched. Luna: So you can reflow the surface-mount side first, then use selective soldering for the through-hole components in a separate step. Lucas: Yeah. And for a hardware startup, that's huge because it means you don't have to redesign your board to avoid through-hole parts. Some components — like high-current connectors, transformers, or large electrolytic capacitors — are actually better in through-hole format for reliability. Luna: I've seen medical device startups that stick with through-hole for certain connectors because the mechanical strength is higher. A usb c port might be fine surface-mount, but a high-density circular connector for a patient monitor? You want those pins soldered through the board. Lucas: Exactly. So the question becomes: how do you assemble those boards cost-effectively when you're doing maybe 100 or 500 units a month? Hand soldering is an option, but it's slow and inconsistent. A good hand solderer might take three to five minutes per connector, and that adds up fast. Luna: And if you're paying someone $25 an hour, even three minutes of labor per connector adds real cost. A board with two connectors might have $2.50 in labor just for soldering. Lucas: Right. But there's also the quality risk. Hand soldering depending on the operator's skill. You can get cold joints, bridges, or just inconsistent fillets. Selective soldering eliminates that variability because the machine controls the solder temperature, the contact time, and the amount of solder. Luna: So what's the price of entry for a selective soldering machine? I've heard numbers around thirty to sixty thousand dollars for a benchtop unit. Lucas: That's about right. A basic single-nozzle system from a company like Kurtz Ersa or SEHO runs about $40,000. Add a fluxer and preheater, and you're closer to $60,000. That's not cheap, but for a startup doing regular production runs, it can pay for itself in a year or two compared to hand soldering. Luna: And you can also use contract manufacturers that offer selective soldering as a service. That way you don't need to buy the machine — you just pay per board. Lucas: Yes, that's a smart intermediate step. Many CMs charge around $0.50 to $1.50 per joint for selective soldering, depending on complexity. Compare that to maybe $0.30 per joint for hand soldering — but hand soldering has higher defect rates, so the total cost including rework might be similar or higher. Luna: I want to get into how the machine actually works. It's not just a mini wave solder, right? Lucas: No, it's different. The board is held in a fixture, and the system moves a small nozzle — like a fountain — up to the bottom of the board at each through-hole location. A pump pushes molten solder up through the nozzle, and it contacts the lead and the pad for a precise amount of time, typically two to five seconds. Luna: And the nozzle is surrounded by a nitrogen shroud to prevent oxidation, I think. Lucas: Exactly. Nitrogen atmosphere reduces dross formation and improves wetting. That's important because you want a shiny, consistent joint. Without nitrogen, the solder can look dull and the joint might be weaker. Luna: What about flux? Do you need to apply flux before the board goes in? Lucas: Yes — selective soldering requires flux. Usually it's applied by a spray fluxer just before soldering. The flux cleans the surfaces and helps the solder flow. Some machines have a built-in fluxer that moves along with the nozzle. Luna: So you've got flux, preheat, solder fountain, nitrogen — sounds like a lot of variables to dial in. Lucas: It is. But once you've set the profile for a given board, it's repeatable. The machine can store hundreds of programs. That's the beauty — if you're building the same board every month, you just load the program and go. Luna: Let's talk about a concrete example. A listener of ours, Sarah, runs a startup making a portable ultrasound probe. They have a board with a 40-pin flex connector that needs to be through-hole for durability. They were hand soldering those connectors, but had a 10% defect rate. Lucas: Ouch. Ten percent rework is brutal, especially when that connector is expensive. Let's say the connector costs $8, and rework time is 10 minutes. That's maybe $5 in labor plus the risk of damaging the board. Luna: They switched to a selective soldering service. They send their boards to a CM that does the soldering for $1.20 per connector. Their defect rate dropped to under 1%. Lucas: And the cost? $1.20 versus maybe $0.30 for hand soldering, but with hand soldering they had 10% scrap. So the effective cost per good board was higher. Once they accounted for yield, selective soldering was actually cheaper. Luna: That's the kind of math that convinces founders. It's not just about the direct labor cost — it's about yield and reliability. Lucas: And selective soldering also gives you process documentation. You can prove that every joint was made at the right temperature for the right time. That matters for medical devices and other regulated industries. Luna: True. If you're ever audited by the FDA, you want to show that your soldering process is validated. Hand soldering is hard to validate consistently. Lucas: So when should a startup consider buying their own selective soldering machine? I'd say the tipping point is around 500 to 1,000 boards per month, depending on the number of through-hole joints per board. If you're doing 5,000 joints per month, the machine pays for itself in about 18 months. Luna: And if you're below that, use a CM. That's what we always say — don't buy equipment until you've proven the product-market fit. Lucas: Right. But there's another option: some equipment vendors offer lease programs. You can get a benchtop selective soldering machine for maybe $1,000 a month. That could make sense if you want to keep the process in-house for IP reasons. Luna: What are the common mistakes startups make with selective soldering? Lucas: The biggest one is not designing the board for selective soldering. The through-hole components need to be spaced so the nozzle can reach them. If you put a connector too close to a tall component, the nozzle might not fit. Luna: So you need to think about tooling clearance during PCB layout. Lucas: Exactly. The nozzle diameter is typically 4 to 8 millimeters, and it needs room to approach from the bottom. Also, you should avoid placing through-hole components on both sides of the board — that complicates fixturing. Luna: Another mistake: not using enough flux. I've seen boards with insufficient flux leading to incomplete fill. Lucas: Yes, flux is critical. And the flux type matters — some are water-soluble, some are no-clean. For a startup, no-clean flux is simpler because you don't need a washing step. But you have to make sure the flux residue doesn't interfere with conformal coating or electrical testing. Luna: Let's touch on thermal stress. Selective soldering concentrates heat on the pad and lead. Can that damage adjacent components? Lucas: It can. If you have a temperature-sensitive component like a crystal oscillator or a plastic connector body close to the soldering point, the heat can cause damage. Preheating the board from below — usually to around 100 to 130 degrees Celsius — helps reduce thermal shock. Luna: So the preheat profile is just as important as the soldering profile. Lucas: Absolutely. And that's something a good CM will dial in for you. If you're doing it in-house, you need a thermocouple board to measure actual temperatures during setup. Luna: This show is all about giving you practical advice that saves you time and money. If that's been useful to you, it's worth knowing that we keep this podcast ad-free thanks to listener support. You can help by buying us a coffee — literally — at buy me a coffee dot com slash fexingo. Lucas: Yeah, that's right. A small contribution goes a long way in keeping these conversations running. And we really appreciate everyone who's chipped in. Luna: So back to selective soldering — one more thing I think is important: don't overlook the fixture. The board needs to be held securely and accurately. A good fixture can cost a few hundred dollars, but it's essential for repeatability. Lucas: And if you're working with a CM, they'll usually design the fixture for you. That's part of the setup cost. But if you're buying your own machine, factor in fixture costs — maybe $200 to $500 per board type. Luna: To wrap up: selective soldering is one of those techniques that bridges the gap between prototype and production. It's not as fast as wave soldering, but it's far more flexible and gentler on the board. Lucas: And for a startup that's scaling from hundreds to thousands of units, it can be a game-changer. You get the reliability of a machine process without the upfront cost of a full wave line. Worth exploring if you're dealing with any through-hole components. Luna: Next time, we'll look at something a bit different — how to choose between potting and conformal coating for protecting electronics in harsh environments. That's an episode I'm looking forward to. Lucas: Same here. Until then, keep building.