Technical Library

HOME
Technical Library
Building for Aerospace Means Building Twice: Once on the Line, Once in the Field

Building for Aerospace Means Building Twice: Once on the Line, Once in the Field

Intro

The first time an aerospace electronics assembly gets built, it’s on your shop floor. The second time, it gets built in service: vibrating at altitude, cycling through temperature extremes, sitting in a satellite payload for years, or powering an avionics box across a thousand flight hours. That second build is where defects you didn’t catch the first time finally announce themselves.

This is the part of aerospace electronics manufacturing that doesn’t show up in a yield report. The board passed test. The functional check was clean. The shipment went out on time. And then, three months later, the customer calls about a unit that failed in the field, and the question becomes: what was the condition of the floor when this was built?

For most contract manufacturers and EMS providers serving aerospace, the honest answer is that some part of the year, the floor was drier than it should have been, and some part of the year it was more humid than it should have been. Neither swing produced a problem anyone could see at the time. Both produced problems that arrived later.

Why Aerospace Customers Feel Humidity Differently

A consumer electronics defect costs a return shipping label. An automotive electronics defect costs a recall. An aerospace electronics defect can cost a program. The component is the same. The cost of getting it wrong is not.

That asymmetry changes how aerospace customers evaluate suppliers. They’re not just buying boards. They’re buying consistency, traceability, and the confidence that the unit that ships in October was built under the same controlled conditions as the one that shipped in March. A shop whose environment quietly drifts twenty or thirty points of relative humidity between seasons isn’t building the same way twice. The boards may look identical. The conditions weren’t.

For context, the wider electronics industry has converged on a fairly well-known operating window: most published guidance places the comfortable zone for electronics manufacturing between roughly 40 and 60 percent relative humidity. Standards like ANSI/ESD S20.20 (for electrostatic discharge control programs) and IPC/JEDEC J-STD-033 (for handling moisture-sensitive surface-mount devices) shape how shops think about the upper and lower ends of that range. We’re not making a claim about compliance here. The point is simpler: an industry full of careful engineers spent decades figuring out that humidity matters, and the range they landed on is narrower than what most production floors naturally hold.

What Goes Wrong On The Dry End

When relative humidity falls, the air becomes a poor conductor. Static charges that would dissipate harmlessly at 45 percent RH start to accumulate at 15 or 20 percent. A worker walking across a floor, a board sliding out of a tray, a reel feeder cycling, all build small charges that, in dry conditions, don’t bleed off the way they should.

The damaging discharges are usually invisible. Modern integrated circuits can be wounded by static events well below the threshold a person can feel. The board passes test because the damage is partial: a weakened gate oxide, a stressed junction, a marginal interconnect that still functions today but won’t survive a few hundred thermal cycles in the field. The defect doesn’t fail manufacturing. It fails the customer.

This is the “walking wounded” problem, and it’s the version of yield loss that aerospace customers fear most, because it’s the one that escapes detection until the unit is already installed.

What Goes Wrong On The Humid End

The other end of the range has its own version of latent damage, this time chemical instead of electrical.

Most modern surface-mount components are plastic-encapsulated. The packaging is permeable to water vapor. When components sit on the floor in humid conditions, moisture diffuses into the package and collects at internal interfaces. The component still works. It still tests fine. The problem only emerges when the part hits the reflow oven, where temperatures climb past 400°F (200°C) and any trapped moisture flashes to vapor. The result, in the worst cases, is internal cracking and delamination, sometimes called “popcorn” failure for the audible pop the package makes as it ruptures.

Even when the damage isn’t dramatic, moisture in a package can compromise solder joint integrity in ways that don’t fail at final test but do fail under the vibration and thermal cycling that aerospace service exposes them to.

Moisture sensitivity levels and floor-life conventions exist precisely because the industry already knows this. The numbers that get cited, 168 hours of floor life for an MSL 3 part, 48 hours for an MSL 5, all referenced to 86°F (30°C) and 60 percent RH, are not arbitrary. They’re the industry’s collective acknowledgment that uncontrolled humidity has a clock attached to it.

The Case for Stable, Not Just Adequate, Humidity

The thing aerospace customers value most isn’t a particular humidity number. It’s a flat line. A shop that holds 45 percent RH year-round will outperform a shop that averages 45 percent while swinging between 25 and 65. The average looks identical on paper. The boards built at the extremes do not.

Most production floors in North America have a chronic problem in one direction: winter. When outside air is cold and dry, heating it for indoor use drops the relative humidity sharply, often into the teens. That’s the season when ESD events climb, when latent damage rates rise, and when aerospace customers tend to see field failures show up six to twelve months later.

The fix is to add moisture back. The question is how.

Why The Method of Humidification Matters

Conventional spray humidification produces water droplets large enough to wet surfaces before they evaporate. On a typical office floor, that’s a minor nuisance. On an electronics manufacturing floor, with exposed boards, open component reels, solder paste, and ESD-sensitive workstations, wet droplets are an active problem. They can leave residue on stencils and inspection optics, encourage corrosion at contact points, and introduce the kind of localized moisture excursions that show up later as solder defects.

Dry fog humidification works differently. The AKIMist®E uses compressed air and water to generate a fog with droplets small enough to evaporate before they reach surfaces. There’s no wetting of boards, components, or floor. The humidity setpoint is reached by adding water vapor to the air, not water to the room. For an electronics environment that needs to stay inside a narrow humidity window without introducing any new contamination risk, that distinction is the whole point.

The AKIMist®E is widely used in SMT lines, component storage areas, stencil printing rooms, and final assembly bays. We’re not claiming a role in composite layup, aircraft assembly, or any process outside the electronics manufacturing context. What it does, reliably, is help an electronics production floor hold its humidity flat through the seasons, so the boards built in February come off the line under the same conditions as the boards built in July.

The Takeaway

Aerospace electronics get built twice. The first build is the one you see, on your line, under your conditions, with your test coverage. The second build is the one your customer sees, in service, where the latent damage you didn’t catch the first time finally becomes a failure you have to explain.

The variables that drive that latent damage are well understood. Static at the dry end. Moisture at the humid end. Inconsistency across the year, which is worse than either extreme on its own. None of them are exotic. All of them respond to the same intervention: holding the humidity of your production floor inside a stable window, and using a method of humidification that doesn’t introduce new problems while solving the old one.

Aerospace customers don’t ask for much that other customers don’t also benefit from. They just ask for it twice, and they notice when they don’t get it.