Humidity, Condensation and Moisture Protection of Assemblies

Moisture reaches an assembly in two ways: as vapour that condenses on a cold surface, and as liquid that enters through a vent or a seal. Both produce the same result, and the design decides which one happens.

How Condensation Forms

Air holds a limited amount of water vapour, and the limit falls as the temperature falls. When a surface is colder than the dew point of the air around it, the excess condenses on that surface.

In an enclosure the surface that cools fastest is usually the metal housing or a metal part of the assembly, which is exactly where the sensitive circuitry tends to be. Our migration notes describe what the moisture then does to the board.

Breathing and Pressure Equalisation

A sealed enclosure is not constant. The air inside expands and contracts with the temperature, and the pressure difference drives air in and out through the seal, a gland or a cable entry.

Each exchange brings moisture in. A vent that allows the pressure to equalise through a membrane limits the exchange to vapour and keeps liquid water out. Our encapsulation notes describe the alternative approach of protecting the assembly itself.

Where the Water Enters

The paths are the seal, the cable entry, the connector, the vent and the material of the enclosure itself. A polymer housing that is thin enough will transmit moisture vapour over time.

Each path should be identified and treated, because sealing four of five leaves the enclosure as wet as sealing none of them.

Design Measures at the Assembly

Where moisture will be present, the assembly should tolerate it. That means a coating on the surfaces that are at risk, a spacing that is wide enough for the voltage, and a layout that avoids trapping water.

A coating must cover the whole surface, including under components and at the edges, or the moisture finds the gap. Our coating notes describe the preparation that this requires.

Layout for Water

Water collects in the lowest point and it follows gravity along a surface. A board mounted vertically drains better than one mounted flat, and a board with a slot in the right place drains better than one without.

Where water can pool around a component, the pooling accelerates any corrosion and it changes the impedance of a high frequency structure. The orientation should be considered at the layout stage. Our high speed notes describe the impedance sensitivity.

Heating and the Dew Point

A product that generates heat while it is running keeps its own surfaces above the dew point, and the problem appears when it is switched off and cools.

Where the cycling is frequent, the assembly sees many condensation events. The test should reproduce that cycle rather than a steady humid condition. Our thermal cycling notes describe how the exposure is arranged.

Verification

The verification is a humidity test with a temperature cycle that crosses the dew point, with the assembly powered and monitored.

The result should be measured electrically during the exposure rather than only inspected afterwards, because the leakage that causes the failure is present at the time. Our quality notes describe how the result is recorded.

Process Control and Verification

The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.

Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting.

Verification and Records

Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record. Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected.

The first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected.

FAQ

Does a sealed enclosure solve the problem? It reduces the exchange and it does not eliminate it. The pressure difference still drives air through the seal, and the moisture that enters stays.

Is a desiccant pack inside the enclosure enough? It absorbs a finite amount of water and it becomes saturated. It delays the problem rather than removing it.

What does gopcb provide for moisture protection? We provide enclosure breathing and venting review, identification of every moisture path, coating coverage at edges and under components, layout that drains and avoids pooling, powered humidity and dew point cycling tests, and electrical monitoring during the exposure.