Damp heat testing at 85/85: what it is, how it is used, and what to measure

Moisture is one of the slowest ways a product fails and one of the hardest to predict. A circuit board that works on the bench can lose insulation resistance after weeks in a humid enclosure. A connector can corrode, a coating can lift, a laminate can swell. A damp heat test finds those failures in the lab by holding the product at high temperature and high humidity long enough for moisture to get in.

The most widely used version in electronics is 85/85: 85 °C and 85 %RH, held for a long time. This article covers what the test does, why it is run the way it is, one piece of chamber behaviour that catches people out, and what has to be measured for the result to mean anything.

What the test does

Warm, humid air carries a lot of water, and given time that water finds its way into a product: absorbed into plastics and laminates, adsorbed onto surfaces, drawn into gaps and seals. A damp heat test accelerates that. The specimen is exposed to the condition for a set duration, often days to weeks, and then checked against its acceptance criteria. Typical checks are functional tests, insulation resistance, visual inspection for corrosion or delamination, and whatever performance measurements the product specification calls for.

Measurements are usually taken at a standard reference atmosphere before and after exposure, so that the only difference between the two is what the damp heat did. The ProcessView+ demonstration uses 23 °C / 50 %RH as its reference atmosphere.

Steady state, accelerated and cyclic

The IEC 60068-2 series, which many product and component standards call up, defines several damp heat tests. They are easy to confuse:

• Steady-state damp heat (IEC 60068-2-78, Test Cab) holds one condition, and 40 °C / 93 %RH is a common one. Its purpose is moisture absorption and diffusion, and it is run so that condensation does not form on the specimen.
• Accelerated steady-state damp heat (IEC 60068-2-67, Test Cy) applies the same idea at a much harsher condition, 85 °C / 85 %RH, to get the moisture in faster. It is aimed primarily at small components that are not hermetically sealed. 85/85 is also widely called up in electronics reliability and photovoltaic module qualification.
• Cyclic damp heat (IEC 60068-2-30, Test Db) cycles the temperature while keeping the humidity high, so the specimen “breathes” and water condenses on it. It is a different test for a different failure, and our article on cyclic damp heat and temperature/humidity cycling covers it.

Damp heat testing turns up wherever moisture can end a product’s life: electronics and printed circuit assemblies, automotive components, aerospace equipment, telecommunications hardware and medical devices. The test is usually specified by the customer or the product standard, and the lab’s job is to prove that the condition was applied as specified.

Why the chamber heats dry, then wets up

For a steady-state test, the order of the ramps matters as much as the condition at the end.

The quantity that decides whether water condenses is the dew point: the temperature at which the air would be saturated. Any surface colder than the dew point collects water. At 85 °C / 85 %RH the dew point is about 80.9 °C, so any part of a specimen more than about 4 °C below the air temperature will be wet. (At 40 °C / 93 %RH the margin is tighter still: the dew point is about 38.6 °C, about 1.4 °C below the air.)

A specimen always lags the air, so ramping temperature and humidity together pushes very moist air over a specimen that is still cold, and it condenses. A steady-state test that was meant to avoid condensation has produced it.

Test laboratories avoid this by heating dry first. The temperature rises with the amount of water in the air held constant, and only once the specimen has reached temperature is humidity added. The ProcessView+ 85/85 Steady State profile does exactly that, with the humidity system switched off for the heat-dry legs:

Step	What happens	Condition
Reference	Hold the standard reference atmosphere	23 °C / 50 %RH
Heat dry	Temperature rises in four legs; the water in the air stays the same	to 85 °C, RH falls to about 2.4 %
Wet up	Humidity is added at constant temperature	to 85 °C / 85 %RH
Damp heat	Hold the exposure condition	85 °C / 85 %RH

The drop in relative humidity during the heat-dry legs looks like a fault and is not one. The air holds the same water, and warmer air can hold more, so its relative humidity falls: 19 %RH at 40 °C, 8.9 % at 55 °C, 4.5 % at 70 °C and 2.4 % at 85 °C. The arithmetic shows it is the same water all the way up. The dew point at every one of those points is about 12 °C, the dew point of the reference atmosphere.

What to measure

A damp heat result is only as good as the evidence that the specimen saw the specified condition for the specified time. These are the figures that provide that evidence, and how ProcessView+ reports each.

Was the combined condition held? The test specifies a pair, for example 85 °C / 85 %RH with a tolerance on each. Two per-loop in-band percentages cannot show that both were in tolerance at the same time. Setpoint-pair dwell measures it directly: for each condition the profile commands, the joint window (the share of the commanded soak with both loops in band at once), the longest unbroken hold, and the total held. The reference atmosphere and the damp heat condition are reported as separate rows of the condition table on the QA report cover. Our article on reading temperature and humidity together covers the joint window in detail.

Where would water condense? The dew point of each held condition is printed in the condition table, calculated from the dry bulb and relative humidity using the IAPWS-IF97 formulation, which the report cites. Beside it is the condensation margin: the coldest load thermocouple reading during the soak minus the highest dew point during the soak, a deliberate worst case. A positive margin means the coldest measured surface stayed dry; at 85/85 there are only about 4 °C to play with. The table also gives the load ΔT, the spread between the warmest and coldest load thermocouple, because a load that is unevenly heated has a cold corner. The margin is shown without a pass or fail judgement and does not affect the verdict.

How much water was in the air? The humidity ratio, in grams of water per kilogram of dry air, is reported once for the run over its soak steps, as a mean and a maximum. It is calculated at standard atmospheric pressure, and the report says so, because the product does not record barometric pressure. It is about 587 g/kg at 85 °C / 85 %RH, against 8.7 g/kg at the reference atmosphere. Relative humidity hides how different those two conditions are; the humidity ratio shows it.

Can you watch it live? The Humidity Diagnostics window plots the joint window, the condensation margin and the humidity ratio while the run is in progress. Its margin indicator turns amber when the coldest surface comes within 3 °C of the dew point and red when it reaches it. That is a live aid for the operator: it judges nothing for the record, raises no excursion and changes no verdict. ProcessView+ does not calculate wet-bulb temperature.

How long was each channel in tolerance, step by step? Time in tolerance is reported per step against the bands captured when the run started, with the time in warn and in alarm, the number of excursions and the longest one. In the demonstration profile, each held condition has its own bands. The humidity band is wider at 85 %RH than at the reference atmosphere, because 85 %RH is near the top of what a chamber can hold and control there is looser. The ramps are monitored but not judged, because the chamber is being moved rather than asked to hold. When steps carry different bands, there is no single whole-run figure; the per-step figures are the answer.

Was the chamber being watched the whole time? Damp heat tests run for days, and over days a communication interruption is likely. Each logged reading is marked as observed or repeated during an outage, and the sample coverage is stated beside the verdict. An outage longer than a short threshold raises the verdict to REVIEW, because a period nobody observed cannot be claimed as clean.

What was the verdict, and why? PASS, REVIEW or FAIL is decided by each step’s criticality and the worst excursion on it, and the reason is printed under the verdict. The joint window, dew point and condensation margin are measurements for the reviewer; they are not part of the verdict.

Mean kinetic temperature is a pharmaceutical storage figure, not something a damp heat specification asks for, and the damp heat demonstration’s QA report leaves it out.

Real tests meet real disturbances

A 1000-hour exposure will see something go wrong, and a lost steam supply is a common one. With no steam, the humidifier stops while the dehumidifying coil keeps running, so the humidity falls while the air temperature holds steady. The temperature chart looks perfect; the condition was lost.

The demonstration shows this. During the hold, the chamber’s steam supply is interrupted twice. Each time, humidity falls about 1.7 %RH a minute, past its warn line but not its alarm line, and recovers when the steam returns. The QA report comes out REVIEW, not PASS. The excursion table lists the humidity warn excursions, the 85/85 row’s joint window is well short of 100 %, and the reviewer notes say what caused them. That is what a good record of a disturbed run looks like: the disturbance found, measured and explained, not hidden in a passing average.

Try it without a chamber

The ProcessView+ free trial includes the 85/85 Steady State demonstration on a built-in controller simulator: the reference atmosphere, the four heat-dry legs and the wet-up, a hold at 85 °C / 85 %RH with two steam interruptions, five load thermocouples on a simulated load of circuit boards, and a judged run ending in a QA report. A run takes about 77 minutes, or about 19 minutes with the demo clock at 4×.

It is a demonstration, not a test. The standard’s severity is 1000 hours; the demonstration holds the condition for about 22 minutes. A real exposure also ends with a recovery period at the reference atmosphere, which the demonstration leaves out. Demonstration runs are simulated and are not test results, and none of them qualifies a product.