Every test report, every chamber spec sheet, every standard document assumes you already know the vocabulary. This glossary defines 50 terms the way a senior test engineer would explain them to a new team member — precisely, with the nuance that matters in practice, and with a note wherever the term is routinely confused with something it isn't.
A
Acceleration factor (AF) — The ratio of test duration to equivalent field exposure time. Calculating a valid acceleration factor requires a physics-based model (Arrhenius for thermally-driven degradation, Coffin-Manson for fatigue). A number cited without the underlying model is not an acceleration factor — it's a guess.
Acceptance criteria — The defined pass/fail threshold for a test. Not specified by the chamber or the test method — specified by the engineer, the customer, or the standard governing the product. A chamber can run a perfect test against acceptance criteria that are meaningless.
Ambient conditions — The temperature and humidity of the room in which the chamber is operating. A chamber rated to -70°C at 23°C ambient may only reach -60°C in a summer lab at 35°C.
Arrhenius equation — A mathematical model relating chemical reaction rate to temperature. Used to calculate acceleration factors for thermally-activated failure mechanisms. Using the wrong activation energy produces an acceleration factor with no connection to reality.
B–C
Burn-in — A production screen in which powered product is held at elevated temperature for a defined period, with the goal of precipitating early-life failures. Burn-in is effective for thermally-activated failure mechanisms and poorly matched to mechanical failure modes.
Calibration — The process of comparing a chamber's sensor readings against a traceable reference standard and documenting the difference. Calibration establishes whether the chamber is reading accurately — it does not adjust the chamber. Adjustment after calibration is a separate step.
Coffin-Manson relationship — A fatigue model relating plastic strain amplitude per cycle to cycles to failure. The foundational model behind temperature cycling test acceleration factor calculations.
CTE (Coefficient of Thermal Expansion) — The fractional change in a material's dimension per degree of temperature change, in ppm/°C. The mismatch in CTE between joined materials is the mechanical driver of fatigue failure in solder joints under thermal cycling. Copper ~17 ppm/°C; FR-4 laminate ~14–18 ppm/°C in-plane; alumina ceramic ~6–7 ppm/°C; silicon ~2.6 ppm/°C.
D–G
Destruct limit — In HALT, the stress level at which damage to the product is permanent and non-recoverable. Distinguished from the operating limit, where malfunction occurs but function returns when stress is removed.
Dew point — The temperature at which air — at a given absolute moisture content — becomes saturated and water begins to condense. During transitions from cold to warm in a humid environment, surfaces below the dew point collect liquid water.
DUT (Device Under Test) — The product, component, or assembly being tested. Every test report should specify the DUT precisely: part number, revision, serial number, assembly state.
Dwell time — The time a product spends at a temperature extreme before the next ramp or transfer begins. Dwell must be sufficient for the product — not just the chamber air — to reach thermal equilibrium. Insufficient dwell makes the test less severe than intended.
Grms — Root mean square acceleration. The standard measure of vibration intensity. Real-world vibration environments for most products are 0.5–5 Grms. HALT chambers apply 20–60 Grms.
H–M
HALT — Highly Accelerated Life Testing. A reliability discovery methodology applying escalating combined stresses until product failure, to find design weaknesses before production. Not a standard. Not a compliance test. A methodology.
HASS — Highly Accelerated Stress Screening. A production screen using stresses derived from HALT operating limits to precipitate latent manufacturing defects in shipped product. Requires prior HALT data to establish valid screen levels.
Humidity (Relative Humidity, RH) — The ratio of the actual water vapour content of air to the maximum it could hold at that temperature. At 100% RH, air is saturated — further cooling causes condensation. Humidity specs always reference a temperature range.
IEC 60068 — The International Electrotechnical Commission's family of environmental testing standards for electrical and electronic equipment. The most widely referenced environmental testing framework globally.
IP rating — Ingress Protection classification defined in IEC 60529. Two digits: first indicates protection against solids (0–6), second against liquids (0–9K). IP67 means dust-tight and protected against temporary submersion at 1 metre for 30 minutes.
Latent defect — A manufacturing defect that does not cause immediate failure under standard inspection but reduces reliability and will eventually cause field failure under operational stress.
MIL-STD-810 — The US Department of Defense's environmental engineering standard. Structured around environments rather than test methods. Unique in requiring test conditions to be tailored to measured field environments.
MLCC (Multilayer Ceramic Capacitor) — The most common passive component in electronics and the component most sensitive to thermal shock. The ceramic dielectric has a CTE of 7–10 ppm/°C, significantly mismatched to PCB laminate and copper pad.
O–R
Operating limit — In HALT, the stress level at which the product malfunctions but recovers when stress is removed.
PID controller — A control algorithm — Proportional, Integral, Derivative — used to regulate chamber temperature and humidity. PID tuning determines how accurately and stably the chamber holds setpoint. A poorly tuned PID overshoots setpoints and oscillates.
PTH (Plated Through-Hole) — A via or component hole in a PCB whose barrel has been electroplated with copper. PTH barrels are vulnerable to fatigue cracking under thermal cycling due to z-axis CTE mismatch between copper and laminate.
Ramp rate — The rate at which a chamber changes temperature, in °C per minute. Published ramp rates on data sheets are measured in an empty chamber. Loading the chamber with product reduces the achievable ramp rate.
Relative Humidity (RH) — See Humidity above.
S–Z
Salt spray test — A corrosion test in which product is exposed to a continuous fog of 5% sodium chloride solution at 35°C, defined by ASTM B117. Defines the test method only — not what constitutes a pass.
Setpoint — The target value of a controlled variable that the chamber controller is programmed to achieve. Not necessarily the same as the actual condition at the DUT.
Stability — The ability of a chamber to maintain a setpoint over time at a fixed location. Distinguished from uniformity, which measures variation across locations simultaneously.
Thermal equilibrium — The state in which the DUT has reached the same temperature as the chamber environment, with no ongoing net heat transfer. The only way to verify thermal equilibrium is a thermocouple attached to or inside the DUT — not the chamber air sensor.
Thermal mass — The heat energy required to raise the temperature of an object by one degree. High thermal mass means slow response to temperature changes. Fixture thermal mass compounds this effect.
Thermal shock — A test in which product is subjected to near-instantaneous transition between temperature extremes, targeting brittle fracture and gradient-induced failure modes. Governed by IEC 60068-2-14 Test Method Na.
TUS (Temperature Uniformity Survey) — A measurement of temperature variation across multiple points within a chamber workspace, conducted simultaneously under defined operating conditions. Required before a chamber is used for controlled testing.
Uniformity — The variation in temperature across different spatial locations within the chamber workspace at a given moment. A chamber with ±2°C uniformity has a 4°C spread between its hottest and coldest zones — invisible to the controller but real to the DUT.
Z-axis CTE — The coefficient of thermal expansion of a PCB laminate measured through the thickness (perpendicular to the board plane). For standard FR-4, z-axis CTE is 50–70 ppm/°C — three to five times higher than in-plane CTE. This is why PTH barrel cracking occurs under thermal cycling.
The three pairs that trip people up most
Stability vs. uniformity. Stability is variation over time at one point. Uniformity is variation across space at one time. Most calibration certificates report stability. Most TUS reports report uniformity. Neither tells you what the other measures.
Operating limit vs. destruct limit. Both are HALT terms. Both involve failure. The difference is reversibility. Operating limit: malfunction that recovers. Destruct limit: damage that doesn't. HASS screen levels must stay below the destruct limit.
Ramp rate vs. transition time. Ramp rate is °C per minute — relevant for temperature cycling. Transition time is total seconds between zones — relevant for thermal shock. The two describe different things and are specified by different standards.