In this test, the component is immersed in a water-based dye at high pressure for a given period of time to see if any liquid will penetrate through any micro-cracks, pores, or bad seals. The component is then dismantled and placed under a UV lamp. Any liquid penetration will show up as the dye fluoresces.
Micro-cracks and seal failures can also occur due to environmental temperature cycling. If a component is manufactured in a warm, humid climate and then transported in the hold of an airplane at -40°C, any internal moisture can shock-freeze and fracture the hermetic seal as the ice expands. The part can go through several thaw/freeze cycles as it is transported through different flight or road traffic distribution centers before arriving at its final destination, thus causing the defect to propagate. A less drastic but more long-term thermal cycle stress can occur if the part is then stored in an unheated warehouse over several summer/winter seasons.
Thus, for instance, if a manufacturer states that their part has a storage temperature and humidity range of -40°C to +85°C @ 50%RH, it does NOT mean that the part can be safely cycled between these storage temperature limits. In fact, if a part is stored at a low temperature to reduce ionic or atomic aging processes (Figure 3), then it must be warmed up very slowly and brought to room temperature before being used. Sustained hot or cold storage temperatures are preferred over several hot/cold cycles.
Fig. 3: Graph of the Arrhenius equation. The rate of chemical reaction, k, is proportional to an exponential function of the temperature T – the higher the temperature, the more aggressive the reaction. A is a constant for the reaction, Ea is the activation energy for the reaction, and R is the universal gas constant. This relationship applies to many chemical reactions including most corrosion, oxidation, and aging processes.
What happens if the storage temperatures are exceeded? SMD components mounted on an internal PCB will have a different rate of thermal expansion or contraction than the substrate itself, so at extreme temperatures, the mechanical stress can cause the solder to break or the component to crack. Encapsulated components (diodes, transistors, etc.) can usually withstand lower temperatures, as the case gives mechanical support to the pins, but they may still fail at temperatures below -40°C, as they often contain metallic lead frames, and copper has a high coefficient of thermal contraction.
At very low temperatures, the most difficulty arises with components that rely on the movement of ions or liquid chemical processes. These include electrolytic and some types of ceramic capacitors. At low temperatures, such activity “freezes out”. Electrolytic capacitors rapidly lose their capacitance during cooling, and at -40°C, they may have only 10% capacitance compared to their room-temperature value. At cryogenic temperatures (i.e., below about −65°C), the electrolyte will freeze, thus causing permanent physical damage.