PTC thermistors used as heating elements are self-adjusting and have no glowing parts. They are mainly used in automotive electronics (diesel preheating, auxiliary heating of passenger compartment, and protection of valves against condensation and icing, etc) and in domestic appliances (washing machine door locks, radiator thermostats). Further applications are found in evaporators for vapour dispensers and insect sprays as well as small household appliances. Passives maker Epcos offers rectangular and disk PTC heating elements for various temperature and voltage ranges in standard and customer-specific geometries.
Component service life is particularly important in automotive electronics and domestic appliances. This is influenced mostly by the type of contact electrodes used. The electrical contacts of PTC thermistors must be built-up in two layers. The outer layer of a PTC heating element is a thick-film silver electrode (about 10 µm thick), which makes possible the clamp contact common in heating applications. The inner metallisation layer serves to overcome the barrier layer between the high-resistance ceramic material and conductive contact surface. Until recently, this was done by silk-screen printing with silver containing a small quantity of zinc. Epcos has now switched to sputter technology to produce the inner metallisation layer. Here the barrier layer is broken down by a thin layer of chrome (less than 1 µm), which is protected by an equally thin layer of nickel.
Sputter films feature outstanding uniformity, density, purity and adhesion (used to make semiconductors, CDs and optical devices). In a plasma discharge in a vacuum chamber, metal atoms are dislodged from a pure metal target. These particles cross a vacuum and are precipitated on the surface to be metallised. This process permits metallisation without the use of contaminating binding agents. Long-term tests have shown that under conditions of continuous load, elevated temperature and humidity, the mean variation in resistance is more than halved thanks to the improved electrode. This is an overriding advantage in critical safety applications in automotive electronics or white goods. The new metallisation also makes it possible to attach a migration-free silver electrode, which improves long-term performance further, especially in ambient humidity.
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