The common effective ways of thick film heating elements are breakdown under pressure, burn off of thick film circuit, burn off of electrical contact point, etc., and the burn off of thick film circuit is the most important and proportional most effective way. Through consulting and experimental analysis to thick film manufacturers, the effective mechanism of thick film heating elements is summarized as follows:
The pressure breakdown mainly occurs in the production cycle of thick film elements. The main reasons for the pressure breakdown of thick film elements are as follows:
1) Influence of substrate materials. The substrate microstructure, composition, surface roughness of dielectric insulation effect has important influence, metallography structure and chemical composition of stainless steel corrosion resistant ability, and the composition of stainless steel to determine its aspect factor, if the size and the substrate from expansion coefficient, in the process of high temperature sintering, may lead to crack of medium size, even stratification, surface roughness will also directly affect the effect of dielectric paste, choosing the appropriate roughness can improve the effect of paste, and the purity of the substrate surface can affect the insulation performance, Therefore, the substrate material must be strictly controlled, and the chemical composition, surface roughness, and surface cleanliness should be ensured to be consistent.
(2) Influence of sintering process and sintering equipment. Each size has a specific sintering curve and the sintering temperature and time are usually determined by the properties of the size. On the one hand, the high precision of sintering equipment is required; on the other hand, the sintering process must meet the sintering curve; otherwise, cavitation and other phenomena will occur, affecting the insulation performance.
(3) The influence of production environment, environmental temperature,humidity and air dryness, all will affect the insulation performance of the product, the production process must be strictly controlled environmental factors.
(4) During the use of the product, due to liquid leakage or moisture attached to the surface of the heating element, insufficient creepage distance will also lead to breakdown phenomenon
Thick film circuit burning occurs mainly in the use stage of the product, which is caused by the following reasons:
(5) Uneven thickness of heating resistance. Both silk-screen printing and sintering processes will affect the thickness uniformity of thick film resistance. When thick film heterogeneity occurs, the weak links will have high resistance and high heat, which will easily form hot spots and eventually burn out.
(6) Uneven distribution of temperature field on the surface of heating body. Generally, the wiring mode of the surface of thick film heating element and the structure of the product have great influence on the temperature field distribution. When the temperature field distribution is not uniform, it is easy to cause different expansion coefficients of each part of the substrate and the heating filament to damage the circuit. In the design of thick film heating element, the uniformity of the temperature field is the most important index.
(7) The temperature controller is not timely protected. When the appliance is under dry burning or other conditions that lead to abnormal temperature rise, the heating element will rise at a speed of 100℃ C per second or even higher, If the humidity protection element is not timely protected, it will eventually lead to the damage of the heating resistor. So the choice of protective opening is critical for the lifetime of thick film heaters.
(8) High power density or current density. Usually, the power or current density of thick film heating element should be left with enough margin, which will lead to the failure of thick film resistance in the case of time accumulation or other abnormal conditions.
(9) Substrate deformation. If the substrate material or structure design is not reasonable, or the substrate deformation due to other structural reasons, the thermal resistance attached to it will also undergo phase transition and be damaged. Due to the large working current of the thick film heater, the contact point of the electrical connection is also a weak part of the heater.
In the working environment of alternating of humid environment under the influence of factors such as, easy to cause fever burned contact oxidation, usually when the design thickness and area of thick film conductor material should have allowance, the thickness of the contact with silver content must meet the requirements of actual use, and contact with thick film conductor by means of elastic connection, shrapnel material generally choose bronze as well, because the bronze has good flexibility and electrical properties.