The invention relates to a method of soldering a conductor on to a ceramic resistor body having a positive temperature coefficient.
Ceramic resistor bodies having a positive temperature coefficient are often abbreviated to PTC. Such PTCs are used predominantly as self-regulating heating elements. This type of PTCs generally consists of a sintered material of predominantly barium titanate which is doped with, for example, a rare earth metal (such as lanthanum), niobium, antimony and/or bismuth. Said dopants provide the sintered resistance material with a semiconducting character.
A sintered material of the above composition exhibits an electrical resistance characteristic which is typical of PTCs. When the resistance is determined as a function of temperature, said resistance is found to increase substantially at a temperature near the so-called Curie temperature of the PTC material. In a graph showing the electrical resistance as a function of temperature, this effect results in a typical S-curve. In the range of the Curie temperature a relatively small temperature increase leads to a relatively large increase in resistance. As the heat conduction of the sintered material is realtively low, this situation is attained at a relatively low electric power. The relatively large increase of the resistance at a temperature near the Curie temperature sets a practical upper limit to the power which can be stored in the PTC and dissipated as heat.
The Curie temperature of barium titanate can be influenced in a simple manner by partly replacing barium by strontium or lead. The replacement of a part of the barium by strontium or lead brings about an increase or a decrease, respectively, of the Curie temperature.
The known PTCs usually comprise two conductors for effecting the supply of electric power. Said conductors are customarily soldered on to the ceramic PTC body. To this end, the ceramic body is provided with two metallic contact layers. Said layers may be provided in various manners such as, for example, by vacuum evaporation or sputtering. The contact layers may alternatively be provided by means of screen printing. In that case, a thin layer of conductive paste is screen printed on the ceramic body and is subsequently annealed. In this manner, an electrically conductive layer is formed. After the contact layers have been provided, the conductors are soldered on to said layers.
Conductors are to be understood to mean, first of all, electrically conductive metallic connection wires of, for example, copper or a copper alloy. However, conductors are to be understood to mean also electrically conductive tracks of a so-called printed circuit board (PCB).
Applicants have found that the conventional soldering methods used for securing a conductor to a PTC are not optimal. More in particular it has been found that the ceramic material of PTC's which were provided with connections in the conventional manner may explode. Said explosions of the ceramic material occur, in particular, when the known PTC's are subjected to high-voltage tests at voltages of from 200 Volts. Under said conditions, also indications of a short-circuit in said PTC's were observed.