Annealing

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The term tempering generally describes the heating of a material over a longer period of time. With such a heat treatment it is possible, for example, to control the distribution of mechanical stresses in components made of glass or acrylic . It is also possible to selectively change the structure of a solid, for example the structure of components made of cast iron or the transformation of the crystal structure of thin layers .

Tempering is used in glass and plastics technology, in the iron and steel industry (see also malleable cast iron ) as well as in thin-film technology.

A heat treatment during or after the galvanic process in the manufacture of parts of the connection technology (e.g. screws) is also referred to as tempering.

description

In the physical sense, tempering means that a solid is heated to a temperature below the melting temperature. This happens over a long period of time (a few minutes to a few days). The increased mobility of the atoms can compensate for structural defects and improve the crystal structure in the short-range and long-range order. In this way, the process of melting and (extremely) slow cooling to adjust the crystal structure can be avoided.

Areas of application

Glass

The component is brought to a temperature just above the lower relaxation limit and held there for a long enough time until the entire component has heated up evenly to this temperature. The temperature must not exceed the upper cooling point in order to avoid uncontrolled changes in shape.

The subsequent cooling is carried out in two different ways, depending on the intended result:

  • If the component is then to be free of internal mechanical stresses, the cooling process takes place slowly and steadily down to the lower relaxation limit . Below this critical temperature, temporary temperature gradients no longer lead to frozen deformations and permanent stresses and the further cooling rate is now only limited by the tensile strength of the glass. Especially with optical components , the freedom from frozen internal stresses is a decisive quality feature, since such stresses lead to - mostly unwanted - birefringence in glasses .
  • Rapid cooling by quenching means that tensile stresses initially build up in the surface, which cools faster, which are relieved by plastic deformation. This deformation is frozen and, after compensating for the temperature differences, causes compressive stress in the surface which, if correctly expressed, significantly increases the strength of the tempered glass obtained in this way against mechanical and thermal loads during use compared to relaxed glass.

cast iron

In cast iron, tempering means long-term annealing at temperatures between 700 ° C and 1050 ° C to improve the mechanical properties ( strength , toughness ). The result is malleable cast iron.

Manufacture of fasteners (e.g. screws)

If fasteners made of high-strength steel (Rm> 1000 N / mm²) are subjected to galvanic post-treatment for reasons of corrosion protection, this heat treatment must be carried out at a temperature of 200–230 ° C during or immediately after galvanizing to avoid hydrogen embrittlement .

Electronics manufacturing

In electronics production , the tempering process is used to prepare components as well as bare circuit boards . The aim is to remove unwanted moisture that creeps into the components or the circuit board over time. Otherwise, the rapid heating during soldering (to over 200 ° C) can lead to deflagrations , which may destroy the component. A bare circuit board that consists of several glued layers can possibly delaminate, that is, the layers separate from one another.

Semiconductor and microsystem technology

In the semiconductor and microsystems technology annealing are widely used. As a rule, these are process steps in which the properties of thin layers or the semiconductor substrate (a wafer ) are changed. These include:

  1. the removal of crystal damage, for example after an ion implantation step , often referred to as "annealing".
  2. increasing the rate of diffusion for
  3. increasing the flow rate of certain layers, for example borophosphosilicate glass (BPSG), to smooth the wafer topography

The tempering steps are usually carried out under forming gas or another inert atmosphere. The temperatures used for this range from a few hundred to almost two thousand degrees Celsius, depending on the application and material. In a broader sense, surface reactions in which material from the gas space is built into the layer or the substrate, for example the thermal oxidation of silicon , are sometimes referred to as a tempering step.

The tempering steps often take place in tube furnaces, as they are also used in the thermal oxidation of silicon. These ovens have relatively long heating and cooling times because they have to bring the entire wafer to the desired temperature as evenly as possible. They are therefore either too slow or cannot be controlled sufficiently well for certain applications. For this reason, so-called rapid thermal processing has been developed, in which mostly only part of the wafer or the layer is heated. Since less material has to be heated here, significantly shorter process times can be achieved. However, this is mostly a single wafer processing and not a processing of 100 or more wafers at the same time as in the furnace process.

Plastic painting

Residues of release agents, which are used in injection molding or reaction injection molding , to enable the parts to be removed properly, adhere to plastic parts. These residues must be completely removed before painting so that they do not react with the later paint build-up. For this purpose, the plastic parts are heated or tempered to 60 ° C for approx. 1 hour.

See also

Web links

Individual evidence

  1. ↑ a b Dietrich Widmann, Hermann Mader, Hans Friedrich: Technology of highly integrated circuits . Springer, 1996, ISBN 3-540-59357-8 , pp. 37-40 .
  2. ↑ Plastics and their paintwork. Page 14, Standox
  3. ↑ Exam questions for vehicle painters, Lackiererblatt, July 3, 2007