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Constant temperature -- that's one secret to producing high-quality, reliable solder joints. A new soldering technology provides constant temperatures even when thermal loads and application time vary. This means that one variable of the process is brought under very tight control.
The technology provides a self-regulating temperature source (SRTS). Not only does the heater strip not overheat or underheat, it does not rely on feedback to adjust for fluctuating temperatures. Feedback, after all, responds to variations. With SRTS, temperature is held constant by the material, so there is no need for feedback.
Materials Make the Difference
The SRTS system relies on basic electrical and metallurgical properties of materials: specifically, the skin effect of a conductor and the Curie point of a magnetic material.
Skin effect is the well-known property of a conductor carrying an alternating current. With a DC power source, current density is uniform across the cross section of the conductor. For an AC source, current is restricted to the outer region of the conductor. As frequency increases, the depth that the current penetrates the conductor becomes shallower. In other words, less cross-sectional area is available to carry current.
Permeability is a measure of a material's magnetic properties. As temperature rises, the permeability falls, dropping off to zero at high temperature. The point at which the material loses its permeability is the Curie point.
At temperatures below the Curie point, skin effect restricts current to the magnetic layer. At the Curie temperature, skin depth increases so that current flows in the high-conductivity material. As changes in the thermal environment occur, current shifts from layer to layer to maintain a constant temperature independently at every point along the heater strip.
The SRTS heating strip uses a two-layer structure of a magnetic material over a nonmagnetic material. The heating depends on the strip's resistivity and permeability. And since permeability is a function of temperature, the temperature is self-regulating. It works like this:
Because SRTS heats faster and maintains a constant temperature, it removes temperature as a variable in the process.
When a constant-current of energy source is first applied to the strip, skin effect confines the current to the magnetic layer. This layer has high resistance and narrow skin depth, so heating occurs rapidly. As the magnetic layer approaches the Curie temperature, its resistivity drops and skin depth increases and current flows into the highly conductive nonmagnetic layer. As a consequence, the generation of Joule heat decreases and all points along the heating strip reach an equilibrium temperature -- the Curie temperature.
As thermal loads change, the current shifts from layer to layer to maintain temperature at every point along the heater's length. By carefully choosing the magnetic material with the proper Curie temperature, an ideal soldering temperature can be maintained.
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