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+44(0)1822
613555 |
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+49(0)77
24 934 125 |
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info@aci-ecotec.co.uk |
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info@aci-ecotec.de |
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| Technical notes... |
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| Thermodynamic fundamentals |
Any metallurgical junction using soft soldering
is always in conjunction with the supplying and dissipation
of heat. A soldering point, therefore, always passes through
a temperature cycle that is determined, on the one hand, by
the melting behavior of the solder and, on the other hand, by
the physics of the running process. Heat is thermal energy that
manifests itself in solid objects in the form of atomic oscillations.
The higher the temperature, the more intense are the movements
between atoms. Heat can be transferred in three ways: by heat
conduction, when objects of differing temperature come into
contact; by heat flow, when the media (fluids or gases) transport
heat, and by thermal radiation, when heat is transferred via
electromagnetic radiation (e.g., infrared radiation). All three
kinds of transportation play a part in soldering technology.
Individual methods distinguish themselves by the kind of heat
transfer used – but this will be considered later. The
behavior of every soft soldering typically follows a temperature
/ time diagram. (Fig. 3)
As well known, heat always flows from higher to lower temperature
levels. The quantity of heat transported depends upon the time
and the differences in temperature. The temperature change follows
strictly an e-function with a decreasing rate of change. The
diagram shows a typical temperature / time curve of a soldering
created by the contact with a soldering tool (e.g., soldering
iron) and supplying solder. At this point, the temperature regulated
on the soldering tip is considerably higher than the required
melting temperature of the solder. In
practice, a soldering tip temperature is selected that lies
above the melting temperature of the solder by a factor of 1.5
- 1.8. This is necessary in order to take the dynamic conditions
into account as well as to keep the time needed for the soldering
cycle as low as possible. |
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| Fig 3: Temperature/ Time Diagram |
| Let’s observe the course
of the diagram: |
The soldering tip touches the parts to be joined
– we are now a point A. The large difference in temperature
between the “hot soldering tip” and the “cold
parts” cause the temperature
of the parts to increase rapidly up to point B that lies above
the melting temperature of the solder by 20 - 30%. At point
B, the solder begins to be supplied. The supplying, depositing
and in-flow of the solder require further heat energy that not
only flows into the solder from the soldering tip, but also
from the parts. For this reason we have a temperature decrease
at
point C. Still, it is also important that C remains significantly
above the melting temperature. By means of additional heat supply
from the soldering point, the temperature rises again
and reaches point D in which the heat supply is interrupted
, i.e., the heat transfer medium is retracted. The cooling-off
phase thereby begins. At point E, the soldering point temperature
falls below the solidus point. The solder changes its condition
from liquid to solid. Now the actual physical soldering cycle
is completed, and this is the earliest possible moment that
the soldering point may be placed under stress mechanically
or electrically (electrical test or work piece movement).
The soldering processes using contactless heat transfer or that
use internal heat generation with, however, an injected primary
energy (radiation, induction or resistor heating) generally
follow this principle as well. It is of great economical concern
to keep the time necessary for the soldering cycle down to a
minimum. Thus, the question is often raised concerning what
possibilities there are to optimize the time. The temperature/time
diagram of a soldering point can, within certain limits, be
varied, influenced and thereby be optimized from an economical
point of view. The following table (Fig. 4) gives information
about various measures that offer advantages and disadvantages
with regard to shortening the cycle time. The limits must be
carefully selected to avoid that a serious negative factor cancels
the advantage you are trying to attain. The optimal ratios are
determined through experiments made prior to the automation-planning
phase. Only the empirical determination of data and parameters
with contemporary result control leads you to the optimal production
ratios. Every spot soldering point has its own characteristics
and very often its own set of parameters. The chemical effects
of the flux during the thermodynamic processes are also to be
taken into consideration. |
| Measure |
Advantages |
Disadvantages |
| Raising of the temperature of the heat
transfer medium, e.g., the soldering tip |
• Shortening of pre- and after heating
times by means of a higher temperature difference |
available for working with the flux
is shortened
• Diffusion depth is lessened
• Heat transfer medium wears out faster
Flux splashes |
| Increasing the mass of the heat transfer
medium |
• Shortening of pre- and after heating
times due to a larger available heat volume |
• Accessibility to the soldering
point is restricted
• Higher loss of solder |
| Preheating of the parts |
• Pre-heating time decreases |
• Larger technical investment
• Higher energy need
• Additional heat
stress placed upon the parts |
| Use of very aggressive types of flux |
• After-heating time decreases |
• Possibly leaves corrosive
residues |
Cooling the soldering point during the
cooling-off
phase |
• The solidus point is reached faster |
• Diffusion depth is
lessened
• Liquid solder can be
deformed |
Improving heat transfer by enlarging the
cross section,
e.g., by pre-tinning the soldering tip.
|
• Pre-heating time decreases |
• Larger technical
investment |
Advantages
Soldering within a deoxidized atmosphere
(nitrogen) |
• Flowing behavior is improved
• Scale forming is reduced |
• High operating costs
•
Larger technical
investment |
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| Fig 4: Measures for shortening cycle time |
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| Chemical fundamentals |
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