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| Technical notes... |
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| Chemical fundamentals |
The chemical processes during soldering
concern mainly the effect of the flux that, with few exceptions,
are used for every soldering application. The use of flux is
the means by which the solderability – or, more exactly,
the capacity of the surface to be wetted – is brought
about.
A metallic surface is never “clean”. Most metals
react with the oxygen in the air – they oxidize. Even
precious metals, which react little with oxygen or not at all,
are layered with materials
from the atmosphere which contain a large number of impurities.
We only have to think about air pollution caused by combustible
gases, containing exorbitant amounts of chemical substances
and causing problems especially in industrialized areas. The
binding forces of the atoms and molecules (covalent bonds) that
react on the surface, form a coating very quickly. The internal
connection between the parent metal and the solder is inhibited
thereby.
A soldering that qualifies as an inter-metallic joint between
solder and parent metal can therefore only take place when the
oxidation and/or surface coating is removed. Here we generally
talk about deoxidization and for this reason we make use of
the so-called fluxes. These are applied either previously on
the piece to be soldered or added to the solder (cored solder
with flux core). The effective phase of the flux always lies
immediately prior to the
soldering phase. The flux covers the surface before the solder
flows. The flowing solder repels the flux, thereby wetting the
“cleaned” surface, and can thus be joined with the
parent metal. A standard flux is conceived to unfold its chemical
effectiveness within the curve of the soldering temperature.
Its effectiveness is of no significance in a normal temperature
range, whereas its full effectiveness unfolds at about 150 °C
and, at about 250 °C, the effectiveness dwindles on account
of thermal stress. |
| Flux should fulfill an entire
series of different requirements: |
• Remove the oxide and superficial coatings as quickly
as possible
• Form no residues after soldering
• Be effective in an appropriate temperature range
• Not reveal any effects damaging to health
• Make the appropriate handling possible, e.g., use
in cored solder
• Be applicable for a large number of parent metals
(alloys)
• Be resistant to aging and permit a long storage time
• Not have a tendency to splashing und the influence
of temperature
• Not damage various substrate surfaces such as solder-stop
varnish
|
| To take these requirements into
account, the manufactures use various formulas. Principally,
it always involves a single carrier substance, loaded with so-called
activators. Flux agents used for soft soldering are summarized
in the DIN 8511 – part 2. The following excerpt shows
the flux types most commonly used in electronics, which mostly
deviate from modern-day formulas. Rosin (colophony) is only
used in single cases because of its forming residues that are
hygroscopic or that create subsequent splints. |
| Type group |
ISO-KZ |
Main components |
Used for |
Measure for removal of flux residues |
| F-SW 24 |
2.1.1
2.1.3
2.2.3 |
Amines, diamines |
Fine soldering, electrical
engineering (intended for non-residual flame
soldering's) |
Check if case arises |
| F-SW 26 |
1.1.2 |
Natural resins (colophony) or
modified natural resins with additional organic activators
containing halogens (e.g., glutamic acid hydrochloride) |
Electrical engineering,
electronics, electrical
appliances, metallic
merchandise |
Generally not required |
| F-SW 32 |
1.1.3 |
Natural or modified natural resins
(colophony) with organic halogen-free activating additives
(e.g., stearic/salicylic/adipic acid), but excluding amines,
diamines or urea |
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| F-SW 34 |
2.2.3 |
Halogen-free organic acids with
natural resins (colophony), but excluding amines, diamines
or urea.
|
Electronics, miniature
technologies, printed circuits
|
Generally not required |
|
| Excerpt from DIN 8511 part 2 flux agents |
| Explanation: |
F-SW 24 means: Flux for heavy metals
soft welding The higher the type group number, the “milder”
the effect of the flux agent. The first job of the flux therefore
consists in the chemical
activity of the deoxidization. It breaks up the oxygen bonding
of the metallic surface, thereby removing the impurities and
coatings on the surface, and thus enables the wetting with the
solder. At the same time, it protects the surface from becoming
oxidized again during the soldering process and absorbs the
broken off particles. The effectiveness of the flux is adjusted
to a certain temperature range. It should be most effective
in the soldering temperature range, hence between approx. 200
and 300° C in standard situations. Thus, the application
of high temperature solders, for example, requires a different
flux than the low temperature solders. Within the range of the
soldering temperature, the flux must be significantly less viscous
(easier flowing) than the solder in order to be repelled by
the solder and not tend to encase the solder. |
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| Fig 5: Schematic representation of the wetting
process |
| |
The schematic represented wetting process (Fig.
5) shows the interaction of flux and solder as it occurs in
the case of cored solder with integrated flux. The behavior
is similar to when the flux was applied separately, or deposited
as a reservoir. A reservoir is when pre-tinned parts have received
a flux coating through immersion in flux and then laid to dry.
Modern flux formulas are made in such a way that a high percentage
of the flux substances, especially the activators, decompose
chemically throughout the temperature cycle and the
carrier substance itself evaporates partially or completely.
Such flux – also called “no clean flux” –
forms only minimal residues around the soldering point into
what is called a “halo” that can only be seen with
a microscope. The cleaning formerly done after the soldering
is only performed today in exceptional cases. The selection
of the flux quantity is also very important. Thus, for example,
cored solders with a flux proportion of 1 to 3.5 percentage
of their weight are most common, making experimentation the
only way to determine the least necessary
dosage empirically. Since flux must, on the one hand, prevent
re-oxidation during the flowing phase of the solder, it should,
on the other hand, decompose during the temperature cycle, it
is necessary to coordinate two parameters apposed to each other.
The feeding-in of nitrogen or another inert gas can be very
effective in this regard. The soldering's of the flux are bettered
and the soldering cycle is accelerated. A flux formula can be
selected with a fast decomposing characteristic that also shortens
the cooling-off phase. The concept of wetting also plays a role
with the flux. This has to do with a purely physical behavior
that in practice, however, is very strongly connected with the
geometrical conditions of a soldering point. For this reason,
an entire chapter has been reserved for this topic. |
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| Geometrical fundamentals |
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