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Bright
reflowed electrolytic tin-plating on copper based metal strip |
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Introduction
General
Description
Main
Characteristics of PEM Processes
Tables |
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 Introduction
:
The tin deposits are mainly intended to protect the base metal against
corrosion, simplify brazing operations, improve connector insertion
and extraction forces, or to enhance the quality of contact at electrical
junctions. |
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General
Description:
Reflowed
electrolytic tin-plating on copper and copper alloy
This type of process, which is applied reel-to-reel, consists in
subjecting the strips to various surface treatment operations within
one installation, through which they pass part by part, from an
unreeling system and, generally, to a reeling system.
Some of these operations can be chemical or thermal (degreasing,
scouring, finishing, etc), but the tin or tin alloy deposit is obtained
by cathode-based electrolysis.
The tin is usually applied on the strip. However, under certain
conditions of use (temperature exceeding 120 °C for example), the
inter-metal diffusion phenomena that occur are undesirable (separation;
deterioration in aptitude for soldering; etc). So it is recommended
to insert an appropriate sub-layer between the base metal and the
final deposit. This sub-layer is itself obtained by electrolysis.
The thickness of the deposit depends mainly on the intensity of
the electrolysis and the speed with the strip passes through the
treatment installation (for a given "electrolysis length").
The control over these parameters allows one to regulate the thickness
with a precision of a few tenths of a micrometer.
If necessary, appropriate technologies allow one to obtain different
thicknesses on each face and/or a partial deposit.
The strips can be tin-plated over the full width and then split
to the dimension desired by the user. In this case, the edges will
not be coated on the final product.
Electro bright reflow finish is produced reel-to-reel, following
chemical and electrolytic treatments. It is obtained by heating
the strip, which will have been coated beforehand with a dull electrolytic
deposit, to above its fusion point and then cooling it before any
contact with an element of the installation (guidance, return, etc).
The deposit remains bright after cooling.
The time during which the bright reflow process takes place is generally
short: from a fraction of a second to a few seconds, during which
nevertheless the diffusion phenomena occur between the tin and the
base material, thereby reinforcing the adhesion of one to the other.
However, the quality of surface preparation before tin-plating (degreasing,
scouring, etc) is essential in preventing defects that are generally
considered to be unacceptable, such as bubbling or de-wetting (phenomenon
of partial or total shrinkage of the deposit observed on the treated
surface). During this operation, the temperature of the strip reaches
values approaching those during fusion deposition, i.e. 250° C.
In practice, bright reflow on strip is not applied for deposits
exceeding 5µ.
Application
on brass strip
In the case of brass, the presence of Zn gives rise to undesirable
consequences, due to the diffusion of this metal towards the surface
of the deposit:
 loss
of brazeability
deterioration
in appearance
deterioration
in contact quality
This phenomena, which is perceptible cold within a few months, is
accelerated with increased service temperature. The insertion of
a sub-layer capable of acting as a diffusion barrier prevents or
considerably diminishes this modification. |
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Main
Characteristics of PEM Processes:
Thickness
of sub-layer
This is commonly from 0.5 to 1 µ.
Thickness of tin deposit (pure tin)
This is linked with the conditions of use.
0.8 - 1.5 µ favourable to sliding, low insertion force
1 - 3 µ Maintenance of aptitude for brazing over time. Seizures
possible beyond 2 µ.
3 - 5 µ Only recommended for static connections (contact
surface increased due to the ductility of the deposit; effect of
impermeability on the contact area due to the crimping of the deposit).
Bright
reflow finish (brightening by fusion)
This is obtained by heating the coated strip. Two techniques are
used:
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radiation
within a tunnel oven (gas or electric); |
| electromagnetic
induction (the strip passes between two induction coils of
which the field gives rise to eddy currents). |
This latter process allows precise control over the fusion temperature
/ time parameters. In both cases, the operation is performed under
normal atmosphere and the cooling of the strip is obtained by blowing
with compressed air.
Table 1 shows
a summary of the processes employed in the case of a reflowed tin
coating on brass.
Table 2 shows
the performances of coatings of reflowed tin applied to brass by
PEM processes.
Because of the sub-layer, the bright reflow finish allows the formation
of stable inter-metal compounds. These compounds ensure good conservation
of the aptitude for brazing and the adhesion during ageing of the
coated metal.
Abradability
of deposits
The nature and composition of the tin-plating baths and the operating
conditions of bright reflow finish play a part in the formation
of abrasive compounds (notably oxides) at the surface of the coating.
These compounds can give rise to abnormal wear of the cutting tools.
The parameters involved in this are kept under surveillance so that
the thickness of this layer does not exceed a few nanometers. The
measurement is obtained using a potentiometric method.
Thermal
effect of bright reflow on the base metal
The metallurgical state of copper-based strips can be deteriorated
by the bright reflow operation: there can be a loss of hardness
and elasticity.
Such is the case with pure or only light alloyed copper, and brasses.
Bronze is little subject to the phenomenon.
This annealing effect depends on temperature and time factors. The
bright reflow processes -- which are "induction" type, of which
the temperature parameter is fully controlled and the variation
in temperature of the high-speed treated area are necessarily used
when such an effect must be maintained at a low level. In practice
this is controlled by measurement of the hardness of the base material,
before and after treatment. On thin strips, it is essential to use
a micrometer; the measurement can be taken on the face of the strip
(provided that, after treatment, the coating is totally eliminated
in order not to invalidate the result which can, in the case of
brasses, be increased due to the formation of hard inter-metallic
compounds during bright reflow). |
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Tables:
Processes employed in the case of a coating of reflowed tin on
brass
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| Coating
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Rôle
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Effect |
Applications |
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| Sub
layer |
Diffusion
barrier |
Modification
of speed of growth of inter-metallic compounds.
Defer the diffusion of Zn within the final layer |
Delayed
ageing
Maintain
the aptitude for brazing during ageing at high temperature |
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| Pure
Sn |
Protection
against corrosion (oxidisation)
Brazing aptitude |
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Small
change in pure tin during ageing |
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| Fusion
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Fusion
at controlled temperature |
Formation
of stable inter-metallic compounds (re-crystallization of
tin) |
Best
resistance to ageing with regard to:
- surface oxidization;
- aptitude for brazing;
- appearance of cleavage* |
*Cleavage: separation of coating by breaking-up of fragile inter-metal
layers formed at the coating/base metal interface |
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| Performances
of reflowed tin coatings (with sub-layer in case of brass) |
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Ageing at 155°C * |
Ageing
at 200°C * |
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Cu
Thickness |
Sn
Thickness |
Sliding
Insertion force |
Brazing
Aptitude |
Coloration
(Oxidisation) |
Cleavage |
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| 0.5
- 1 µ |
0.8
- 1.5 µ |
Good |
16
h |
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| 0.5
- 1 µ |
1
- 3 µ |
Seizure
possible |
48 h |
500
h |
>
500 h |
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| 0.5
- 1 µ |
3
- 5 µ |
Bad
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48 h |
500
h |
>
500 h |
*Tests in damp
heat at 155° C and 200° C are performed in accordance with standard
NFC 20720 or IEC 68. |
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SOFIPEM,
Siaugues St Romain 43300 SIAUGUES STE MARIE - FRANCE
Tel: +33 (0)4.71.74.21.09 - Fax: +33 (0)4.71.74.26.57
- Email : contact@sofipem.com
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