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Immersion Tin
— Immersion Tin —
sponsored by
MicroCirtec Micro Circuit Technology (MIC)
The procurement of PCBs often
leads to the question as to which surface protection is the right one. This is partly due to the
changing technical demands on PCBs in further processing. Also, as of 1. 6. 2006, EU-Regulations
restricting the use of dangerous materials in electronic appliances are to include the prohibition
of materials containing lead, and is, therefore, another reason why alternative surfaces have been
developed. One of these is immersion tin.
A layer of tin is applied to the PCB by means of a special process,
in which the copper
atoms of the exposed conductors and contact areas are exchanged for tin atoms
by
adding thiourea. The chemical reaction ends when the copper surface is 0,6 to 1,2 my.
Between
the tin and copper layers there is an inter-metallic copper-tin phase after this
process, i.e.
blended metal.
There are a number of reasons in favour of applying tin chemically. One advantage is the
uniform
smoothness of the surface. This feature enables the application of fine pitch
components with
a <0,5 grid. Such a fine grid sorely tests the limits of classical lead-tin
surfaces (SnPb),
especially when Fine-Pitch technology is necessary on both sides of
the PCB.
Characteristics
We can expect PCBs to require a combination of assembling technologies
in the future. Immersion tin could also satisfy
the demands of such a development. Components can
be stuck to the PCB as well as soldered. Immersion
tin demonstrates a good soldering
ability, but is slightly at a disadvantage compared with SnPb, because
the thin immersion
tin layer cannot be completely dissolved. Multi-soldering is possible, as well as
pressing
techniques, thanks to the thinness of the layer.
The thin tin layer does, however, has its disadvantages. The inter-metallic copper-tin
phase is
approx. 0,25 mµ thick when delivered. During storage, the diffusion of copper
in the tin surface
continues, causing the thickness of the pure copper or tin layers to
decrease. If the blended metal
reaches the surface it becomes coated with a non-
removable oxide, making soldering impossible. Storage
possibilities of immersion tin
PCBs are, for this reason, considerably limited compared with conventional
SnPb
techniques: stored PCBs should be used up within 3 months and if time exceeds 6
months, heavy
processing problems can occur.
The price of immersion tin is between that of the classical SnPb and the immersion
nickel gold surfaces.
However, thiourea, which is used in the production, does not
conform with environmental or effluent
requirements, therefore the future costs for
disposing of wastes are difficult to calculate, particularly
in view of the upcoming, stricter
environmental regulations.
Conclusion
Immersion tin can be seen as a viable alternative, as the surface
offers many advantages under the conditions described. The costs are also slightly lower than Immersion
Nickel Gold. However, until lead is prohibited, immersion tin will never be able to completely
replace the old lead tin. In many cases SnPb perfectly satisfies the assembling requirements and
offers definite cost advantages, provided that environmental costs do not increase.
