Sur-Tech A/S
Surface Technology
TECHNO-TIN
A tin plating for technical
purposes in the electronic industry
DS/ ISO – 9001
ISO – 14001
Quality Assurance System
Environmental Management System
Sur-Tech A/S – Bygmarken 1 – DK3520 Farum – Phone: +45 44 95 65 66 – E-mail: [email protected]
TIN
Atomic weight
Density at 20oC
Melting point
Re-crystallizing temperature
Electrical conductivity at 20oC
118,7
7290 kg/m3
232oC
Under 20oC
8,3 m/Ohm-mm2
Tin is a soft, shining metal, it is white, having a yellowish appearance. Tin and its salts are
relatively non-poisonous. This is important in the application as a coating material on steel sheets
for tin cans. The classic coating was a layer of 5-10µm applied in several steps to avoid the
formation of penetrating pores (tin plate). The most common layer thickness used recently is 0.1-1
µm tin coating plus a clear lacquering.
Tin is often used as a roll metal, in layers of 0.1 – 0.3 mm on steel pans.
Tin plating is used in many electronic articles to make them suitable for soldering, and tin plating of
cast iron, and aluminum pistons in combustion engines are made to ease the running in of the
engines. Metallic (tetragonal) tin is stable only to approximately 13oC. Below this temperature the
metal slowly converts to gray tin with a density of 5750 kg/m3, having diamond structure and is
non-metallic.
Tin-lead, soldering alloys
Tin and lead forms a eutectic point as shown in fig. 1. It is possible to solder using pure tin, as is
common at certain applications in the electro-technical industry and in the tin-can fabrication; but
the eutectic alloy between tin and lead is easier to use for soldering as the melting point is lower and
the alloy is floating easier. Common soldering-grade tin hold 50% Sn and 50% Pb and is cheaper
than the eutectic alloy.
Fig. 1: Existance of eutektikum through tin and lead
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The corrosion of tin
Tin is reacting in acidic as well as alkaline environment. In neutral or almost neutral environment
tin is quite stable. It does not corrode in soft water and has been used in many years for tin plating
of copper pipes for distilled water.
Pot and pans made of copper used for cooking purposes, have traditionally been tin coated to
protect against corrosion. The coating is applied as hot, melted tin.
Tin cans made of steel sheet material is electroplated with tin in very thin coatings and, as the
competition with aluminum as tin can material is hard, it is sought to achieve a better corrosion
protection applying even thinner coatings. Heat treatment and additional lacquering are used to
improve the corrosion protection. During the heat treatment the tin is meting and many of the
formed pores are removed. During the following cooling of the metal it becomes solid again –
almost without pores and having a very bright and beautiful appearance.
Tin-lead coatings
Tin is like nickel the cathode when in contact to steel. If it shall serve as a corrosion protecting
coating on steel it therefore has to be free of pores or alternatively be applied on a dense under
coating of either copper or nickel.
For outdoor applications tin is less stable compared to copper, but better than nickel. For indoor
applications tin is more stable than copper, as it forms a passive fine layer of oxide.
Mostly tin plating is used on electronic components, to enable soldering on the surfaces. In these
applications the well-known but dangerous whisker formation may occur. Whiskers may be the
reason to short-cuts in electrical installations.
The so-called whiskers consist of fine crystals of hair-structure having a diameter of 1-2 µm and a
length of 1 cm.. The formation of whiskers happens momentarily from the tin surface, often as a
unit-crystal.
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The energy used for the formation of the unit-crystals is probably coming from micro-tensions in
the metal surface (see fig. 2). The problem seems to appear mostly in coatings from bright tin
plating baths and on tin coatings applied to zinc containing base materials.
Fig. 2. Whiskers grown from a tin alloy coating on steel. The shown whiskers have a length of approximately 0.5 mm.
Besides the ‘true’ whiskers, other types of irregularities are obvious. At the end of the hair-structured whiskers (Sshaped at right in the picture), on the edge of a line (at right , in the middle) or even randomly. The magnification is 180
times.
To minimize the risk of whisker formation it is suggested to use a tin alloy holding more than 1% of
lead. On zinc containing base materials the problem can be minimized by the application of an
under coating of copper or nickel. This barrier layer prevents diffusion of zinc to the tin layer.
To achieve a suitable surface for soldering on a base metal, which is not suitable for soldering, it is
of utmost importance that the adhesion of the coating (on which soldering can take place) to the
substrate (base material) is perfect.
References
Soldering in electronics, R.J. Klein Wassink
Metallurgi for ingeniører, Conrad Vogel, Celia Juhl, Ernst Maahn
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