Tin alloy is sulpher so good

Silver alloys and pure silver have long been used in the manufacture of industrial

control gear. Although silver surfaces will tarnish even under normal environmental

conditions, such deposits do not usually have an adverse effect on the performance

of electrical contacts or conductors. The deposit form is argentous oxide (Ag2O)

which, although resistive in nature does not usually form in sufficient quantities

to cause problems. The deposit is also relatively soft and is easily worn away

by continuous contact operation. This is particularly true of wiping type contacts,

which would normally be expected to break through the surface film completely

at the first operation.

Under these conditions, sulphur is normally present in the form of hydrogen

sulphide (H2S) or sulphur dioxide (SO2)

and a reaction takes place with the silver to produce a silver sulphide layer.

The coating can be recognised by its dull grey or blue/ grey colour. This layer

can become very heavy, has a higher electrical resistance than silver and usually

results in abnormal temperature rise, followed by ultimate failure of the device

due to thermal stress. In some cases, the layer can become so heavy that it

delaminates and flakes fall into the bottom of the switch or cubicle. Although

the sulphide layer is relatively high resistance compared with silver, it still

represents a short-circuit or earth path hazard if it accumulates in the wrong

place.

An additional and probably even greater hazard is that of whisker growth, which

can take place over a period of months, days or even hours. Although somewhat

unpredictable, whisker growth generally initiates at corners or stress points

and is usually accelerated by higher temperatures. Whiskers can take the form

of straight or curled monofilaments or multiple whiskers growing in clumps with

a resemblance to steel wool. Monofilaments can grow to many centimetres in length

and as such, are able to easily bridge between phases or phase to earth. Although

the sulphide filament is many times the resistance of pure silver, it still

represents a potential conducting path.

Figure 1 illustrates an extremely long monofilament whisker found in a fuse-switch

in use in a paper mill. This whisker spans a gap of just over 2.5cm, but the

actual total length of the filament is in the order of 4cm. Finding examples

like this is a rarity as they are normally vaporised in the faults they initiate.

Figure 2 illustrates parts of the contact assembly in a fuse-switch removed

from the coking ovens in a steel mill. Multi-filament growths can be clearly

seen and, as previously mentioned, are reminiscent of steel wool both in texture

and colour. There is also

evidence of flaking of the surface deposit.

The basic problem can be solved by simply removing one of the basic elements

causing the reaction which creates silver sulphide, ie either the silver or

the sulphur. Removing the sulphur or preventing it from contaminating control

gear enclosures is easier said than done and is not normally an option. We therefore

need to find an acceptable substitute for silver. Gold would be a good option

as it does not readily react with sulphur, but its softness and prohibitively

high cost, renders it impractical for use in power switching devices.

Other options

Of the materials commercially available, tin is the most likely contender. Although

it does react with airborne sulphur, the reaction only takes place at elevated

temperatures approaching that of molten tin. However, it does have two problems.

Firstly, like silver, tin also produces whisker growths in the presence of

sulphur. However, the whiskers are generally single crystal growths rather than

multiple whiskers. They are also metallic in nature, which makes them seven

or eight times more conductive than silver whiskers. Hence they provide an even

greater risk of short-circuit.

Much research has been carried out aimed at overcoming this problem and a number

of non-whisker tin alloy plating solutions have been developed. There are two

main solutions available which provide non-whisker characteristics, these are

tin/lead and tin/copper. For power switching applications, the tin/copper process

would be the preferred alternative. Unfortunately, this process is extremely

difficult to control and is not offered on a commercial basis by any plater

at present. This leaves tin/lead as the best practical solution.

A second problem associated with tin and tin alloy plating is that a diffusion

process can take place between the copper and the tin. When this occurs, it

creates an inter-metallic layer between the copper substrate and the plating

layer. This intermediate layer has a resistance roughly six-times higher than

that of pure tin. Once again, this produces the probability of excessive temperature

rise and ultimately the probability of failure of the switching device. The

diffusion process can and does take place at normal room temperatures, but is

accelerated by high temperatures and hence can lead to thermal run-away.

Putting up barriers

The most effective way of inhibiting this diffusion process is by the introduction

of a thin barrier layer between the copper and tin, using a metal with which

tin or copper will not react.

Figure 3 is a final illustration, which shows the effects of both kinds of

sulphur contamination on the moving contact assembly from a fuse-switch. Here

the multiple whisker growths and flaking of the surface coating of silver sulphide

can be clearly seen.

After considerable investigation and research, the system introduced by Hubbell,

for use in sulphur laden atmospheres, is based on the use of a bright tin alloy

as the final plating layer, with a thin interposing plating barrier of dissimilar

metal. To be fully effective the alloy ration in the primary plating layer needs

to be carefully controlled along with the actual thickness of both primary

and barrier layers.

Hubbell fuse-switches using this construction have been in production since

1992 and to date there have been no reports of failures due to silver sulphide

surface contamination or whisker growth.

The entire range of Hubbell fuse switches are available in this form, from

the UFS32 up to and including the UFS800C3. The maximum current ratings are

generally reduced by 10% below that of the equivalent unit with silver-plated

contacts and conductors. To assist recognition, a suffix ‘T’ is added to the

product reference