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Electronic packaging. (Special Feature).

In the final article in this series John Burgess, EIF Manager UK, Shipley Europe takes a look at electroplating for electronic packaging, including applications from simple flat strip for the forming of lead frames used in the manufacture of chip packages, to the plating of connectors and chip capacitors.

Typical plated metals are gold, tin, tin/lead and copper. Nickel is often used as an undercoat or as a solderable finish, and in total or selective plating of components.

Typical plated metals are gold, tin, tin/lead, copper and nickel. Nickel is often used as an undercoat to tin and tin/lead. Tin and tin/lead are normally used as a solderable finish and are plated totally or selectively onto the component. This is common in reel-to-reel applications (see below).

Components may be plated using traditional rack or barrel operations. For high volume production, a continuous process known as reel-to-reel plating is common.

Reel-to-reel plating

In the reel-to-reel process, the continuous strip of material to be plated is unwound from a reel, passed through a series of cleaning, pickling, plating and rinsing cells, before being rewound onto another reel.

The speed with which the material passes through each part of the process, and the current density applied in the plating cells, together determine the thickness of the final plate.

Plating solutions are normally contained in a plating tank positioned underneath the plating equipment. The solution is pumped from this tank to the plating head, or trough. The solution in the head overflows and is piped back to the tank forming a continuous cycle.

Heating, auto dosing, cooling and level are normally controlled from the plating tank. The plating cell has anodes along its length and a cathode strip running through the middle of the trough head.

Because there tends to be a lot of movement of the liquids within the plating cells, these solutions are designed so as to minimise foaming which can cause loss of solution. High current densities are often employed, for fast plating and high production output.

Selective plating

It is common practice in the plating of strips of connector pins, to selectively plate a proportion of the pin.

In order to achieve this, the pins pass through a protective mask, which allows the plating solution to come in contact with only that part of the material which is to be plated. As long as the masks are properly maintained, very precise edge definition can be achieved.

Typical solutions used in these machines are soak and electrocleaners (low foaming), copper plating (less common nowadays) and sulphamate nickel. Methane sulphonic acid-based tin and tin-leads are also used in preference to fluoborate types because they can be run at much higher current densities and produce very bright deposits.

Environmental issues

The environmental issues raised by the use of lead in plating are cause for concern. The options are to replace the lead with a different alloying material such as copper or bismuth, or to use pure tin as the deposit. Chemical suppliers are currently working to develop solutions with similar solderability characteristics to that of tin/lead.

The reel-to-reel process is an efficient method for the plating of high production volumes. Machines require considerable capital investment and a high level of maintenance. Temperature must be closely controlled to optimise plating, with extra cooling to make sure the mechanical action of the pump does not over-heat the solution.

Rack and barrel plating

Rack plating is not often used for plating electronic packaging materials. Bright tin and tin/lead rack plating baths are sometimes employed, for example for the plating of copper bar for high voltage heavy duty electrical equipment.

Barrel plating is much more common and is used on small individual components. Barrels are usually much smaller in size than those used for the zinc plating of nuts and bolts. Again, the metals used are, typically, nickel, gold, tin and tin/lead.

Often, the work will be cleaned off-line in a basket, in soak cleaners and electrocleaners. It will then be acid-activated and loaded into the barrel for plating.

Alternatively, a fully automated barrel plating process can be used, where the load is placed into the barrel and passed through the cleaners, pickles and plating solutions.

Chip capacitors are plated using a special process designed to prevent deposition of metal onto the sensitive dielectric parts of the component. Tin or tin/lead plating of the tips of the terminations is carried out in a special solution which has a high pH, around 2.5 -- 4.5. This prevents attack of the ceramic/glass materials, which would render the components useless.

Chip capacitors are normally undercoated with nickel prior to being plated with tin or tin/lead.

Best practice

It is important to ensure that all solutions are analysed regularly and additions maintained.

Correct maintenance of temperature control and solution levels help to produce consistent results.

In reel-to-reel machines, it is extremely important to ensure that all the tanks and cell hoses are secure to prevent leaks, that pumps are pumping effectively to maintain correct level control, that the temperature is controlled correctly, and carry over is minimal.

The rinse cells must be maintained for efficient rinsing as these can often can reduce efficiency especially in hard water areas.

For further information:

Shipley Europe Ltd

Tel: 024 7665 4400

Fax: 024 7665 4458

E.mail: infouk@shipley.com

Shipley Company serves decorative and general metal finishing markets with a wide range of metal and precious metal solutions.

The company is a world leader in the manufacture and supply of imaging technology, specialist chemicals and proprietary plating process additives to the finishing, connector and semiconductor packaging and printed circuit board industries.

For the demanding semiconductor and connector markets, Shipley provides gold, palladium and other precious metal plating solutions, complemented by imaging and plating processes for lead frames and other packaging applications. Technologies for printed circuit boards include liquid and dry film photoresists for imaging, electroless, electrolytic and direct plating systems, etchants, soldermasks, inks and final finishes.

Shipley Company's European headquarters Shipley Europe is located in the UK, with local facilities in Germany, France, Italy, Switzerland, Sweden and Benelux. A complete network of distributor partners ensures full coverage of all countries in Western and Eastern Europe.

Shipley Europe is a division of the Shipley Company, a wholly owned subsidiary of Rohm and Haas. Shipley Company sales are expected to exceed $1.2 Billion in 2000.
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Author:Burgess, John
Publication:Finishing
Geographic Code:4EUUK
Date:Apr 1, 2001
Words:1072
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