Electrocodeposition of lead free tin alloys

Abstract

A method of electrocodeposition of lead free tin alloys is disclosed. The method includes the following steps: electrocodepositing tin metal and particles of a noble metal to form a composite; and converting the composite to an alloy by heating the composite.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS [0001] The present application claims the benefit of provisional patent application Ser. No. 60 / 587,350 filed Jul. 13, 2004, which is incorporated herein by reference.TECHNICAL FIELD [0002] The technology relates to tin alloys, and, more particularly, to electrocodeposition of lead free tin alloys. BACKGROUND INFORMATION [0003] Tin-lead (Sn—Pb) solder is used in connections for electronic assemblies. Applications of tin-lead coatings include protective solderable finishes as well as masking of printed boards, soldering of leads, and C4 (controlled collapse circuit connection) or “flip chip” packaging using solder bumps. Sn—Pb solders are electrodeposited in some electronics packaging applications because electroplating provides superior thickness control in comparison to application of the melt, and it is easily scaled down to small pitch sizes. Solder bump technology can allow a bare integrated circuit to be mounted on a next level package, w...

AI Technical Summary

Benefits of technology

"The present invention relates to a method of electrocodeposition of lead-free tin alloys. The method involves electrocodepositing tin metal and particles of a noble metal to form a composite, and then converting the composite to an alloy by heating it. The resulting alloy has improved properties, such as high solderability, low melting point, and good electrical conductivity. The method can be used for various applications such as protective solderable finishes, masking printed boards, and controlled collapse circuit connection packaging using solder bumps. The technical effects of the invention include improved properties of the resulting alloy and simplified and efficient methods for its production."

Problems solved by technology

However, industrial processing and consumer use of lead-bearing products is a recognized public-health hazard, and the international trend toward regulation is driving demand for lead-free alternatives such as the Sn / Ag (Tm=221° C.) and Sn / Ag / Cu (Tm=217° C.) eutectics.

Lead-bearing solder presents a number of problems.

The alpha particle can cause significant problems.

Lead-bearing solder also represents a disposal problem in finished products.

Because lead represents a well-documented threat to public health, many governments are seeking to restrict its use in consumer products.

Even with additives, the baths remain more difficult to stabilize and control than pure-tin baths.

Because silver is deposited at the transport limited rate, morphological instability leads to undesirable structures over a range of current densities.

A second deficiency is that the alloy composition depends on current density (tin deposition rate) and transport conditions (silver deposition rate), both of which may vary over the substrate.

This introduces problems with process control and deposit quality.

As a result, insoluble anodes are usually required, introducing additional chemical monitoring and control workload.

However, a thorough rinse is required each time the substrate is transferred between baths, so that this approach introduces mechanical and material complications.

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