Chonglun Fan

Driving force for the formation of Sn whiskers: compressive stress-pathways for its generation and remedies for its elimination and minimization

Compressive stress is widely accepted as the driving force for tin whisker formation. There are several pathways for compressive stress buildup in Sn coatings, which include the following: residual stress generated during plating; stress formation due to interfacial reactions between tin and copper substrate; mechanical stress; and thermal mechanical stress due to coefficient of thermal expansion (CTE) mismatch between the tin layer and substrate during thermal cycling. In order to prevent or reduce whisker growth in tin deposits, compressive stress has to be eliminated or minimized. This paper discusses the pathways for compressive stress formation and various remedies for its elimination and minimization. Particularly, a novel approach for dealing with thermal mechanical stress due to CTE mismatch is discussed.

Gold and aluminum wire bonding to palladium surface finishes

Palladium surface finishes are utilized on leadframes, printed wiring boards and automobile sensors. Their superior functional performance and the considerable environmental impact of plating lead‐free finishes for packaging processes have been increasingly recognized by the electronic industry. Wire bondable and solderable palladium finishes meet military and industrial standards at no extra cost in the overall assembly processes when compared to traditional packaging techniques. In addition to the development of palladium plating chemistries and technologies, the functional properties of the surface finishes including their wire bonding performance have also been investigated at Bell Laboratories. In this study, gold and aluminum wire bonding to palladium finishes was tested and the wire bond pull force and break position were examined in order to optimize the bonding processes. The results of the study are reported in this paper.

Electroplating and Properties of SnBi and SnCu for Lead-Free Finishes

SUMMARY Tin alloys such as Sn-0.7% Cu and Sn-2%Bi were identified as viable alternatives to tin-lead finishes. Electroplating of these alloys is challenging because of the great difference in reduction potentials of the individual elements and the usual problems associated with strong immersion deposition and poor alloy control. To overcome these problems we utilised metal-specific non-complexing organic additives, which can significantly slow down or completely inhibit deposition of the alloying element in a certain range of potentials. In this paper, we present the recent developments for SnBi and SnCu plating chemistries. The plating electrochemistry and materials properties of deposited alloys are discussed.

The materials properties and contact reliability of palladium-cobalt

SUMMARYPalladium and its alloys have become the standard precious metal finishes for high reliability connectors. While pure palladium is preferred for high temperature automotive applications, palladium-nickel has become the preferred finish for ambient temperature, high insertion applications such as edge card connectors. However, palladium-nickel, when plated over nickel, is not without its problems. Quality control issues related to the measurement of composition and thickness by simple, non-destructive XRF analysis remain a significant concern. Palladium-cobalt does not suffer from this shortcoming, and furthermore has been found to outperform palladium-nickel for high durability applications.In this study, the metallurgical structure of palladium-nickel and palladium-cobalt electrodeposits is characterized by X-ray diffraction and electron microscopy. In addition, the hardness, porosity, wear performance and thermal stability, among other properties, are discussed.