Key reliability concerns with lead-free connectors

Abstract

This paper presents the key reliability issues of lead-free connectors. The paper first discusses electrical contact resistance of lead-free and lead-based solder-dipped contacts under various aging conditions. Then, the fretting corrosion of lead-free contacts is discussed. Finally, the reliability due to tin whiskers induced by mating pressure between connector contact elements is presented.

Introduction

As the interconnect density increases and the circuit current decreases (as low as microamperes) in electronic systems, the reliability of electronic connectors has become more critical [1]. In the past several decades, major progress in the study of electrical contacts and connectors has been obtained. For example, mixed flowing gas laboratory systems have been developed for studying environmental corrosion, a wide range of surface analysis approaches have become available, fretting corrosion of electrical contacts has been better understood, and new connector designs and lead-free finish have been developed.

A contact finish is used to provide protection for the base metal of a connector against corrosion, diffusion, and contact wear, and to help establish and maintain a stable resistance at the contact interface. There are two kinds of contact finish materials for electrical connectors: noble metals and alloys, and non-noble metals and alloys. Noble metals and alloys include gold, gold alloys, palladium, and palladium alloys. Noble metals and alloys are the preferred coating materials for separable connectors in the electronics industry because of their low contact resistance and corrosion resistance. Nickel, nickel alloys, silver, tin, and tin alloys are the most common non-noble contact finishes. They exhibit a surface film due to natural aging mechanisms. The surface films are primarily responsible for the failure of electrical contacts in the field. Abbott [2] reported that surface films, together with the superimposed effect of interface motion, might account for a major portion of intermittent failures.

Tin–lead solder alloys have been the most commonly used non-noble contact finish materials in the electronic industry due to their low cost and ease of manufacture. With appropriate design considerations, tin–lead alloys may often be successfully utilized as cost-effective alternatives to gold under lower requirement of reliability. Since the tin–lead alloys are soft, the brittle oxides of tin–lead alloys can be easily displaced by proper mechanical deformation and the wiping action of contact surfaces, low contact resistance can be obtained. Extensive studies have been made on the electrical performance of tin–lead solder coatings. Cowieson et al. [3] conducted aging experiments on hot dipped tin and tin–lead alloy coatings. They found that the hot dipped tin–lead coating had fairly low contact resistance in the non-aged condition, but the resistance increased substantially after the steam aging test for contact normal loads below 150 g. They also found that the intermetallic compound, which did not reach the surface of the tin or tin–lead solder coatings, did not affect the contact resistance of the coatings. Hammam [4] studied friction, wear, and electric properties of different tin coating techniques (hot dipping, electroplating, and reflow), and reported that property differences among the techniques correlate to the thickness of the pure residual tin layer and the intermetallic compound.

The relative motion of the contact interface during its service life is a main cause of failure of tin and tin-alloy-coated contacts. This failure mechanism is called fretting corrosion. Fretting is small amplitude oscillatory motion, which can result from vibration [5], shock, or differential thermal expansion of materials in contact, usually tangential between two solid surface in contact. Fretting corrosion is a corrosion (oxidation) form during fretting. In the case of tin alloy coated electrical contacts, the corrosion is usually the oxidation of wear debris. Tin and tin alloys are soft metals on which a thin but hard oxide layer is rapidly formed. Supported by a soft substrate, this layer is easily broken and its fragments can be pressed into the underlying matrix of soft, ductile tin. Fretting disrupts the oxide surface layer and exposes fresh tin to the environment. As fretting continues, exposed metal oxidizes, and oxide debris accumulates at the contact interface. This sequence of events often causes an increase in contact resistance and leads to contact opens.

Driven by legislative requirements and market forces, lead-free solders replace tin–lead in soldering processes, lead-free solder alloys are also being used in electronic connectors. The electronics industry has widely adopted pure (matte) tin and high-tin alloy finishes as a lead-free option. However, both silver and copper, which are present in popular lead-free alloys (Sn–Ag–Cu, Sn–Ag, Sn–Cu), are subject to film formation and corrosion in atmosphere, especially with corrosive pollutants like H₂S and SO₂ [6], [7]. The high tin content (>96%) in the alloys may exhibit poor fretting corrosion behavior.

The adoption of high-tin content finishes has also created a reliability concern pertaining to the formation of conductive tin whiskers, which can bridge adjacent conductors and lead to current leakage and electrical shorts. Since the spacing between conductors is of the order of a few hundred microns, and since tin whiskers have been known to grow up to a few millimeters in length [8], tin whiskers pose a serious risk to its reliability.

In this paper, the key reliability issues of lead-free connectors are discussed. First, there is a discussion of electrical contact resistance of lead-free contacts subject to various aging conditions (dry heat, steam aging and mixed flowing gas environment), and subject to micro-motions at the contact surfaces (fretting corrosion). Comparisons are made with lead-based solder-alloy-coated electrical contacts. Then, the pressure-induced whisker, at separable interfaces of connectors plated with pure tin and lead-free finishes, is presented.