Symposium Conference Proceedings

Investigation of Oxidation on Electronic Connector Leads Through Multiple Heat Cycles

Authors: F.Che Ani, A.S Harun, S. E Lim
Company: Celestica (M) Sdn. Bhd.
Date Published: 4/18/2012   Conference: Symposium

Abstract: During production build using X's Connector, line has high fallout for non-wetting phenomenon and lead observe severe black signature on connector lead after wave soldering process. Thus, fallout rate was 33%. There was severe black signatures were observed on the connector lead due to presence of unwanted elements on the surface of pre-tinned layer during the wave soldering process. The base metal oxidation (zinc oxide) due to low nickel thickness on the inner row pins of ‘failed’ connectors identified as one of the key factors that attributed to the darkening (black) of contacts. In addition, the solid state diffusion is a subject of basic importance for understanding mass transport phenomena in surface of the connector leads. In the particle aspect of surface diffusion of single atoms and simple molecules, information of interest is the detail atomic diffusion mechanisms and the activation energy of the various atomic processes (atomic diffusion occurs during heat treatment), and also the binding energy of atoms at different surface sites. Surface of materials, the top few atomic layers of the connector lead, are of critical importance because it is here that a material interacts with its environment. The surface properties of materials dominate their corrosion behaviour, wettability, electrical contact resistance, frictional behaviour, adhesion properties and chemical reactivity. Therefore, surface analyze techniques was performed in terms of X-ray Photoelectron Spectroscopy (XPS) on the surface composition of electronic connector lead specimens following various mechanical, chemical, electrochemical or thermal treatments has been studied. It was found that as received pre-tinned surface of the connector lead was contaminated by the oxides of zinc due to the step-wise migration of atoms from lattice site to lattice site (diffusion at atomic level). In short, Zinc/Zn present on the Nickel/Ni surface during soldering can easily oxidize and may prevent wetting. For experts’ point of view, even though the atomic % of Zn is small, but it is a true root cause for the oxidation and lead to the black signature [it will slowly oxidize when subjected to multiple heat cycles]. Then, the degradation can start from the first dip in molten Sn-Pb solder and followed by the next heat cycles. The higher the temperature and the longer the time at this temperature, the more Zn atoms will diffuse out from a lead frame material through the Ni layer [especially if the layer is thin – porous]. Moreover, the enlargement of porosity obtained naturally when exposing to any heat cycles. So, more pre-tin steps we have, the higher the risk of Ni, Zn, Sn, intermetallic oxidation, solderability may be significantly affected, and Zn will exaggerate this problem. On the other hand, the electroplating of Ni and good values of plating efficiency needs enhancement in eliminating such defect prior to multiple heat cycles involved during pretinning and soldering.

Key Words: 

Ni, Zn, Sn, intermetallic oxidation, heat cycles

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