SMTA China Conference Proceedings

Qualification Test for Creep Corrosion Using Flower of Sulfur Chamber

Authors: Prabjit Singh, Dem Lee, Jeffrey Lee, Karlos Guo, Julie Liu, Simon Lee, Geoffrey Tong, Chen Xu, Haley Fu
Company: IBM Corporation, iST-Integrated Service Technology, Inc., Lenovo, The Dow Chemical Company, Nokia, iNEMI
Date Published: 8/28/2017   Conference: SMTA China

Abstract: Electronic hardware can be susceptible to creep corrosion in corrosive environments. Creep corrosion is the corrosion of metallization (generally copper and silver) and the migration of the corrosion product (typically copper and silver sulfides) across the printed circuit board (FR-4 and solder mask) surfaces. In environments high in sulfur-bearing gaseous contamination, the extent of creep corrosion may be so high as to electrically short circuit adjacent pads and traces, causing the circuit boards to malfunction.

The iNEMI (International Electronics Manufacturing Initiative) project team on creep corrosion has developed a flowers-of-sulfur (FOS) based test that is sufficiently well developed for consideration as an industry standard qualification test for creep corrosion. The test chamber consisted of a 300-mm cube acrylic box placed in an oven. A bed of sulfur provided the corrosive sulfur vapors. The concentration of the sulfur was kept constant at a predefined value by maintaining the chamber at a constant temperature. The humidity was maintained constant at a predefined value by selecting and using an appropriate saturated salt solution. CloroxTM solution was used to generate chlorine gas.

Since the Electroless Nickel on Immersion Gold (ENIG) finished PCBs are very popular in the consumer electronics industry due to their excellent solder-ability and life time, there is an urgent need to determine the root cause of their susceptibility to creep corrosion and to provide a solution that will make them more robust against creep corrosion. The morphology of the ENIG finish and the creep corrosion products were analyzed using a range of analytical tools including optical microscopy (OM), focused ion beam (FIB), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). Preliminary cross-section analysis has shown that there is a gap formed between two nickel layers at the edge of the solder mask. The gap provides an easy path for copper ion migration and for copper sulfide creep. In an effort to improve the robustness against creep corrosion, ENIG finishes with various phosphorus content and post-treatments were studied. The paper will present the root cause of the severe creep corrosion on the ENIG finished PCBs and ways to improve their robustness against creep corrosion through Flower of Sulfur chamber.

Key Words: 

Creep Corrosion, Flower of Sulfur (FOS), Electroless Nickel on Immersion Gold (ENIG), Copper Ion Migration

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