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Testing PCBs for Creep Corrosion
Testing PCBs for Creep Corrosion
This paper covers the iNEMI technical subcommittee test for creep corrosion using a flowers-of-sulfur based qualification test.
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Authored By:
Haley Fu, iNEMI, Shanghai, China
Prabjit Singh, Levi Campbell and Jing Zhang, IBM Corporation, Poughkeepsie, NY, USA
Wallace Ables, Dell Corporation, Austin, TX, USA
Dem Lee and Jeffrey Lee, iST-Integrated Service Technology, Inc., Taiwan
Jane Li and Solomon Zhang, Lenovo (Beijing) Limited Corporation, Beijing, China
Simon Lee, The Dow Chemical Company, Tao Yuan Hsien, Taiwan

Summary
The iNEMI technical subcommittee on creep corrosion is developing a flowers-of-sulfur (FOS) based qualification test for creep corrosion on printed-circuit boards (PCBs). The test setup consists of a 300-mm cube chamber with two means of mounting the test specimens and flowing air over them to expose them to constant, predefined humidity and temperature conditions and sulfur and other contaminants. The FOS chamber performance has been evaluated using copper and silver foils and preliminary test runs have been conducted on PCBs from a manufacturing lot known to have failed in service.

The effect of air velocity on the copper and silver corrosion rates was quite linear. The effect of humidity on copper and silver corrosion rates in the low air velocity range of less than 0.1 m/s showed a strong dependence on relative humidity. In the high velocity range of 1 m/s, there was no clear dependence of humidity on copper and silver corrosion rates. A means has been developed for applying controlled concentration of ionic contamination on selected local areas of test PCBs.

Preliminary test runs have shown that ionic contamination found in fine dust may be a necessary condition for copper creep corrosion. Printed circuit boards from a manufacturing lot that suffered creep corrosion in service, with and without dust contamination applied to them, were tested in a FOS chamber at 60oC with 1 m/s air flowing over them. The PCBs with no dust contamination did not suffer creep corrosion in the 3-day test; whereas, the PCBs with dust contamination suffered creep corrosion with morphology similar to that occurring in the field.

Conclusions
The following conclusions can be drawn from the present phase of the iNEMI project on developing a cost-effective qualification test for creep corrosion on PCBs:

In the flowers of sulfur (FOS) test at 60oC, at low air velocities less than 0.1 m/s, copper corrosion rate was lower in the lower humidity range, rising in a somewhat step-wise manner to a higher corrosion rate above about 80% relative humidity. The silver corrosion rate remained quite independent of relative humidity. At the higher air velocity of 1 m/s, the corrosion rates did not behave in as well-defined a manner as a function of humidity.

In the flowers of sulfur (FOS) test at 60oC and 80-82% relative humidity, the copper and silver corrosion rates increased quite linearly with air velocity with no sign of plateauing off even at velocity as high as 1.3 m/s.

A means was developed to locally apply controlled concentrations of salts to PCBs prior to subjecting them to the FOS test.

The presence of salt contamination, similar in composition to the fine dust (PM2.5), was found to be necessary for creep corrosion. PCBs from lots with history of creep corrosion related failures, did not suffer creep corrosion in FOS test unless contaminated with ammonium salts and NaCl. ZnCl2 salt, found in some data centers high in chlorides and with extensive zinc plated ducting, was also found to aid in creep corrosion on PCBs from lots known to suffered creep corrosion in the field.

Initially Published in the IPC Proceedings

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