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Flowers of Sulfur Creep Corrosion Testing of Populated PCBs
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Authored By:Prabjit Singh and Marie Cole IBM Poughkeepsie Poughkeepsie, NY, USA Tibor Kiraly IBM Corporation Hungary Julian S. K. Tan IBM Singapore Raj Rangaraj, Gerry Woods and Tim Chang SanDisk - a Western Digital Brand, USA SummaryCreep corrosion testing of printed circuit boards using a specially designed flowers of sulfur chamber has been pioneered by an iNEMI technical committee. The iNEMI test is based on a chamber that is 300-mm cube acrylic box with 8-paddle wheels rotating at 20 RPM that can accommodate 8 printed circuit boards under test. In one embodiment of the test setup, the sulfur vapor with controlled concentration is provided by two 100-mm diameter petri dishes containing beds of sulfur and by maintaining the chamber temperature at 50oC. The relative humidity is maintained at 81% using two 80-mm diameter petri dishes containing KCl saturated solution. The source of chlorine, while repeatable though time varying in concentration, is provided by 40-ml household bleach in a 100-ml beaker. The iNEMI creep corrosion qualification test has successfully predicted creep corrosion on specially designed and manufactured unpopulated printed circuit boards of various finishes, soldered with rosin or with organic acid flux. Creep corrosion similar in morphology to that observed in the field has been reproduced in the iNEMI tests. This paper describes the iNEMI creep corrosion testing of a number of fully populated printed circuit boards of various technologies and vintages of known field reliability. The results confirm the iNEMI finding that prebaking the printed circuit boards is a necessary condition for creep corrosion to occur in the iNEMI flowers of sulfur chamber. The creep corrosion results on prebaked printed circuit boards of 7 different technologies agreed with the field reliability experiences. The PCBs from lots that suffered creep corrosion in the highly polluted geographies showed creep corrosion of similar morphology in the flowers of sulfur creep corrosion test; whereas, the PCBs from lots that did not suffer creep corrosion in the field, survived the flowers of sulfur test with little or no creep corrosion. The iNEMI PCB creep corrosion qualification test is now sufficiently well developed to be adopted as an industry standard test. The paper will also shows some evidence that long-term storage may eliminate creep corrosion. ConclusionsThe iNEMI creep corrosion test is sufficiently well developed to be an industry standard test for the qualification of PCBs technologies and production lots to ensure that they are free from creep corrosion in the field. There is evidence that the storage of soldered PCB assemblies for 2 years eliminates creep corrosion in the iNEMI test. Initially Published in the IPC Proceedings |
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