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Characterization, Prevention and Removal of Particulate Matter on PCBs
Characterization, Prevention and Removal of Particulate Matter on PCBs
This paper describes a means of measuring the DRH of minute quantities of particulate matter (1 mg or less) found on PCBs.
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Authored By:
Prabjit Singh
IBM Corporation, Poughkeepsie, NY, USA

Patrick Ruch and Sarmenio Saliba
IBM Research, Zurich, Switzerland

Christopher Muller
Purafil Inc., Atlanta, GA, USA

Particulate matter contamination is known to become wet and therefore ionically conductive and corrosive if the humidity in the environment rises above the deliquescence relative humidity (DRH) of the particulate matter. In wet condition, particulate matter can electrically bridge closely spaced features on printed circuit boards (PCBs), leading to their electrical failure. Failures attributed to particulate matter have even been observed in data centers where the gaseous contamination levels are low enough to meet the ANSI/ISA-71.04-2013 G1 severity level.

The combination of miniaturization of electronic components, the reduction of feature spacing on PCBs and the loosening of the data center temperature and humidity envelope to save energy is making electronic hardware more prone to failure due to particulate matter. The characterization of particulate matter on PCBs is challenging because of the small amount of particulate matter available for analysis. The objective of this paper is to develop and describe a practical, routine means of measuring the DRH of minute quantities of particulate matter (1 mg or less) found on PCBs. Data center particle filtration schemes and means of removing the particulate matter from PCBs will also be presented.

In summary, the deliquescence relative humidity test procedure that has been developed for testing field-returned IT equipment or witness coupons is as follows: The field returned hardware is washed with a deionized water and isopropyl alcohol solution, the wash water concentrated by evaporation and 10 drops of the concentrate, 10 ul in volume, deposited on a silver-plated comb pattern. A comb pattern witness coupon, on which data center dust settles directly, needs no further treatment and can be transferred to a controlled humidity chamber as is.

The humidity in the chamber is raised in steps of roughly 10% RH and the leakage current measured at each humidity step using a +/-1 V square waveform with a 2 second period for 2 cycles. The humidity range for which this method is valid is from 30 to 90% RH. The logarithm of the leakage current is plotted versus relative humidity and the deliquescence relative humidity (DRH) is at the intercept of the inversion line and the high humidity asymptote. The critical relative humidity (CRH) is the intercept of the inversion line and the low humidity asymptote. The relative humidity of the air in the data center must be kept below the CRH of the accumulated particulate matter on the IT equipment to avoid failures due to the particulate matter wet and therefore conductive.

Initially Published in the IPC Proceedings

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