Avoiding Particle Deposition in Small-Scale Electronics
The deposition of particles from indoor air on circuit boards in electronic equipment reduces the electrical isolation between conductors, sometimes causing electrical short circuits. This is the finding of EETD researchers, who believe that, over time, deposited particles may be responsible for failures in telephone-switching and other electronic equipment. Circuit failures caused by indoor air pollutants cost U.S. telephone offices roughly $200 million annually, but given the amount of electronic equipment in use throughout the U.S., the overall total costs may be much higher. Protection of electronic circuits against indoor air pollutants is an important issue for industrial economies, which increasingly rely on smaller components in information-processing equipment. As miniaturization increases and electrical conductors are placed closer and closer together, the products are increasingly susceptible to this problem.
To keep spaces clean, air-handling systems remove large (2 to 20µm) particles, including fibers, which are generated by mechanical wear on materials or derived from biological sources. Smaller particles (<2µm), generated by combustion or photochemical gas-to-particle conversions, often pass through filters and are carried into buildings with outdoor air but may not be removed efficiently by the air filters.
Risks to Industry
Such fine-mode particles produce a greater threat to indoor electronics because they can be deposited on both vertical and horizontal surfaces. Moreover, when voltage is applied, electrical forces enhance the deposition on circuitry. Above all, such particles tend to be hygroscopic—i.e., able to attract or absorb moisture from the air. As much as half the fine-particle mass in a building is from hygroscopic particles. When the environment reaches a critical relative humidity (roughly 50 to 65%), the deposited particles deliquesce (become liquid by absorbing moisture) and become electrically conductive.
Some of the electrical conductors in modern electronic equipment, like the "legs" of surface-mounted chips that connect the chip to the circuit board, are directly exposed to ambient air in the workplace. When particles accumulate, producing bridges between adjacent conductors, and relative humidity runs high, electric current may leak or produce short circuits.
Discovering the Nature of Deposition
Andres Litvak, a graduate student working under the guidance of Ashok Gadgil and Bill Fisk of the Indoor Environment Department, conducted an experimental study with a view toward better understanding particle deposition and failure rates of electronic circuitry. The findings will allow the prediction and prevention of failure rates (as a function of ventilation and filtration scenarios).
Eighteen dummy surface-mounted chips and six television sets were installed in an exposure chamber. The chips were connected to surface trace conductors on circuit boards by their conductive legs, then mounted (some vertically and others horizontally) in an environmental chamber. A range of voltages was then applied to the circuits. After careful cleaning of the circuits, the electrical isolation between the legs was measured as a function of the relative humidity of the surrounding air. Particle concentrations inside the chamber were then increased using a system that generated ammonium sulfate particles (average diameter of 0.48 µm) and were maintained at 500 times greater than "normal" for 281 hours, yielding the equivalent of 16 years' worth of normal particle exposure.
Surprisingly, no television malfunctions were observed at any relative humidity. The lack of deterioration may possibly be explained by the elevated temperatures inside the television sets, which prevented the relative humidity from exceeding the critical relative humidity for deliquescence. Also, the spacing between electronic conductors in the TV sets was larger than that in most modern electronics.
On the circuit boards, the highest levels of accumulated particles were observed between the legs of surface-mounted chips with a voltage differential—especially on the sharp edges and in the sharp bends of the legs. However, particle accumulation was also observed between legs that were grounded. Particles bridged adjacent legs, but in uneven patterns. Fibers were observed within some of the finer agglomerations of particles, apparently facilitating the formation of bridges between conductors.
Electrical isolation between the adjacent legs of chips decreased after particle exposure. The isolation between legs with a differential voltage decreased by approximately three orders of magnitude. In some instances, the electrical isolation decreased to a point known to cause failures of electronic circuits. On one circuit board, a short circuit caused visually obvious damage. Researchers proposed a change in the geometries of the legs that eliminated the sharp bends in the conductors associated with particularly high localized particle deposition.
As equipment shrinks to meet new demands from industry, particle deposition will cause a certain percentage of equipment failure. Understanding this process under varied conditions can help avoid catastrophic and costly failures in an industry that will be increasingly developing unique electronic solutions.
For more information, contact:
- William Fisk
- (510) 486-5910; fax (510) 486-6558
This research is sponsored by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy.