What are the key characteristics of dust? Are some dust types worse than others? This paper presents some results towards answering these questions. Analysis Lab
Bo Song, Michael H. Azarian and Michael G. Pecht
Center for Advanced Life Cycle Engineering (CALCE)
University of Maryland, College Park, MD
Transcript
Atmospheric dust consists of solids suspended in air. Dust is well known for its complex nature. It normally includes inorganic mineral materials, water soluble salts, organic materials, and a small amount of water.
The impact of dust on the reliability of printed circuit board assemblies is ever-growing, driven by the miniaturization of technology and the increasing un-controlled operating conditions with more dust exposure in telecom and information industries.
A fundamental and systematic study on the impact of dust is needed, since not much research has been done in this area. Since dust is always present in the atmosphere, under what conditions is dust a reliability concern for electronics?
What are the key characteristics of dust? Are some dust types worse than others? Should there be classifications of dust?
How will different combinations of dust, voltage, relative humidity, temperature, and other factors affect electronic materials and circuits? This paper presents some results towards answering these questions.
Summary
Atmospheric dust consists of solids suspended in air. Dust is well known for its complex nature. It normally includes inorganic mineral materials, water soluble salts, organic materials, and a small amount of water. The impact of dust on the reliability of printed circuit board assemblies (PCBAs) is ever-growing, driven by the miniaturization of technology and the increasing un-controlled operating conditions with more dust exposure in telecom and information industries.
A fundamental and systematic study on the impact of dust is needed, since not much research has been done in this area. We started by asking some basic questions on dust. Since dust is always present in the atmosphere, under what conditions is dust a reliability concern for electronics? What are the key characteristics of dust? Are some dust types worse than others: e.g., dust that is more hydrophilic? Should there be classifications of dust? How will different combinations of dust, voltage, relative humidity (RH), temperature, and other factors affect electronic materials and circuits? This paper presents some results towards answering these questions. We designed a group of experiments using real life dust collected from both indoor and outdoor areas. AC impedance spectroscopy (IS) was employed as the measurement technique for this research.
We designed test coupons with adjustable spacing between electrodes and measured their electric properties under different relative humidities. We analyzed aqueous solutions produced from dust samples were using their pH and conductivity. And we further analyzed the compositions of the dust samples. We found that dust had a significant impact on the reliability of PCBA. Relating the test results to the analysis results, indoor dust is more sensitive to the change of relative humidity compared to outdoor dust due to the water soluble salts and particle size. At the same dust deposition density, indoor dust is more susceptible to induce moisture related failure, such as loss of surface insulation resistance, electrochemical migration, and corrosion.
Conclusions
We designed a group of experiments to gain some fundamental understanding of the physics of failure of dust contamination. We used the dust collected from the field instead of a mixture of chemical compounds to simulate the real dust. The test results were more realistic representation of the dust impact on the printed circuit assembly. AC impedance spectroscopy (IS) was employed as the measurement technique for the dust research, as it can provide interfacial and bulk properties of the overall electrochemical system without losing the SIR information measured by DC voltage. To our knowledge we are the first ones to use IS to analyze the effects of dust. The key findings of this paper are listed below.
The data showed that the presence of dust had a significant impact on the reliability of PCBA. There was negligible change of the impedance spectra of control samples at different relative humidifies, while there were orders of magnitude changes observed in the samples in presence of indoor or outdoor dust.
Comparison tests were performed on indoor and outdoor samples under the same test conditions and dust deposition density. Bode plots showed that indoor dust was more sensitive to the change of relative humidity compared to outdoor dust. We believed the difference is due to weight percentage and species of the dominant water soluble salts and dust particle sizes in the dust sample.
At the same dust deposition density, we observed that test coupons with indoor dust are more susceptible to induce moisture related failures. These failure mechanisms include loss of surface insulation resistance between electrodes, electrochemical migration, corrosion, etc. In the real life usage conditions, indoor dust often has much lower deposition density compared to outdoor dust on the printed circuit assemblies due to the usage of air-filtering system in buildings. We should put real use-case scenarios into consideration when evaluating the impact of dust.
Initially Published in the IPC Proceedings
Comments
I highly recommend your readers to review Mil-Std-810G section 510.6 for sand and dust testing. It sa lot of insight on different failure modes and what to watch for. This was required by one of our customers.
George Kopacz, PNY Technologies, Inc.
An interesting paper. However, IMO, airborne particles are not the worst dust contamination but particulates in the printed circuit manufacturing and assembly processes. These are still very common because of the mechanical operations and these are exacerbated by encapsulation under solder masks and conformal coatings. I have previously evoked these problems in various publications. The worst airborne cases I found was with dust resulting from sea spray, ~50 km inland under gale conditions in Brittany and that thrown up by tyres from passing vehicles within an hour after the road was de-iced with salt, found indoors some tens of metres from the road.
That having been said, if an assembly is likely to malfunction in service because of airborne dust - no matter the origin - then it should be in a hermetic housing or, at the very least, be conformally coated. I live in a country where particulate contamination from various sources is common; as I write this, visibility is under 1 km due to dust, a mix of exhaust pollution, tyre and road dust, mineral dust from the Sahara, atmospheric photochemical reactions etc.
