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REAL TIME WITH... IPC APEX EXPO 2019 SHOW & TELL MAGAZINE I I-CONNECT007 89 For instance, one of the air quality index mea- surements is PM 2.5 concentration, which is the concentration of dust is smaller than 2.5 microns. You also have PM 10, which is the concentration of dust smaller than 10 microns. In a few minutes, we will talk about why PM 2.5 is more important for electronic products. Due to the ubiquitous presence of dust in the environment, it's unavoidable for the dust to deposit onto the electronic equipment. Even so, our products are designed to tolerate cer- tain degrees of dust deposition. Following the Telcordia standard, we need to do a qualifica- tion test to make sure our products can sur- vive in certain dusty environments. If the dust concentration becomes too high, then it can impact the quality, performance, and reliabil- ity of the product. There are different ways how the dust can impact the reliability of elec- tronic products mechanically, chemically, or electrically. For instance, if you have some moving parts like fans in the electronics, the dust accumula- tion can interrupt the motion of the moving parts. Also, it can impact the heat transfer, thermal performance, and cooling efficiency of your product if you have dust deposition on a heat sink. In terms of the chemical way, because of the composition of the dust, typi- cally, it contains ionic compounds such as salt or some other corrosive agent, which can cor- rode your product. Electrically, for a water-soluble com- pound like ammonium sulfate, nitrate, and any kind of ionic salt compound, when the humidity becomes high enough or above the critical relative humidity, the ionic compound can pick up moisture from the environ- ment and form an electrolyte solution, which is conductive. This can cause impedance reduction on the surface and all kinds of issues such as electri- cal failures and so on. Holden: I know what you're talking about. I once had to troubleshoot a problem at Hewlett-Packard when var- ious RF amplifiers they had designed would stop working at certain times during the summer. When we finally found out that the designer didn't take into account the humid- ity effect of the dielectric constant of laminate, and when the humidity got very high in Louisi- ana, the dissipation factor would go up. Then, the RF amplifier wouldn't work, but if we took it back into the building in a laboratory, then it would work. But out in the field, it wouldn't. It wouldn't work only during certain times of the year. Not taking into account the humid- ity effects on laminate materials is why people use conformal coatings and things like that. Xu: Right. That is exactly the problem we are facing with the dust. It very often becomes a no-trouble-found issue. In the lab, when you get the product returned, you do diagnostics, and it works just fine. Eventually, we real- ized if you run the testing at the high relative humidity, then you can reproduce some of the failures. Holden: Now, it's interesting because you have a co-author—Jason Stafford of Imperial College London. Since he's not here, can you explain his contribution? Xu: When I first observed this preferential deposition of dust at certain locations of the circuit board, we had some speculation on why it happened. Then, we thought we needed

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