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92 I-CONNECT007 I REAL TIME WITH... IPC APEX EXPO 2019 SHOW & TELL MAGAZINE then the dust deposition rate becomes much higher. When you design your circuit board or cooling process, you need to take that into consideration. You want to try to keep the flow parallel to the surface as much as possible. Also, people typically use air filters to reduce the dust infiltration. One of the downsides of using air filters is the air restriction due to the air filter. You will have some air pressure drop. For example, in a typical air filter with a MERV rating of 7 or 8, we will reduce the air flow by a factor of 50%. That would certainly reduce the cooling efficiency. Then, you need to increase the air flow and increase the fan speed, which means higher energy costs. When you select an air filter, you have to strike a balance. On one hand, you want to have a high enough efficiency to reduce the dust concentration, but you also want to mini- mize the air flow reduction. It's about finding a balance between the two. What we also found was that when you use air filters, it actually increases the dust deposition by changing the air flow pattern, so you have to consider that. This work also highlighted the importance of maintaining a low concentration of PM 2.5 dust at the data center, and the most practical and cost-effective way for removing PM 2.5 is at the room level. Holden: I think it's important for a lot of designers of mechanical enclosures to read this paper because in your conclusion, you prove that putting in air fil- tration could actually make the problem worse instead of bet- ter. That is not common knowl- edge or intuitive. Xu: Correct. Holden: Why did you single out 2.5 microns? Xu: There were a couple of reasons. If you look at the composition of the particles, typically, the particles can be divided into larger and smaller particles. The larger particles are typically par- ticles larger than 2.5 microns, and smaller par- ticles usually have a diameter smaller than 2.5 microns. That's called PM 2.5, and PM 2.5 par- ticles tend to contain a higher content of water soluble ionic compound between 20–50%. For the larger particles, the ionic compound con- tent is much lower between 5–20%. Due to the higher ionic content, PM 2.5 particles are more detrimental to electronics than the large par- ticles. Also, PM 2.5 particles are much more difficult to remove than large particles using air filters. Thus, PM 2.5 particles have the most impact on electronic products. Holden: Thank you for your time, Dr. Xu. Hope- fully, we can look forward to publishing your paper eventually. If people don't have the IPC proceedings and want to contact you, where can they reach you? Xu: You can email me at chen.xu@nokia-bell- labs.com. Holden: Thank you very much. It was good to see you again, and congratulations on your award. Xu: Thank you. S&T