SMT007 Magazine

SMT-Apr2015

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68 SMT Magazine • April 2015 figure 2: Materials used for shielding study, clockwise from upper left: copper tape, ceramic non-woven, clay/water, stainless steel and polyimide tape. are typically qualified at 260°C for 40 seconds (though component body temperatures can reach 260°C during reflow). This work examined the current state of shielding options that rework technicians have in protecting neighboring devices. The purpose of this study was to determine the shielding ef- fectiveness by measuring the temperature at se- lect neighboring devices based on a set distance from a rework source location. The options in- cluded polyimide tape, 304 stainless steel, and copper tape with an adhesive backing as com- mon rework shielding materials. Non-tradition- al materials such as a ceramic non-woven fiber and an organic water/clay gel, both of which are utilized outside of the electronics industry, were also part of this study. While protection against conductive heat transfer will have to be left up to the design en- gineers, shielding against radiation and convec- tive heating sources is a function of the shield material properties. Shown below in Figure 1 is a list of the materials used in this study as well their respective thermal conductivity values at 300°C. From the chart one would expect ceramic fiber to be the best performing thermal shield material. There are a variety of materials (Figure 2), namely stainless steel, copper tape, polyimide tape, clay/water gel or a ceramic non-woven material which can shield the aforementioned heat energy from being transferred from the re- work area to other areas of the PCB. Stainless steel metal shields are designed to shield a com- ponent from absorbing excessive heat either by dissipating, reflecting or simply absorbing the heat. The physical properties of stainless steel's reflectivity and emissivity, thermal conductiv- ity and specific heat capacity make it the ideal material for the fabrication of heat shields. The copper tape allows the copper shield to be flex- ible and be easily applied to a PCB in the rework area. The adhesive side allows it to adhere to a board surface and stay tacked down. Polyimide tape is the most commonly used (and misused) methodology for masking areas on a PCB. The ability of polyimide tape to maintain excellent physical, electrical, and mechanical properties over a wide temperature range makes it an ideal "duct tape" for the PCB assembly industry. The one drawback with this material is its relatively poor thermal insulation properties. The clay/ water gel shielding material is a product which Figure 1: Thermal conductivity Watt/m K @ 330°C. FeAture pOlyiMiDE TapE iS NOT THE ONly aNSWEr DuriNG rEWOrK continues

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