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April 2015 • SMT Magazine 69 has been used in welding applications due to its extreme resistance to heat. The gel contains de- ionized water and clay. The product is easily ap- plied and, due to its consistency, able to work its way into small spaces on a PCB. The gel residue can simply be "washed off" using water. Ceram- ic fiber non-woven shields, previously relegated to aerospace, nuclear energy and high tempera- ture processing industrial environments, offer users several properties which makes it well- suited for PCB rework shielding. This ceramic fiber material offers several advantages includ- ing the properties of high-temperature stability, low thermal conductivity, high heat reflectance and the ability to be easily wrapped and cut to shape. These final (2) materials are not com- monly used in the PCB assembly area but are new materials requiring further testing of their impact. Experimental procedure In order to determine the thermal shielding effectiveness of various materials, a controlled heat source simulating a rework process was placed onto an IC reference site of a (10) layer mixed technology printed circuit board. The board was fabricated with a SAC305 solder alloy with a HASL finish. A reference site component (U3) was then chosen as the sample rework lo- cation. It was not connected to the same cop- per layer as the components of interest. Various heat-sensitive components on the board were then identified, and the distance from the refer- ence site to these locations was measured with a Figure 3: Sample output reflow profile. FeAture pOlyiMiDE TapE iS NOT THE ONly aNSWEr DuriNG rEWOrK continues