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November 2014 • SMT Magazine 21 and is usually split up into 4 or 8 quadrants; this helps stop the package swiveling during the re- flow process and ensures solder coverage to be equal. Most manufacturers require 60–80% sol- der contact to the PCB ground pad in the cen- ter. This ground pad keeps the package cool and stops overheating, as heat is transferred to the PCB via the center ground pad. Special 0.4 mm pitch stencils have been developed to better align the package to stencil print locations. QFN/BTC with a pitch of 0.5 mm have been in mobile de- vices since 2005 and are much easier to rework. QFNs/BTCs with a 0.4 mm pitch are the most dif- ficult and challenging. These components often have two rows of connections on the perimeter. Each type of component requires careful consid- eration in regards to the techniques needed to place the part on the circuit board. Once the part is placed on the circuit board, the new components require the development of thermal profiles. Thermal profile repeatabili- ty is very important. The thermal profile should start with a linear ramp rate of 0.5°C–2.0°C/ second which allows the gradual evaporation of volatile flux constituents and prevents defects such as solder balling/beading and bridging as a result of hot slump. It also prevents unneces- sary depletion of fluxing capacity when using higher temperatures. Next, an optional soak stage between 160°C and 180°C may be implemented for a few rea- sons. This includes minimizing the ΔT across the board to <10°C and allows for full volati- zation of the flux solvents to reduce flux in- duced voiding. Finally, the recommended peak temperature is 225°C–235°C and the total time above liquidous (TAL) should be less than 120 seconds above 217°C. A rapid cool down of 4°C/ second is desired to form a fine grain structure. Slow cooling will form a large grain structure, which typically exhibits poor fatigue resistance. If excessive cooling 4°C is used, both the com- ponents and the solder joint can be stressed due to a high coefficient of thermal expansion (CTE) mismatch. Fixtures can be used to improve pro- cess yields. Fixtures are very important in mo- bile device rework due to the size and shape of many mobile device PCBs. A fixture will ensure consistency by locating the circuit board in rela- tion to the heating source (Figure 11). The fixture should be low mass and made of a material which does not act as a heat sink. A metal fixture will absorb thermal energy dur- ing use and will alter the profile. Proper circuit board profiling and the right fixture will ensure a repeatable process. After the part has been reflowed, the circuit board is tested. Testing at this stage ensures the reworked component functions properly before applying the epoxy. Typically, the PCB will be subjected to detailed inspection and a rework cHaLLenges For smartPHones anD tabLets continues feaTure figure 9: PoP/Bga paste dip. figure 10: Multi-row QfN/BTC.