SMT007 Magazine

SMT-Nov2014

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14 SMT Magazine • November 2014 rework challenges The removal of a glued component from a PCB assembly requires a specific order of opera- tions. The first step is to remove the glue fillet (Figure 1) located between the component and the circuit board. Mobile products generally have many types of components glued to the PCB in order to meet the industry standards outlined in JESD22-B111 Board Level Drop Test Method of Components for Handheld Elec - tronic Devices and JESD22-B110 Subassembly Mechanical Shock. The epoxy is applied to the components to prevent the common failure modes of cracks in the laminate, cracks near the intermetallic, and cracks in the bulk solder. The addition of the epoxy increases the robust- ness of the design and enhances reliability for the user. One of the issues associated with underfill epoxy is the glass transition temperature of the material. The glass transition temperature is the temperature at which a sudden change of phys- ical properties occurs. In the case of underfill epoxies, they generally soften at approximate- ly 208°C (exact temperatures dependent on manufacturer's specifications). Lead-free solder, used to form an intermetallic bond between the component and the pads attached to the circuit melts at 217°C with peak temperatures of 235°–245°C common. The closeness of the epoxy and solder temperatures creates a narrow window of opportunity for removal of the ep- oxy and removal of the component. Removal of the surface mount device requires the removal of the epoxy fillet from around the sides of the device without damaging or disturbing the ad- jacent discrete components. Failure to remove the fillet increases the chances of damaging or removing the adjacent components (Figure 2) adding time and cost to the rework process. rework cHaLLenges For smartPHones anD tabLets continues feaTure Figure 1: SMD with epoxy fillet. figure 2: adjacent components within 10 mils of SMD. Figure 3: Densely packed components commonly in use for mobile devices.

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