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66 SMT Magazine • July 2015 by Bob Wettermann bEST inC. KNOCKING DOWN THE BONE PIlE A Rework Dilemma: PCB Shields Column RF shields minimize radio frequency (RF) noise to prevent it from affecting the sensitive and critical electronic components beneath the shield. They also prevent such noise from in- terfering with neighboring devices or other sys- tems in the vicinity. RF shields typically have a unique design and conform to the layout of the PCB. Often, the shields are not regular- shaped designs; rather, they are designed and are shaped by the PCB layout. Typically these shields can be found on handheld wireless de- vice PCBs such as smartphones, netbooks, tab- lets, portable medical devices and audio/video players, to name a few. There are two basic construction types for these shields. Some of them are a two-piece construction with a "fence" soldered to the PCB and a cover fitted over this fence. Other shields are a one-piece construction with an open-sided can overlaying the compo- nents and soldered to the PCB. These one-piece shields are a serious challenge to rework and will be the focus of this rework discussion. Challenges of Shield Rework There are numerous challenges associated with reworking shields, including, but not lim- ited to: 1. Devices are very tight and close to the shield itself—many times within a few millime- ters. This means that the rework process, if not precisely controlled, can present problems in disturbing nearby devices. 2. Shield trace solder excavation and PCB board prep are challenging as well, especially for odd-shaped shields. Manual methods re- quire extreme dexterity as the distance between the land of the shield and neighboring devices Figure 1: Example of multiple shields on handheld wireless pcb. Figure 3: Example of one-piece shield. Figure 2: Example of two-piece shield.

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