A Rework Dilemma: PCB Shields

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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 interfering with neighboring devices or other systems 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 device PCBs such as smartphones, netbooks, tablets, 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 components 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 limited to:

1. Devices are very tight and close to the shield itself—many times within a few millimeters. 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 require extreme dexterity as the distance between the land of the shield and neighboring devices is very small. This means that neighboring devices can be disturbed.

Resoldering of the shields can be very challenging as the gap between the shield lip and the board needs to be closed so as to not allow for any ingress or egress (in some cases) of RF signals. This means there is little room for error.

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Editor's Note: This article originally appeared in the July 2015 issue of SMT Magazine.


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