Assembly Innovations: Plastic EMI Shield Outperforms Metal
DEK International (Flemington, NJ) and W.L. Gore & Associates Inc. (Newark, DE) have developed a new board-level, electromagnetic interference (EMI) shield for wireless communications devices.
Dubbed the Snapshot, the lightweight, removable shield is made from a metallized, high-temperature plastic that can be thermoformed into intricate shapes and multicavity configurations. The plastic shield weighs 80 percent to 90 percent less than a metal shield of similar size.
The plastic's electrical insulating properties allow engineers to design printed circuit board (PCB) assemblies with virtually no gap between the components and the EMI shield. It also enables engineers to reduce the minimum spacing between components by 25 percent to 50 percent. In cavity-to-cavity testing of shielding effectiveness, the Snapshot outperformed metal cans and removable metal lids.
The shield is secured to the PCB with snap fits. A series of holes around the shield's perimeter snap over solder spheres that have been reflowed onto the PCB. The spheres provide both mechanical and electrical connections.
The solder spheres are placed on the board in a two-step process. First, a screen printer deposits solder paste onto pads that have been defined by solder mask. Then, a second printer places the solder spheres into the paste through a stencil. The spheres are contained in an enclosed transfer head. The head can place solder balls as small as 0.3 millimeter in diameter onto substrates or wafers with fine-pitch accuracy and a first-pass yield of more than 99 percent. The head can hold 50 million solder balls.
Once the PCB has been fully populated with components, it proceeds to reflow, where both the components and the spheres are soldered in place. After reflow, the Snapshield can be installed manually, semiautomatically or fully automatically using a standard cell for odd-form component assembly. If necessary, the shield can be removed and replaced with a new one, without desoldering or resoldering the board.
For more information on EMI shields, call W.L. Gore & Associates Inc. at 888-914-4673, visit www.wlgore.com, or Reply 21. For more information on solder sphere placement equipment, call DEK at 908-782-4140 or visit www.dek.com.