Medical Device Link.

Originally Published MEM Fall 2004

SHIELDING

A thermoformed, metallized plastic shielding offers an alternative to gasketing and metallic shielding enclosures.

Ross Livington

Portable handheld wireless devices used in today's medical applications feature smaller form factors that weigh less. These devices include wireless PDAs, laptop data-connect cards, and handheld data-connection devices. The increasingly higher frequencies that these devices operate at, combined with components that are closer to one another on the printed circuit board (PCB), creates a potential problem with crosstalk and electromagnetic interference (EMI). So, the ability to better isolate the components and improve EMI shielding has become more important than ever.

Shielding PCB components can be accomplished is several ways. In all methods, the ground plane of the PCB makes up one side of the six-sided metal enclosure required to create a Faraday cage or shield (an electrical apparatus designed to prevent the passage of electromagnetic waves). One technique uses the housing of the device as the EMI enclosure.

The cavities on the inside of the cover are metallized on the surface and electrically sealed to the PCB via a conductive elastomeric gasket. The front and back housing are brought together by mechanical fasteners, which apply the necessary force to the gasket interface. However, when using this method, the device is not completely functional until the PCB and covers are assembled together.

Another shielding method involves the use of metallic enclosures that attach directly to the PCB, resulting in a finished PCB that has the shielding integrated into it. This method does not require that the device housing be part of the shielding solution. The PCB-level shielding technique has primarily employed solder-attached perforated metal cans (with and without removable lids).

PCB Manufacturing and Shielding

Surface-mount technology (SMT) is the preferred manufacturing technique for assembling virtually all modern handheld wireless devices. This process involves screen printing solder paste down onto bare PCBs, picking and placing the components onto the board, and then running the boards through a solder reflow oven.

Metal cans used as shields are normally stamped with a hexagonal series of round holes (or apertures). These holes provide more even heat transfer during reflow. Typically stamped from sheet metal and formed into individual rectangular boxes, the cans are mechanically and electrically attached to the PCB using the same solder process as the components. The cans are placed over the components as the last step before the board goes through the reflow oven.

Because the cans cover the components during the solder reflow process, this may have adverse effects on the solder joints of the components underneath them, which can result in cold solder joints. Once the can is soldered in place, it greatly hinders the ability to inspect component solder joints after reflow and to rework the area if it is faulty. To work around this problem, some metal can manufacturers have incorporated removable lids into the can design. This feature often adds size and complexity to the shield and could compromise the shielding effectiveness of the can. Also, depending on how well the lid fits on the can's frame and the snapping mechanism used to hold it in place, there may be issues associated with the lids popping off during mechanical, shock, or vibration tests.

As designs become more compact with higher density and lighter weight, however, stamped metal cans are becoming less attractive as a shielding option. As the components are forced closer together, the ground traces separating these components are necessarily narrower. This makes the use of multiple, single-cavity cans impractical when several cavities are required. And, the boxy shape of these shielding cans does not work well with the low-profile, more stylish contours of today's modern device housings. These factors make an alternative to stamped metal cans desirable. One recently developed alternative is a thermoformed, metallized plastic shielding technique that solves the problems associated with gasketing and metallic shielding enclosures.

Improved Board-Level Shielding

Figure 1. Metallized plastic material can be formed into any shape.

The recent development of a thermoformed, metallized plastic shield offers the promise of overcoming the problems associated with both types of shielding. It also improves performance over typical metal cans. Called SnapShot, this new technology incorporates a high-temperature, high-performance plastic shield material that has been metallized to provide the requisite EMI shielding.

When the material is formed into a shield, it can weigh 80–90% less than a metal can of the same size: 0.3 g versus 1.5–3.0 g for 0.1–0.2–mm gauge steel. Because the metallization is on the outside and the insulating plastic base material is in contact with the PCB, a lower profile can be achieved by virtually eliminating the component-to-shield gap. It also reduces minimum component-to-component distance by 25–50%, freeing up valuable space on the board.

For design flexibility, the material can be formed into virtually any shape, rather than being limited to rectangles. It can incorporate multiple cavities and accommodate intricately shaped components and devices (see Figure 1). The benefits and cost of the metallized plastic is similar to metal cans; the primary difference is in the installation process.

The Snap-Attach Mechanism. Instead of being soldered directly to the PCB, a shield made of the new material uses a snap-attach mechanism that allows it to be placed on the board after reflow. It can be removed easily for rework, if needed, and a new shield can be snapped back into place.

Figure 2. Cutaway drawing of snap-on shields. (click to enlarge)

A series of solder spheres is deposited into solder paste on the board around the outline of the shield and reflowed along with the components. Each shield has a series of holes around its perimeter that snaps in place over the solder spheres on the PCB. The spheres provide both electrical connection to the shield and mechanical retention on the board (see Figure 2).

Process Implementation. To deliver such benefits to devices produced at high volumes, the new product uses a high-speed, high-volume method to mount ball-grid array (BGA) spheres to PCBs. During prototyping and preproduction cycles, the BGA spheres can be attached to the PCB using standard SMT equipment. The BGA can be supplied in EIA 481-tape format or in bulk for use with surface-mount bulk-feeding equipment. These methods are effective for quick and successful implementation of the process.

To develop a high-volume implementation process, the developers of the shield formed a joint applications team with an industry equipment supplier to produce an effective process for the new product. Although the system was initially designed for use by semiconductor manufacturers to place spheres into flux on wafer surfaces, it operated on a platform that was a familiar piece of SMT equipment: a screen printer. The system, designed within the framework of a screen printer, consists of an enclosed reservoir for solder spheres and a distribution device.

As the system traverses the stencil surface, the spheres exit the reservoir and migrate into the distribution device, where a positive placement force ensures that they are securely deposited onto the substrate. In the course of developing the new process, the only modifications required were in stencil design. One stencil had apertures for solder pads (to hold components) and for solder dots on which the spheres would be placed.

A second stencil was electroformed with apertures in a pattern designed for the solder spheres only. A laminate layer on the underside of this stencil created an offset that allowed the spheres to be placed into the solder paste without affecting the solder pads on which the components would be placed.

Placing Pads, Dots, and Spheres. To implement the process, two screen printers are placed together at the start of an SMT assembly line. The first deposits solder onto surface-mount device pads for both components and spheres. A conveyor moves the board to the second screen printer, which places the solder spheres into the solder paste. The board is conveyed to component placement and then reflow, where both components and spheres are reflow-soldered to the board.

After inspection, a semiautomated installation device is used to attach the shields onto the spheres. The installation could also be performed manually for prototype or low-volume applications. For high-volume production, this task could be automated using a standard SMT odd-form cell, along with tape-and-reel or tray-packaged parts and a task-specific modular placement tool.

Following the success of the development team's initial efforts, the next step was to verify the new procedure under actual production conditions. A feasibility study conducted at the facilities of a major multinational electronics manufacturing service provider confirmed that the process was a viable solution for EMI shielding attachment in an SMT assembly environment.

A New Shielding Technology

This snap-on shielding device solves many of the problems associated with current PCB-level shielding solutions, such as soldered cans. The fully integrated shielding consists of a metallized, thermoformed shell that is attached to the PCB using BGA solder balls as snap-attachment features. The shell is removable. This technology offers a PCB-level shield that is applied to a populated board after it has gone through the solder reflow process. This enables manufacturers to inspect and rework the PCB unimpeded by the shield.

The inside of the shield is nonconductive plastic, whereas the outside of the shield is fully metallized. The BGA balls make contact with this outside surface by snapping through holes in the shield, thereby electrically and mechanically connecting it to the PCB. The periodicity of the ball placement is determined by the highest-frequency component and performance required. Because the inside of the shield is insulative, any components that come into contact with the shield's inner surface are not electrically shorted.

Figure 3. Shielding-effectiveness data of the snap-on shield. (click to enlarge)

This shielding can be thermoformed into just about any shape, profile, or contour. Multiple heights, rounded edges, and curved shapes can all be readily incorporated into a shield design. After the shield is attached, it can be removed easily if rework or repair to the components is needed. The shielding's flexible nature makes the coplanarity of the PCB significantly less critical than with standard metal cans. Shielding-effectiveness (SE) testing shows excellent SE data for this new technology (see Figure 3). The SE is about 60 dB through 9 GHz.

Figure 4. Shielding-effectiveness data of removable-lid perforated can. (click to enlarge)

For comparison, five equivalent-sized perforated metal cans with removable lids were evaluated using the same test method. The can was completely soldered around its perimeter to the test PCB, which represents a best-case scenario. Figure 4 shows that the SE data for the metal cans is far inferior, with much less consistency and far more visibility, than the results for the snap-on shields.

The snap-on shield provides approximately 30 dB better SE than the removable-lid metal can. Part of this difference is attributable to the fact that the metal can has apertures in its shield and seams between the lid and the metal frame. The snap-on shield, by contrast, is constructed from a solid sheet of metallized materials and does not have any perforations on its surface. Also, the snap-on shield's BGA attachment mechanism provides a robust mechanical and electrical connection all along its perimeter.

In accelerated life testing (ALT) and thermal shock testing, the shield and its snap-attachment mechanism were shown to provide greater overall and cavity-to-cavity EMI shielding effectiveness than metal cans with holes or removable-lid metal cans. Even when every other solder sphere was removed (to simulate possible effects of a missing solder sphere), the shielding effectiveness of the board-level snap-on shield outperformed typical metal cans.

Conclusion

Portable wireless device designers—particularly those developing medical-related applications—face significant challenges in the area of PCB-level shielding. Challenges include size, weight, geometric flexibility, and performance. The ability to evaluate the shielding performance of board-level shields is critical to developing today's wireless products.

This new PCB-level shielding technology solves many of the problems that designers have had to deal with when using traditional stamped metal cans. The metallized plastic snap-on EMI shield provides a high-performance, lightweight, cost-effective alternative to perforated metal cans. In demanding applications, such as portable medical wireless devices, this new shielding offers design engineers a new level of flexibility that has not been possible until now.

Many of the problems associated with perforated metal cans—such as inspection and rework of PCBs and the extra weight of the cans themselves—are no longer an issue with the new thermoformed snap-on EMI shielding. Designers can now be assured of exceptional shielding performance at high frequencies, flexibility in shape and profile, and a rugged and reliable attachment mechanism. As wireless medical devices continue to shrink in size and increase in complexity, this new PCB-level shielding technology will make it easier for design engineers to stay on top of industry trends and provide an effective shielding solution.

Ross Livington is global product manager for W. L. Gore & Associates (Elkton, MD). He can be reached at 410-392-3800 or via e-mail at rlivingt@wlgore.com.

Copyright ©2004 Medical Electronics Manufacturing