3G Causes Huge EMI Headaches for Handset Designers

As mobile phones begin to pack more functionality, the need for good shielding schemes becomes vital during the design process.

Contents
What to do?
Metal cans
Size/weight issues for metal cans
Closing the housing gap
Pre-formed gasket problems
Form-in-place
Conductive paints
Emerging shielding techniques
Surface-mount gasket technology
The early bird wins

By Robert Keenan, Managing Editor

The dawn of third generation (3G) wireless technology is upon us. With higher data rates being installed and standards being ironed out, the wireless industry is racing toward the next age in its existence.

The emergence of 3G wireless systems will bring many changes to the wireless market, especially at the handset level. Handsets, that once provided only voice connectivity, will now deliver web browsers, advanced messaging services, personal digital assistant (PDA)-type operations, e-mail access, and other data-centric wireless functionality.

With additional functionality, comes increased challenges at the design level. Engineers must pack increased functionality into their handset designs while continuing to decrease the size and weight of their products. But, by packing more functionality into these smaller footprints, engineers are also opening themselves up to increased electromagnetic interference/RF interference (EMI/RFI) problems (Figure 1).

Joe Rowan, global business leader for W.L. Gore and Assoc. (Newark, DE), says that engineers are trying to place 10 lb of stuff in a 5 lb bag. "Something has to give. EMI is one of those things."

Ray Borgersen, marketing manger at EMI shielding manufacturer Tecknit (Cranford, NJ) agrees. "Today's wireless engineers face an extreme design challenge when providing EMI/RFI protection in their systems."

What to do?
Traditionally, engineers have employed a host of techniques to counterattack EMI problems. These approaches included metal cans (a.k.a. Faraday cages), filtering techniques, pre-formed parts, metal fingers, conductive paints, grounding planes, spacer gaskets, and form-in-place approaches.

Today's engineers have an array of EMI shielding approaches to choose from. And as time goes on, they will have even more techniques to select.

Metal cans
One of the most common shielding approaches used in wireless devices today is the metal can. Under the approach, the printed circuit board (PCB) within the wireless handset is segmented into sections. Metal cans are then soldered on top of these sections to prevent EMI from leaking out into the rest of the system, causing crosstalk and compliance problems.

Although good at curbing EMI, the metal can approach faces a few problems. The first is maintenance.

By soldering metal cans to the PCB, Rowan says design engineers will find problems inspecting and accessing the components underneath. Therefore, if a problem occurs underneath the metal can, engineers may have a difficult time correcting that problem.

Palatine, IL-based Boldt Metronics International (BMI), which develops metal cans for wireless designs, has identified these past maintenance problems. According to Jack Black, director of sales, the company has developed several approached to help eliminate this problem. First, according to Black, the company has developed a one-piece shielding product that can be easily removed when heated up. Thus, to access a component, the engineer heats the one-piece shield and then pops it off.

In many situations, however, the extra steps of heating the shield can be cumbersome to the system designer. To avoid these problems, BMI has also developed a two-part shield that allows engineers to remove a cover on the metal can to access the components underneath. Once the repairs are finished, the engineer pops the shield back on and continues working.

According to Black, these approaches eliminate many of the maintenance concerns generally associated with metal cans.

Size/weight issues for metal cans
Some other concerns for engineers employing metal cans is size and weight. According to Joe Butler, marketing manger at Chomerics, metal cans can increase the size and weight of a wireless product. Therefore, as wireless systems move to smaller and lighter footprints, Butler says metal cans become a less attractive solution during the design process.

BMI's Black says size and weight are not as much of a drawback as people think when choosing cans. Through the proper design of holes and apertures, Black says manufacturers, such as BMI, can develop surface-mount shields that weigh next to noting.

One final concern with cans is manufacturability. Traditionally, metal cans were placed on the PCB using a second soldering approach. Therefore, the components on the board are exposed to heat for a second time. According to Butler, this second exposure can degrade the performance of the components, and in turn, degrade the performance of the overall wireless handset.

To avoid these manufacturing problems, some manufacturers, such as BMI, are developing shields that can be integrated with the handset manufacturers current pick-and-place process (Figure 2). By integrating the metal cans into the current pick-and-place process, BMI says engineers avoid running PCBs through the soldering process a second time. In addition, this approach eliminates any manual labor associated with placing other shielding techniques.

Closing the housing gap
One of the hot EMI areas in a handset occurs where the PCB and plastic shield housing meet. If any spaces or gaps occur at this interface, radiated energy can seep out and cause standards compliance and functionality problems.

To eliminate interference in this problem area, engineers can apply a gasket at this shield interface. Right now, there are two common approaches to developing these gaskets. These two techniques include pre-formed parts and form-in-place approaches.

Under the pre-formed gasket approach, which is employed by companies like W.L. Gore, a gasket is precisely die cut to the shape and contour of the plastic shield housing and supplied on a release liner. The gasket is then transferred onto the shield, and the shield is subsequently held in place against the PCB using screws (Figure 3).

According to Rowan, W.L. Gore can lay a gasket, with a thicknesses down to 10 mils thick, on a housing that is as narrow as 30 mils wide. A shield of that size, Rowan says, can still deliver high levels of shielding with proper design.

Pre-formed gasket problems
There are a few problems with the pre-formed gasket approach. First, according to Butler, companies developing pre-formed gaskets can not develop products that offer thicknesses down to the levels of other gasketing approaches, such as form-in-place. In addition, Butler says placement is a big concern. If pre-formed gaskets are not properly placed, they may not provide the shielding effectiveness needed in today's designs.

To address the placement problem, W.L. Gore has developed three automated installation platforms, called Staccato, Allegro, and Crescendo. According to Rowan, these systems use a vacuum fixture technology to pick up a pre-formed gasket from a release liner and attach it to a housing. According to the company, the fully-automated-automated version of this approach accurately places the gasket on the handset at high speeds, eliminating the placement concerns associated with pre-formed gaskets.

Form-in-place
For designers looking to avoid the pre-formed gasket approach, form-in-place technology maybe a good solution. Under the form-in-place scheme, a liquid conductive elastomer is robotically dispensed, in a bead form, onto the housing of the handset in. Once in place, the housing is placed into an oven where the gasket is cured (Figure 4).

According to Butler, Chomerics has been ale to deposit an elastomer on a flange that is as small as 30 mils wide. In addition, he says that form-in-place gaskets can deliver a shielding effectiveness of 80 dB or better.

One of the main drawbacks to form-in-place technology is shipping. In order to employ form-in-place gaskets, engineers must ship products to an off-site location where the elastomer is applied and cured. This additional shipping can create huge expenses and slow down the manufacturing process.

The curing process also causes problems for the handset designers. Form-in-place gaskets have a tendency to run or slump during the curing process. In addition, the extreme heat used in the curing process can change the housing, causing new EMI problems.

Chomerics and other companies employing form-in-place technology are addressing both of these concerns. To prevent the curing problems, form-in-place manufacturers are turning to materials that can be cured using lower-temperatures.

On the manufacturing side, form-in-place manufacturers are setting up manufacturing facilities throughout the world. Thus, they are providing more localized manufacturing support which greatly reduces shipping costs and speeds manufacturing time.

Conductive paints
In addition to gasketing and metal can technologies, today's system engineers are also using conductive painting techniques to improve EMI shielding performance in their handset designs. Under this approach, which is mainly used when a plastic housing is involved, manufacturers robotically apply a conductive paint to the inside of the housing. This solvent-based paint is designed to prevent radiated energy from escaping the handset. Similar to the form-in-place approach, once the paint is applied, the housing is then cured so that the paint will dry.

Overall, Butler says the painting is a good approach to providing shielding. He says that painting delivers a good shield that is only a few mils thick. In addition, it is straightforward to do and easy to apply.

But, painting is not a full replacement other shielding techniques. According to Butler, painting is one method in the overall shielding scheme.

Emerging shielding techniques
Similar to other areas of design, handset manufacturers are continually trying to find new approaches to solving EMI problems. Therefore, EMI manufacturers are continually improving processes and developing new techniques to shield wireless products.

Conductive-loaded housings are one approach members of the EMI industry have explored over the past few years. Under the scheme, manufacturers will develop a plastic housing that has conductive elastomer material molded into it.

By integrating the shield directly into the housing, many engineers feel they can eliminate many of other shielding products housed in a wireless product. The problem is, manufacturers have not found any success developing conductive-loaded housings.

According to Borgersen, engineers have not perfected the conductive-loaded plastic housing approach. He says that engineers are having trouble evenly dispersing conductive materials throughout the plastic housing.

Surface-mount gasket technology
Another approach W.L. Gore is exploring is a product that combines its PTFE shielding technology with a surface-mount platform. According to Rowan, W.L. Gore hopes its efforts in surface-mount gasketing will deliver products that deliver low installation cost while allowing the original equipment manufacturer (OEM) to control the EMI installation process.

The early bird wins
One thing is clear, as wireless functionality increases in handsets, EMI problems will also increase. Therefore, the only way for a system designer to effectively stop interference problems is by dealing with EMI issues in the early stages of a design. If an engineer waits too long, they may find themselves employing costly solutions at the end of a design, or, even worse, scrapping a whole design all together.

About the Author…
Robert Keenan is the Managing Editor of Wireless Design Online (http://www.wirelessdesignonline.com). He can be reached at 3257 N. Sheffield, Ste. 109, Chicago, IL 60657, Tel: 773-477-3574, Fax: 773-477-3904, e-mail: rkeenan@wirelessdesignonline.com.