Laser Soldering Applications for RF Shield Rework -- By using one-piece RF shields and lasers for rework, throughput can be increased while significantly reducing the number of rework technicians.

Continually shrinking package sizes, reduction of pitch and reduction of clearance areas available around components have made rework of radio frequency (RF) shields increasingly difficult. RF shields are used to separate areas of a printed circuit board (PCB) assembly from each other and to reduce noise and prevent radio signals from escaping from the product and affecting the operation of other devices. Antenna products, cell phones and wireless Internet electronics require some RF shielding that is either mounted to the PCB or incorporated into the packaging that surrounds the electronics (Figure 1).

More demanding process requirements and the increasing complexity of PCBs have prompted the development of new techniques for removing and replacing RF shields. Photonic soldering is one new technique that offers advantages for this application. Briefly, photonic soldering is a method of heating and reflowing solder using the energy of photons generated by a laser source.

With photonic soldering, the surface temperature of an RF shield is precisely controlled and monitored using a pyrometer, which is linked via computer to a laser that is controlled in a precise pattern to reflow the solder at the base of the RF shield. This technology reduces the amount of tooling required for soldering shields of different geometries, prevents the reflow of adjacent components and components under the shield, and increases the speed at which rework may be performed.

RF Shielding Methods

Two common methods are used to attach shielding to a PCB. Both methods attempt to completely cage a specific area by soldering to surface connections to ground planes internal to the PCB. In one method, a metallic "fence" is attached around the perimeter of the area to be shielded. A spring-loaded metallic "lid" is then placed over the fence to enclose the area. This method relies on mechanical contact of the lid and fence to create the RF shield.

The fence may be added anywhere in the process before the reflow oven. Typically, the fence is manually placed or machine placed using an odd form assembly system. The lid is manually placed after reflow or by a second automated system. It can easily be removed by hand if other parts must be replaced inside.

The second method is a one-step process that adds the shields during fine-pitch placement. By using one-piece shields presented in tape-and-reel or component trays, RF shield attachment requires no changes in the assembly process. The shields are placed and then soldered directly onto the board using a standard process. No operator or additional inline machinery is required. A complete metallic bond surrounding the entire area provides the most reliable RF shield and prevents the end user from tampering with the device. The shield itself must be desoldered if defective components are found during in-circuit or functional test or if a device upgrade is required.

Photonic Soldering

Photonic soldering has been a successful alternative to hot air convection soldering for several applications that require precise process control, optimization of process time, and have physical constraints that prevent the use of shielding devices to protect adjacent components (Figure 2). Originally developed for personal computer assemblies, photonic soldering has been well received by the telecommunications industry where the density of the component real estate is essential to the value of the final product. Using lasers to rework a component removes tooling costs and delays in beginning the process for new components. By changing its path, speed and intensity, the laser can be programmed for different components.

Laser specifications

A yttrium aluminum garnet (YAG) laser, with a beam diameter adjustable between 1.0 to 4.0 mm, is used to remove and replace components on the PCB. With a wavelength of 1064 nm, the laser beam heats the solder joints. For FR-4 substrates, 75 percent of the heat is absorbed by the solder joints and 25 percent is absorbed by the FR-4 material. Once completely isolated, the laser is considered to be Class 2, and neither safety glasses nor special facilities are required to operate the equipment under normal operating conditions.

Beam positioning

Computer-controlled positioning mirrors are used to control the path of the laser. They can be programmed in an infinite variety of paths. Two reflectors are used to control the path in the x and y axes. X and y galvanometers are programmed for each component type, and the paths are stored in the computer interface. The distance the beam travels and the time it takes the path to be completed are programmable, providing the flexibility to modify the velocity of the laser beam path.

Visual alignment laser

Because the YAG laser beam is in the infrared spectrum, it is invisible to the human eye. By coupling a visible helium-neon (HeNe) laser with the YAG laser, the path can be shown during programming. The x-y galvanometers deflect the HeNe laser, giving the programmer an exact visual representation of the path of the YAG laser.

Temperature monitoring

To successfully remove or replace a component during rework, the YAG laser must be controlled so it does not overheat the components by applying heat too rapidly. The objectives are to simulate the thermal profile of a reflow oven and maintain thermal uniformity across the component while preventing adjacent components from being overheated or reflowed. To accomplish these objectives, a third optical device called a pyrometer is coupled with the HeNe and YAG lasers. The pyrometer continually monitors the surface temperature of the solder every 10 milliseconds for optimal process control. As the information is fed back to the computer, the power to the YAG laser is adjusted. Continuous process monitoring through a closed-loop thermal regulation system ensures a repeatable and reliable process.

Bottom heating

To reduce thermal stress and prevent localized warping in the rework area, bottom heating should be used to preheat the board before applying the laser to the top. Bottom heating can be accomplished by using a large capacity convection heater with programmable temperatures ranging from 100 degrees to 300 degrees C.

Rework Procedure

By affixing thermocouples to the solder joint and shield at strategic locations, the thermal uniformity across a shield can be measured and a specific profile can be determined if required. Thermally conductive adhesives or high temperature solders are used to secure the probes in place and prevent any movement during the process. Care must be taken to measure the solder temperature and not the air temperature under the shield. Three thermocouples are sufficient to monitor the process.

By using a pyrometer, the surface temperature of the solder is continually monitored during the process. As the heat transfers from the top of the package to the solder joints, the laser continuously compensates for higher density areas that may absorb the energy more efficiently. If a particular shield section is connected to the ground plane of a dense assembly, the laser power is increased.

Laser path definition

For standard rectangular-shaped shields, simply defining the x and y dimensions is sufficient for defining the laser's path. Because only the perimeter of the shield is heated, internal components are not heated up to reflow temperatures.

Odd form shields must be processed using custom designed paths based on the shape of the shield itself. Hardware and setup variables, such as nozzle design, incoming air pressure and temperature, and the distance of the nozzle from the component being reworked, add to the challenge of process control. By applying a CAD-like interface on the rework machine, any shape or pattern can be generated. This technique requires no tooling change to remove or replace different shields, thereby considerably reducing rework time. Lead times associated with additional tooling requirements for new parts and assemblies are also reduced.

Shield removal

For this specific application, high temperature tape is required to increase the vacuum force when removing the component. Several holes in the top surface of the shield provide access to test points inside. Hole diameters and their locations are designed for each assembly to provide the correct access to test pins under the shield.

After the high temperature tape is applied, the assembly is loaded into the rework system. The bottom heater preheats the assembly as the operator roughly aligns the component to be removed. The center of the YAG laser path is defined by a reference point shown by the HeNe laser.

Fine adjustment of the board's location under the laser beam can be achieved by using a vision system. The vision system's cameras are positioned using a single axis stepper motor that is driven to the correct location based on the shield dimensions. The operator does not need to adjust the location, focus or field of view to align the component. This adjustment is accomplished by looking at a monitor as the HeNe laser jumps from corner to corner around the shield perimeter. The component is positioned under the laser using fine-resolution micrometers.

Once aligned, a safety door closes and the laser fires on its preprogrammed path. The laser heats only the perimeter of the shield and the solder joints. No heat is added to the center of the shield, the components under the shield or the adjacent components. The laser's scanning rate may be adjusted so that the entire solder interface is heated simultaneously. All of the solder must be liquid before the part may be removed.

At this point, a vacuum pick-up automatically removes the shield from the board. Computer controlled stepper motors drive the pick-up. The safety door opens, and the PCB is removed.

Site preparation

Residual solder is removed using a vacuum desoldering station. Normally the entire assembly maintains a temperature of 100 degrees C or less when it is removed from the laser system. This time is the best for removing the residual solder.

The board must cool down below 65 degrees C before adding paste. Additional solder is added using a syringe filled with eutectic solder paste. In this case, the shield is placed by hand after the paste is applied to the site. Through-hole pins locate the shield in the correct position. Miniature stencils or automated dispensing can also be used for higher volume applications.

Reattachment 

Reattachment uses the same process as for removal. The board is loaded in the rework machine and positioned roughly with the HeNe laser. The fine adjustment of the component under the laser path is done with micrometers. The process may be defined to match the original reflow process of the assembly simultaneously or soldered in a slow path that traces the outline of the shield. Both methods yield reliable solder joints.

Conclusion

RF shields may be removed or replaced by applying a controlled source of heat using a laser system. Pyrometer control maintains the proper process temperatures to ensure that the YAG laser transfers the desired power to the specified site. Either point-by-point soldering or uniform heating may be accomplished by varying the scanning speed.

One advantage of laser soldering over convection systems is the speed at which rework can be conducted. Process times of 30 seconds or less can be achieved with smaller components. Fast process times dramatically increase the volume of boards processed through a single rework station.

RF shielding continues to be integrated into PCB assemblies at an accelerated pace. The technology of laser soldering has increased the flexibility of design and allowed RF components to be integrated into standard, automated assembly lines. By using a one-piece shield and laser technology, the number of rework technicians can be significantly reduced and throughput increased.

Laser soldering increases the flexibility of design by reducing the clearances required for rework. As component densities increase, the clearance areas required for rework usually stated in design for manufacturing (DFM) specifications are being reduced. The laser rework method heats only the component's perimeters where it is soldered to the PCB. Adjacent components under the shield experience minimal heat.

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Edgar Cerda is a RF engineer for Solectron Corp., Apodaca, N.L., Mexico. Neil Watson is the European thermal product manager of Vision Inspection Technology SA, Yverdon-les-Bains, Switzerland; e-mail: [e-mail].

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Copyright [copyright] 2001 Miller Freeman LLC