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ABSTRACT

Continually shrinking package sizes, reduction of pitch, and reduction of clearance areas available around components has made it increasingly difficult to rework RF shields. More demanding process requirements and the increasing complexity of printed circuit boards has prompted the development of new techniques for removing and replacing RF Shields. Photonic soldering shows advantages for this application using inventive control tools and a computer interface for programming. By precisely controlling and monitoring the surface temperature of the shield using a Pyrometer, a Nd:YAG Laser may be focused and controlled in a precise pattern to reflow the solder at the base of the 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.

Photon: A quantum of electromagnetic energy of a single mode; i.e., a single wavelength, direction and polarization. As a unit of energy, each photon equals hn, h being Planck's constant and n, the frequency of the propagating electromagnetic wave. The momentum of the photon in the direction of propagation is hn/c, c being the velocity of light. – Photonics Dictionary

Photonic Soldering: Method of heating and reflowing solder using the energy of photons generated by a laser source which is controlled by a pyrometer to determine surface temperature for closed loop thermal process control.

RF SHIELDING APPLICATIONS
Several standards and guidelines have been determined by the FCC for the U.S market for products emitting radio waves and by CE for similar products in the European market. RF or Radio Frequency shields are used to separate areas of an assembly from each other and to reduce noise and to prevent radio signals from escaping from the product and effecting the operation of other devices. Antenna products, as well as cell phone and wireless internet electronics, require shielding to some extent that is ether mounted to the PCB or incorporated into the packaging that surrounds the electronics.

“Fence and Lid” or One Piece Shielding
There are two common methods of attaching shielding to a circuit board. Both methods attempt to completely cage a specific area by soldering to surface connections to ground
planes internal to the circuit board. One method includes the attachment of a metallic “fence” around the perimeter of the area to be shielded. A spring-loaded metallic “lid” is
then placed over the “fence” to enclose the area. The “fence” may be added anywhere in the process before the Reflow oven. Typically, it 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 there are parts to be replaced inside. This method relies on mechanical contact of the “lid” and “fence” to create the RF Shield.

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 real or in 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 is required and no 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 components are found to be defective during incircuit test or functional test or if a device upgrade is required.

PHOTONIC SOLDERING THEORY
Photonic soldering has been a successful alternative to hot air convection soldering for several applications that require precise process control, and optimization of process time,
and have physical constraints that prevent the use of shielding devices to protect adjacent components. Originally developed for personal computer assemblies, this technology 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 delay in beginning the process for new components. By changing the path, speed, and intensity of the laser, the laser is programmed for different components.

YAG specifications
A YAG (Yttrium Aluminum Garnet) laser is used to heat the solder joints. The YAG wavelength is 1064 nm, which is 75% absorbed by solder joints and 25% absorbed by FR4
materials. It is the ideal laser to remove and replace components on a printed circuit board. The beam diameter is adjustable from 1.0 mm to 4.0 mm. It is considered a Class IV laser. Safety requirements of interlocks and shielding are required for operation. Once completely
isolated, the laser is considered Class 2 and does not require safety glasses or special facilities to operate the equipment under normal operating procedures.

Beam Positioning
Computer controlled positioning mirrors, called Galvanometers 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. These 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
The YAG laser at 1064 nm is in the infrared spectrum and is invisible to the human eye. A visible HeNe (Helium Neon) 632.8 nm laser is coupled with the YAG laser to show the path during programming. The HeNe laser is positioned in the same optical path as the YAG laser so that the YAG will follow the exact path of the visible laser. The X-Y Galvanometers deflect the HeNe laser to give the programmer a visual representation of the path of the YAG.

In order to successfully remove or replace a component for
rework, the YAG laser must be controlled so as not to overheat the components by applying too much heat too quickly. The goal is to simulate the thermal profile of a reflow oven, maintain thermal uniformity across the component, and prevent adjacent components from being overheated or reflowed. To accomplish this a third optical device is coupled with the HeNe and YAG lasers: a Pyrometer is calibrated to read the surface temperature of the component. By reading the 3.5 micron light wave emission, the Pyrometer continually monitors the surface temperature every 10 milliseconds during the process 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
A large capacity convection bottom heater is used to preheat the board prior to applying the laser to the top. This reduces the temperature differential between the top and bottom of
the board to reduce thermal stress and prevent localized warping in the rework area. The bottom heater is programmable and can be adjusted from 100°C to 300°C.

REWORK PROCEDURE

Thermal Process development
By affixing thermocouples to the solder joint and shield at strategic locations, it is possible to measure the thermal uniformity across a shield and determine a specific profile if required. Thermally conductive adhesives or high temperature solder is used to secure the probes in place and prevent any movement during the process. It is important to measure the solder temperature and not the air temperature under the shield to ensure that proper measurements are being made. Three thermocouples are sufficient to monitor the process.

By using a pyrometer, the surface temperature of the solder is monitored every 10 milliseconds. 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 section of the shield 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 dimension is sufficient for defining the path of the laser. Only the perimeter of the shield is heated. Internal components are not heated up to reflow temperatures.

Odd form shields may be processed using custom designed paths based on the shape of the shield itself. A CAD-like interface on the rework machine allows for any shape or
pattern to be generated. The special shape for different components allows for the path to be nozzle linked to the process file. No tooling change is required to remove or replace different shields. This reduces the total process time of rework considerably. It also reduces the lead times associated with additional tooling requirements for new parts and assemblies.

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 there 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 starts to preheat the assembly immediately as the operator roughly aligns the component to be removed. The center of the laser path is defined by a reference point shown by the HeNe laser.