Fine adjustment of where the board is located under the
laser beam is achieved by a vision system employing two
CCD cameras and a video mixer. The cameras are
positioned using a single axis stepper motor that is driven to
the correct location based on the dimensions of the shield.
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 the 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, the safety door closes and the laser fires on a
preprogrammed path. The laser heats only the perimeter of
the shield and the solder joints. There is no heat added to
the center of the shield, the components under the shield, or
the adjacent components. The scanning rate of the laser
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.
Site preparation
Residual solder is removed using a vacuum de-soldering
station. Normally the entire assembly maintains a
temperature of 100C or less when it is removed from the
laser system. This is the best time to remove the residual
solder. The board need to cool down below 65C 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.
Re-attachment
Using 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 as shown in Figure 6 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 have proved to yield reliable solder joints.
Added value of Laser Technology
One of the greatest advantages to using laser soldering over
convection systems is the speed at which rework can be
carried out. Process times of 30 seconds or less are
achieved with smaller components. This dramatically
increases the volume of boards processed through a single
rework station .
RF shielding continues to be integrated into printed circuit
board assemblies and are being integrated into new designs
at an accelerated pace. The technology of laser soldering
has increased the flexibility of design and allowed for the
integration of RF components into the standard, automated
assembly line. By using a one-piece shield, and laser
technology to rework, four technicians per shift per line are
no longer required.
This Laser soldering technology increases the flexibility of
design by reducing the clearances required for rework. As
component densities increase, the clearance areas required
for rework that are usually stated in DFM (Design for
Manufacturing) specifications are being reduced. The laser
rework method heats only the component perimeters where
it is soldered to the circuit board. Adjacent components
under the shield experience minimal heat.
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
the YAG transfers the desired power to the specified site.
Either point by point soldering or uniform heating may be
accomplished by varying the scanning speed.
The use of Photonic Soldering Techniques for RF shield
applications allows “one-piece” shields to be used on
assemblies and RF shield integration into the standard
assembly process. This reduces the number of operators
required in manufacturing of specific products significantly
and increases the throughput and repeatability of high
volume rework. |
