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Selective Soldering by Laser
Firefly robotic laser soldering system.

The development and improvement of selective soldering systems using laser technologies within the past few years has happened for several reasons — the wide range of electronic equipment on the market, the extreme miniaturization and integration of electronic components, along with the restrictive environmental directives, have all driven the rapid growth of this technology.

Today's soldering systems are capable of reaching the most critical and difficult-to-reach parts of electronic circuits. But the use of lead-free alloys, which are less aggressive than tin-lead solder, have created additional difficulties when completing soldering operations without interfering with adjacent components.

Laser Technology
The laser, which dates back to the 1960s, provides radiation that is highly directional, monochromatic, with a high level of brightness and coherence. It emits its radiation in a single direction, at a known and steady frequency and at much higher energy levels than traditional light sources.

Lasers are available in an extremely broad range of power, wavelength, type of radiation and the optics used to transmit the beam. Different arrangements of these parameters make the laser suitable for different sectors, such as medical, military and industrial. The power control of the laser source and the steadiness of the output power are essential to guarantee a stable and repeatable process. Optics developed for the soldering process produce conically-shaped beams which can be adjusted and moved, focusing an intense amount of heat on the solder joint with no distortion.

This kind of performance comes as the result of years of R&D, leading to today's extraordinarily high power and functionality of laser selective soldering systems. The use of laser technology for selective soldering enables precise heating, avoiding the placing of high thermal stresses on heat-sensitive components, while permitting soldering of such sensitive components at high temperatures. The transfer of heat and energy through the laser beam completely eliminates the need for mechanical contact; the components are not subjected to heat damage, thereby decreasing the need for rework repairs. The use of focusing optics permits soldering in small spaces and of fine pitch components, and the use of motorized optics optimizes the focal point for each solder joint. In addition, laser soldering systems consume less electric energy than conventional soldering systems, and do not require any warmup time.

Process Parameters
Several variables affect the laser selective soldering process — such as the pad geometry or the size of the crown, which can affect the formation of the solder joint; in particular, the ratio between the crown size and the through hole are important. If the ratio is not optimal — the hole is too big or too small in relation to the pin diameter, problems in the reflow of tin on the components side may occur. If the pin is either too long or too short, problems of poor bonding may occur both on the pin and the pad.

In addition to the physical and geometric variables of the printed circuits and components, it is necessary to take into account the soldering parameters applied during the process. The thermal profile control, assigned to each joint, allows for optimization of these soldering parameters — particularly when the pads are connected to ground planes or the circuit has a large number of layers, or when the components to be soldered have a large thermal mass. The use of temperature-measuring tools, such as pyrometers — capable of detecting the exact temperature of the joint to be soldered with response times lower than 10ms — permits verification of the thermal profile for each solder joint. This is essential to prevent soldering problems, such as voids or cold solder joints. The use of a pyrometer considerably speeds up the process of generating the soldering program and is a fundamental tool for an operator with little familiarity or experience with the soldering profiles for the particular circuit board being soldered. In addition, acquisition of the temperature on the joint and the thermal profile are needed in order to have full traceability of the process.

Laser Selective Soldering
Seica, with experience in axes motion and control of Flying Probe systems, manufactures the Firefly Line of selective soldering systems, an integration of software, mechanics and laser technology. The Firefly soldering system makes it possible to program a specific thermal profile for every single joint. This programming comprises three phases and is characterized by the change of temperature over time.

Pre-heat. The first phase measures temperature and time to pre-heat the pad and pin surface.

Alloy melt. In the next phase, a higher temperature melts the alloy, which is automatically dispensed to the joint. The time of the second phase is proportional to the alloy and dispense speed, thus forming the solder joint.

Reflow completion. The third phase is used to improve the wettability and to complete the reflow of the solder joint. The thermal profile is controlled using a pyrometer which adjusts the power of the laser source based on the detected temperature and the thermal profile that has been previously defined for the specific joint. This couples the real thermal profile with the theoretic thermal profile.

To take advantage of the laser technology used by the Firefly selective soldering systems, all the operating elements are inserted onto a soldering head capable of rotating 180°; this allows targeting the solder joint at any angle, and if necessary, rotating in the soldering phase to improve the wettability, particularly on larger-size joints. The optics of the 50-watt laser source are connected to the soldering head through a fiber optic cable, attached to a motor, which allows focusing the laser beam spot based on the solder joint size. The feeding of solder alloy to the solder joint is handled by a motor and sensor combination, which detects the alloy in the nozzle of the wire feeder as well as the amount of alloy fed to the joint. Because of the varying dimensions of electronic components and circuits, the Firefly system is capable of using 500 gram spools of alloy wire with a diameter that ranges from 0.5 to 1mm.

The laser technology requires that the laser beam must always be perfectly focused on the solder joint. If there is board warpage, the focus may not be accurate. A warpage recovery sensor is located on the soldering head to detect and make adjustments to correct for the warpage and properly focus the optics on the joint. A camera mounted on the center-line of the soldering head provides fiducial recognition and video storage of the soldering phase, monitoring the entire soldering process.

Laser Management Software
Among the basic features of the software, the ability to create a comprehensive soldering program needs to be available, as well as the ability to process CAD/CAM data to create the solder joints. VIVA — the management software for the Firefly laser selective soldering system — can create the required parameters working from CAD/CAM data or data manually entered by the operator, using a Macro designed for the soldering process. The use of the software ensures that there are no surprises, when using lead-free alloys with flux developed specifically for use with laser technology. The VIVA software can manage PCB features, such as the color, surface finish, and the composition of the alloys creating soldering parameters and thermal profiles for all types of boards and components.

Process traceability is another important aspect of the software system. It provides up-to-the-minute results of production accuracy as well as the quality and repetitiveness of results. This monitoring this allows for the detection of possible production deviation. The VIVA software stores the thermal profile in use for each solder joint, the exact temperatures detected, and a video of the soldering process. The automation of such a critical part of the manufacturing process is an important requirement in achieving the highest solder joint quality and repeatability.

Contact: Seica, Inc., 50A Northwestern Drive, Suite 10, Salem, NH 03079 603-890-6002 Web:
http://www.seicausa.com

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