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www.us-
tech.com Selective Soldering by Laser By Alessio Raga and Kris Carlson, Seica S.p.A., Strambino, Italy
within the past few years has hap- pened for several reasons — the wide range of electronic equipment on the market, the extreme miniaturization and integration of electronic compo- nents, along with the restrictive environmental directives, have all driven the rapid growth of this tech- nology. Today’s soldering systems are
T
capable of reaching the most critical and difficult-to-reach parts of elec- tronic circuits. But the use of lead- free alloys, which are less aggres- sive than tin-lead solder, have creat- ed additional difficulties when com- pleting soldering operations without interfering with adjacent compo- nents.
Laser Technology The laser, which dates back to
the 1960s, provides radiation that is highly directional, monochromatic, with a high level of brightness and
At one time described as a “death ray”, high-power
lasers today have become an important part of the
electronics manufacturing process.
coherence. It emits its radiation in a single direction, at a known and steady frequency and at much high- er energy levels than traditional
Firefly robotic laser soldering system.
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
Control solder paste height means to improve process
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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 for-
he development and improve- ment of selective soldering sys- tems using laser technologies
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
placing of high thermal stresses on heat-sensitive components, while permitting soldering of such sensi- tive components at high tempera- tures. The transfer of heat and ener- gy through the laser beam complete- ly eliminates the need for mechanical contact; the components are not sub- jected to heat damage, thereby
mation of the solder joint; in particu- lar, 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 neces- sary to take into account the solder- ing 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 solder- ing program and is a fundamental tool for an operator with little famil- iarity or experience with the solder- ing profiles for the particular circuit board being soldered. In addition, acquisition of the temperature on the
Continued on next page
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