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Laser Soldering: A Turning Point
December 31, 1969 |Estimated reading time: 6 minutes
By Dave Sigillo, Seica
Selective soldering is seeing a rise in popularity coinciding with an uptick in mixed surface mount and thru-hole assemblies. Choosing a selective soldering system that meets your needs requires understanding of the entire production process.
Selective soldering has found wide application as use of SMT devices increases. Selective soldering basically is a choice of process more than of machine; the choice of system comes as a consequence.
The selective soldering technological solutions available to EMS providers looking to avoid manual soldering are somewhat limited: wave soldering with masked carriers, dipping, or selective soldering (mini wave, hot iron, and laser) with robot systems. Every selective soldering procedure is unique and needs specific management for operation, including dedicated tools. To choose a system that meets one’s needs, a whole vision about the assembly to be soldered is required.
Process
As far as process is concerned, there are several elements that must be considered, such as board, pad, and pin size. In automatic soldering with robotic systems, other influencing variables include pad geometry and the size of the circular crown that receives the alloy amount required to realize a sufficient solder joint.
For solder joint formation, other critical items play an important role, such as the size ratio between the circular crown, the hole, the pin size, and its shape; how long the pin is on the soldering side of the board; etc. The alloy flows up in the hole for capillarity; consequently, if the pin diameter is too thin or too big in relation to the hole, there are difficulties in flowing up. If the pin is too long, the heat applied tends to concentrate on its edge. On the top, it tends to sink the component body, so that the alloy flows in the hotter zone and concentrates on the tip. If it’s too short, the alloy cannot adhere.
Selective soldering is an operation that occurs after other thermal phases, as it is generally performed post-reflow on mixed assemblies. Therefore, it is important to foresee any possible board distortion. A planarity sensor indicates suitable adjustments to maintain soldering device synchronized with soldering plane.
When dealing with process considerations, productivity can’t be left as a separate part of the equation. Productivity must be considered not only as number of soldering actions per unit of time, but also as production changeover speed with minimal maintenance and irrelevant stoppages.
The most important element for the final assembly is the surface finishing of components, pads and pins. The lowest performance comes from OSP finishing, followed in difficulty level by nickel/gold, while tin-based finishes have proven easier to solder.
The tin/lead alloys can be defined today without any secret, but the lead-free alloys still present some unknown behavior aspects. Although the group’s main characteristics are known, certain aspects remain unpredictable. Alloys similar in composition, but made by different manufacturers, often present different behaviors in the same working conditions. These diverge completely when operators change the working variables. Every solder wire manufacturer also requires a specific flux. These elements are enough to make a difference in production, evident when considering residues and post-solder cleaning.
Soldering System
The most suitable selective soldering system for Western European production is a robot soldering machine. Robot systems help a gripper bring the board over the mini wave pot, or to move an hot iron together with a wire feeder unit, or point a laser beam for reflow.
Availability of a movable axes system is fundamental to accuracy and repeatability. The soldering head must have wide freedom of movement to face all possible situations. The nozzle, for mini wave and wire soldering, must allow easy replacement and facilitate work on high-density boards. When using a wire, the feeder system is particularly relevant because of the several diameters in use. Its location must be closest to the solder joint. Laser systems’ wire dispensers host large reels with various solder-wire diameters accommodated. Some systems include a sensor at the dispenser exit.
A high-speed reading pyrometer on a small area makes the whole process easier, due to its double function as control instrument and acquiring instrument. A micro camera can be used to manually learn the coordinates where the soldering must be executed, to read fiducials during the automatic process, etc.
The software links all subsystems in the soldering machine and must be simple in terms of programming and use for the operator.Good comprehension of the process and available system capabilities is part of the technician’s knowhow. Besides, knowledge of soldering basic elements and a “sensibility” about the process make learning and using a system easier. In this equation, a good relationship between the technical production team and the equipment’s sales support comes into play.
Among the several advantages of laser soldering, there’s accurate and repeatable thermal transfer. Since this is a contactless soldering technique, it realizes a joint without stressing the board or components, without wearing out the bits – unlike hot iron robots – or producing waste. This prevents any board contamination.
By changing the size of laser spot, it is possible to work on different pads without changing the soldering nozzle, as on mini wave soldering machines.
Consistent quality targets soldering of lead-free alloys without using nitrogen. It also works with traditional wire; users simply change the solder alloy and corresponding thermal profile.
During the soldering phases, interaction between metallic and chemical components influences soldering performance. Inside the process, temperature is the independent variable, but it is also the one having the highest impact on wettability.
Figure 1. Soldering process interactions.
The aim of flux is to take away oxidation from surfaces to be soldered in order to make their wettability easier with the melted alloy. The flux must be able to deoxidate pads and terminals at soldering temperature without decomposing (Figure 1). When considering the influence of temperature on flux, we must focus our attention on three parameters: Evaporation of solvent and of volatile substances; speed of distribution and viscosity of solid substances under transformation; and decomposition of organic material. It is important to take away remaining contamination soon after the soldering process. Ionic residuals left on the board may cause short circuits because of the formation of dendrites or generate corrosion due to the electromigration. Residues from no-clean (NC) processes are hard, non-corrosive, and inert. This can be left on the board or removed with an adequate solvent.
With the help of system software advances discussed earlier, laser selective soldering systems are able to determine the necessary data – thermal profile, laser spot size, volume of tin required (translated as wire length) – for every solder joint. The computations are generated from information received in CAD and Gerber files, or introduced by operators.
Since each joint has its own thermal mass, changed by varied paths on the PCB, it has its own thermal profile. The soldering profile is characterized by the trend of temperature according to time, and is obtained by changing the involved powers in the three phases and the corresponding time of application.
The head is the operating center of the soldering system, since it contains the laser optic, wire dispenser, camera, pyrometer, and planarity sensor necessary for controlled operation. In laser selective soldering systems the laser unit typically is located at the back of the machine. The laser is routed through the optical fiber and focused by the motorized optic group in the required size and such to meet the pad size. The whole head rotates a little more than 180°; rotation allows changes in working angle and rotating during the soldering process, helping improve wettability for a large-sized pad.
Conclusion
The growth of laser soldering is due to its capability to exploit the high energy enclosed in the ray and focalized only on the joint, a characteristic which avoids involving the substrate and nearby components, even with the higher working temperatures required for lead-free alloys.
Dave Sigillo, general manager, Seica Inc., may be contacted at (603) 890-6002; Fax: (603) 890-6003.