A Revolution In Lead-Free Hand Soldering By Ed Zamborsky - OK International Eastern Regional Sales Manager By the time a printed circuit board is ready for the final manual soldering of its odd-form components, it will typically have gone through many lengthy and costly production procedures. It will also have been populated with numerous components, most of which are delicate, many of which are tiny, and some of which may be extremely expensive. It is hardly surprising then that quality managers and line supervisors the world over are under extreme pressure to keep their hand soldering processes under control. While safeguarding the integrity of the assembly, they must guarantee that the soldering process is consistent and repeatable, and that it is producing high quality, reliable solder joints. And, of course, as productivity is a prime objective for any business, they must ensure that all of this happens at acceptable throughputs. This is far from easy. Manual processes, by their very nature, are more variable than fully automated assembly processes carried out under a statistical process control regime. The assembly will also be tightly packed with fine-line circuitry and delicate devices, so simply accessing the areas that have to be soldered requires the appropriate tools, concentration and a skilled hand. The operator must then apply enough heat energy to reflow today s high-temperature lead-free solders, but without damaging heat-sensitive devices or the PCB itself. These challenges re-emerge during rework which, if anything, is even more difficult and time-consuming, and the risk of damaging components is even greater. Heat Management Issues With all this in mind it fast becomes clear that soldering professionals must keep very tight control over their processes. In particular, they must put exactly the right heat energy into the solder joints; too little will impact productivity, while too much could have devastating effects on the assembly s reliability. Yet, as a result of outmoded design, many conventional soldering irons are incapable of providing operators the control they need, thanks partly to inefficient thermal management within the soldering iron. The tip, often distant from the heating element, stores the heat energy supplied by the heater, and then releases it in a surge on contacting the pad. The potential for causing heat damage is evident, but this is not the only issue. As the tip releases its heat energy into the pad, it cools. It must quickly be re-heated, but because both the heat sensor and the heater are remote, this does not happen immediately. Similarly when the tip is re-heated, this same distance induced inertia can cause a temperature overshoot.
In our new lead-free world, this uncontrolled oscillation around the temperature set-point is a real problem. It is further compounded by the fact that the majority of soldering tools now in use transfer far less than 100% of the heat energy from the soldering iron to the pad, thanks to thermally inefficient tips. The result is that, try as they might, operators rarely have their hand soldering processes under control. Frustrated by the thermal delays within the iron itself and between the iron tip and the pad, they often attempt to maintain production rates by increasing tip idle temperature, knocking temperatures even further off target, the consequences of which can be disastrous. Ironically, an overheated tip can also increase soldering time as the flux may be burned off before it can do its job, or in the case of more active lead-free flux, it may char, depositing residues onto the tip that prevent wetting. Excess heat will also promote oxidization, further hampering joint formation and reducing the working life of the soldering iron tip. Improving Thermal Efficiencies Lead-free hand soldering calls for a new breed of soldering tools that, rather than measuring indirect parameters such as tip idle temperature, sense exactly how much thermal energy is needed at the pad, and supply this precise amount the moment it is needed. In this way, soldering can be carried out faster at lower temperatures, and the risk of damage through temperature overshoot is eliminated. With its Metcal SmartHeat technology, leading production assembly equipment supplier, OK International achieves just this by keeping the tip temperature, rather than the power supplied to it, at a constant level. Thus, rather than storing heat energy, the tip behaves as a thermal highway, drawing heat energy from the heater and releasing it as, in turn, the pad draws the energy away. The heat sensing function is within the tip itself, so feedback is instantaneous and, thanks to this and a patented metallurgical design, temperature is controlled automatically and accurately to within ±1ºC of the set point. Thermal recovery is immediate, so there are no more frustrating delays, thereby increasing productivity. Since the temperature of the station is fixed and no calibration is required, users can enjoy a new level of process control in their hand soldering applications.
Conductivity Factor For all of this to be successful, the tip itself must allow thermal energy to flow freely through it, and of course, soldering iron tips are manufactured from thermally conductive materials. As OK International discovered, however, this is not the entire story. From extensive tests conducted on a range of tip styles, the company discovered that thermal efficiency could be improved by optimizing tip geometry. Its studies also revealed that precisely how much heat energy a certain tip will transfer can be predicted by measuring its Conductivity Factor (CF), a mathematical equation that describes the tip s length, width and crosssections. Figure 1 shows how a tip s thermal efficiency, e.g. its Conductivity Factor, changes with its geometry, by illustrating how three different shaped tips with identical chisel ends performed when soldering a 2-layer board (light load) and a 10-layer board (heavy load) using the same source temperature. The benchmark line shows the minimum thermal performance necessary for lead-free soldering. As thermal efficiency is about energy flow, it follows that the shorter and wider a tip is, the more energy that can flow through it, so the higher its Conductivity Factor. Indeed, the tip on the right, with its fine, multi-stage tapering, hampers energy flow and accordingly it has the lowest Conductivity Factor, while the best performance comes from the wide, short, smoothly-tapered tip on the left. Figure 1: Optimising the Conductivity Factor through tip design
Armed with this valuable information, and in a first for the industry, OK International went about designing a range of high-conductivity Factor PowerTips, as show in Figure 2, that, combined with their SmartHeat technology, provide operators with the most thermally efficient tools in the industry. By ensuring that temperatures are within tight tolerances, these tools leave the operator free to focus on the considerable physical challenges of hand soldering, while a range of tip geometries allows easy access to even the most complicated solder joints. The operator is further aided, in delicate tasks, by a lightweight ergonomic design that includes a short tip-to-grip distance for precision control and cool operating temperature handles. Figure. 2: Lead-free assemblies with numerous or large solder pads demand energy levels and recovery rates that call for Conductivity Factor-optimizedsolder tips Conclusion It is a commonly held misconception that by controlling indirect parameters, such as tip idle temperature, it is possible to produce quality hand-soldered joints that are reliable over time. While this approach may have been acceptable with the more forgiving tin-lead solders, it becomes extremely risky in our new lead-free world, especially as operators are often tempted to increase temperature settings in the hope that this will speed up the process. Lead-free soldering operations are carried out at temperatures that are uncomfortably close to levels that can cause thermal shock and heat damage to PCBs and delicate components, so there is no longer any room for this sort of trial-and-error approach.
A new level of thermal efficiency and control is essential if the hand soldering process is to continue producing quality solder joints at acceptable throughputs. Soldering iron and tip technologies are available that automatically, directly and instantaneously deliver exactly the right thermal energy into a solder pad. Combined, they allow operators to work at lower temperatures, while satisfying stringent demands for high quality, reliable solder joints in a repeatable process. They also provide substantial improvements in yields and throughput by automating and speeding up all the thermal control processes. Further, costs are saved on solder tips as their working life is extended by the lower operating temperatures, and, last but by no means least, by simplifying the hand soldering task these technologies will contribute significantly to a reduction in operator training costs. For reader enquiries, please contact: Ed Zamborsky - OK International Eastern Regional Sales Manager T: 714 799 9910 Email: ezamborsky@okinternational.com Website: www.okinternational.com