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Lead-free soldering process constraints have led to an evolution of reflow ovens which are becoming longer and longer whatever throughput is expected, with an increasing number of zones and parameters. With these equipments, the set up of reflow profiles is becoming more and more complex and the control of the process is essential to reassure finalcustomers. Existing in-line ovens are thus giving quite good results as solutions have been developed to answer the constraints enumerated before but the industry is still seeking for the best compromise between performance, flexibility and cost of ownership. The solutions known today are short term solutions which allow current technology to work within the narrow process window required by lead-free but they cannot give entire satisfaction as they all have drawbacks related to quality, flexibility or cost of ownership. Examples of limitations in regard to the thermal dispersion of the newovens include: • Increasing the length of the oven enables the decrease in thermal dispersion but the oven footprint increases in an environment where space is expensive and line layouts are not very flexible. • Increasing the convection speed is another solution but it can cause components to move and it increases nitrogen consumption. Addressing reflow issues
For many years, different companies in the reflow market had tried to find a solution to all of the issuesrelating to current reflow ovens. Requirements included: • Reduced floor space
• Higher throughput with increased flexibility
• Excellent temperature homogeneity
• Elimination of conveyor warpage and distortion
• SPC control with pyrometers
• Reduce electrical and nitrogen consumption
• Self-cleaning system
• Environmentally friendly Phase convection, a technology using convection, developed by ViTechnology, meets all of these requirements and many others. It has been described as being a technology, which has all of the advantages of vapor phase technology (which it is not), but without the disadvantages such as using liquids, low throughput, increase of voiding effect and tomb stoning of small chip components such as 0402 and 0201. System flexibility
An example is the X-600 phase convection system (figure 1) from ViTechnology, designed with two loading and unloading lifts, one at the entrance and one at the exit of the system. These are used to load and unload the boards, boats or strips into and out of each of the two or three stacked phase convection levels. Each of these levels is made up of four phase convection units (figure 2) that are each controlled individually (and is different from atraditional vertical oven). The four phase convection levels can each receive up to four or eight boards at a time depending on the board size, taking one or two in each of the phase units at a time. By loading each level one after the other (FIFO), a total of 12 (24 if small boards and/or boats) can be processed simultaneously. This offers high throughput and productivity. As each of the “phase units” and each of the levels are all controlled individually, different profiles can be run on each level if required, offering total flexibility of the system. Temperature homogeneity
Each level has four convection phases, three heating phases and one cooling phase (figure 3). The PCB moves from one to the other phase thanks to an advanced technology board transport system specifically designed for this application. The PCB remains static during each phase time which corresponds to pre-heating, soak, reflow and cooling phases. The static state of the PCB within a chamber which is uniform in temperature enables the heating of the whole PCB area at the same time (figure 4), thus eliminating the temperature gradient between the beginning and the end of the PCB as are usually demonstrated on standard reflow ovens. This temperature gradient of standard ovens causes board warping, solder joint weakening and adversely affects temperature homogeneity. The physical separation between each of the heated and cooling phases, added to the thermal breaks along the patented board handling system, avoids edge effect from zone to zone and heat transfer between zones. On standard reflow ovens, the conveyor rails cause transverse temperature heterogeneity, although these can be balanced thanks to side heating modules on some reflow ovens. The patented top and bottom forced convention device (figure 5), with reduced cross-flow, enables homogeneous and efficient heat transfer on the PCB area. The layout, design and distribution of the convection nozzles has been developed to make the impact cone of hot air on the PCB as homogeneous as possible and to make sure that the areas of impact of each cone are perfectly adjacent. Thanks to this distribution of the convection nozzles, the air which gets cooler on contact when heating the PCB does not flow on or over the board but returns toward the fan. All of these parameters, including the convection nozzle diameter, their distribution, the distance between the blower unit and the PCB, have been mathematically simulated in order to befully optimized and have been patented. This distribution enables a very low temperature gradient between the heated air and the PCB at the end of each phase (figure 6), and an optimized thermal transfer to limit thermal dispersions depending on the component mass. It enables the processing of a large number of different PCBs with the same profile, thus reducing the number of qualification procedures as well as the requirement for manufacturing control. Quick transfer of boards
A patented board handling system (figure 7) was adapted specifically for the phase convection technology. It ensures quick transfer of PCBs from one phase to the other with no vibration and brings versatility and ease-of-use when positioning the board support systems, as well as the ability to easily use the same system for dual lane applications. Journey through the phase convection units
Figure 8 shows the temperature profile at various stages of a board’s journey through a phase convection system. The board first enters the first phase convection unit where it is heated slowly respecting the temperature ramp up velocity of the components. The board is heated until it reaches the soak temperature required by the programmed profile. Once this is achieved, the gates that separate the four phases open quickly and without any adverse effect on the profile to allow the board to move into the second phase convection unit. The board then enters the second phase convection unit, where it is maintained at the soak temperature for the required time within the set profile. Once the correct time and temperature have been reached, the board will move to the third phase convection unit where convection is used to ramp up the temperature quickly to the reflow temperature. This offers a better thermal segregation between the soak temperature and the peak ramp, as is often necessary according to certain lead-free applications. Once the reflow temperature has been reached and the solder paste is above liquidus, the gate between the third and fourth phase convection unit opens to allow the board to pass into the cooling unit. Once inside the cooling unit, the cooling airflow allows the board to be cooled at whatever speed is required. The whole board area is cooled uniformly which limits risk of board distortion and solder joints weakening. Statistical process control
The convection system is totally closed loop and statistical process control (SPC) is achieved using pyrometers in each phase unit. The pyrometer measures the exact temperature on the PCB and not only the air flow around it. In a normal reflow oven both the air and the board are moving and regulation is made depending on the temperature of the airflow within the tunnel and the speed of the conveyor. The fact that pyrometers are used to measure the exact temperature on the PCB allows this data to be collected in real time and thus providing full SPC capability in the software, which collects the exact temperatures and profiles that each board has been subjected to and stores them for SPC trend analysis and tracking. Reduction in electrical and nitrogen consumption
Phase convection is a flexible technology, not only by the fact that the PCB’s temperature is controlled by pyrometers measuring the board temperature, but also by the architecture of the system. The four phase convection units allow the maximum temperature and profile flexibility for each level and can be programmed totally independently. However each level is also independent to the other two and can be programmed individually. With programmable input and output units, boards of different sizes could be used at the same time. An additional advantage is that if one or two levels are not needed due to a reduction in the throughput requirement, that one or more levels could be turned off, reducing electrical consumption. Another added advantage is that the oven can be started up level by level therefore reducing the current needed for the start-up of the system. Many companies are obliged to increase the power installations in the factories just to handle the start-up currents for reflow ovens which are also expensive to install and maintain. To lower downtimes, one feature of the Phase Convection X600 oven is the ability to set up a new process on one level while working on two others. Because the units are self contained and the convection units are closed loop with no outside connection or perturbation, Nitrogen consumption, when Nitrogen is used, is reduced by up to 50 percent with a very low oxygen levels throughout the oven. Self-cleaning system
A major advantage to phase convection technology is the fact that the system has a built-in flux cleaning system which makes sure that each of the phase units is kept clean and free from any flux contamination. The air is recuperated after the heated phase unit and passes through a flux capturing unit on each independent level. These flux-capturing units (figure 9) collect the flux residues at all times during the use of the oven. Each of these flux capturing systems is connected to the flux cleaning system which is closed-loop and which cleans the flux out of the flux capturing units into an easily changed flux residue collector at the bottom of the oven. The liquid in this flux collection unit needs to be changed once a week, reducing maintenance downtime of the system to less than five minutes per week. Once cleaned completely, the air is then recycled into the cooling phase unit before being sent back into the heated phase convection units. Conclusion
A constant challenge facing our industry is the reflowing of lead-free solders that have two major issues, higher reflow temperatures and increased flux contamination. Using phase convection technology, it is possible to heat the board to temperatures well above those required for leadfree solder paste. Standard ovens have difficulties reaching these higher temperatures without issues with conveyors, reduction of the lifetime of the heaters, temperature uniformity and flux contamination. Phase convection, even at much higher temperatures, is a technology that is controlled and clean. The additional advantages of reduced floor space, self-cleaning, reduced downtime; easy maintenance and being environmentally friendly make it a technology, which will set the standards for many years to come. | Lead-free soldering: Constraints and requirements | The requirements of the lead-free process have revealed new constraints in addition to the existing issues related to the soldering process. All of those constraints come down to the following basic rules: • Reduction of the T°C on assembled PCB’s because leadfree alloys require higher liquidus temperatures which is getting closer and closer to the maximum temperatures allowed by components manufacturers. • Need to improve effectiveness and ease-of-maintenance of the flux trap systems because of higher contamination due to the increase of the organics portion in the alloys composition and to decrease downtimes. • Ensure PCB’s integrity despite the going above and beyond the Tg. • Need for increased flexibility, including that of the manufacturing equipment, to better answer quick shift turnaround in production due to low volume high mix. • Need for enhanced quality control throughout the soldering process due to the narrower process window of lead-free. | Fig 1 Fig 2 Fig 3 Fig 4 Fig 5 Fig 6 Fig 7 Fig 8 Fig 9 About the author:
Jean-Jack Boumendil and Pascal Preti, ViTechnology, Mouans Sartoux, France, can be reached at info@vitechnology.com | | 
