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Next-generation Reflow Technology
December 31, 1969 |Estimated reading time: 11 minutes
Worldwide lead-free solder paste implementation figures to quicken as components become more diverse, ranging from large ball grid arrays (BGA) to ever finer-pitched parts, and requiring the new reflow ovens to provide more precisely controlled thermal transfer.
By Hiro Suganuma and Alvin Tamanaha
Tables 1 and 2 list the characteristics and wetting parameters of typical lead-free solder pastes. Table 1, showing the main metal compositions and characteristics (excluding those containing bismuth) of various lead-free materials, reveal them to have higher melting temperatures than conventional Sn/Pb pastes. From the wetting parameters on the copper data in Table 2, it is clear that they also do not wet as well as Sn63/Pb37 paste, which spreads thin and wide. Further, additional tests have shown that while Sn63/Pb37 pastes have a spreadability of 93 percent, lead-free pastes range from 73 to 77 percent.
Reflow conditions of Sn63/Pb37 solder pastes have a melting temperature of 183°C, with the peak temperature of leads on small components reaching 240°C, and those of large components attaining 210°C. However, this 30°C difference between large and small components does not affect their lifespans. This is because solder joints are formed at 27° to 57°C over the melting temperature of the paste. And since metal wettability generally improves at higher temperatures, these conditions are favorable for production.
Figure 1. Process windows for Sn/Pb (left) and lead-free solder paste. To maintain stable reflow profiles, the ovens must reduce peak temperature differences between large and small components.
With lead-free solder pastes, however, the melting point of Sn/Ag compositions, for example, becomes 216° to 221°C. This results in the leads of large components being heated to higher than 230°C to ensure wetting. If the peak temperature of leads on small components is to be kept at 240°C, then the differential between large and small components must be reduced to less than 10°C. This also drastically reduces the difference between the melting point of the solder paste and the peak reflow soldering temperature, as shown in Figure 1. Here, a reflow oven must reduce the peak temperature differential between large and small components and maintain stable profiles throughout the printed circuit boards (PCB) passing in-line to attain high productivity levels.
Peak Temperature MaintenanceThermal capacity and transfer time to parts to be heated also must be considered. This is especially true for BGAs whose bodies (and the PCB) are heated first. Heat then is transferred to the lands and BGA spheres for joint formation. For example, if 230°C air is applied to the package surface even at significant velocity the lands and BGA spheres will heat gradually rather than instantly. Thus, to prevent thermal shock, packaged devices must not be overheated in the reflow zone while the lands and BGA solder bumps are heated for joint formation.
Reflow Oven Heating SystemsThe two most common heating methods for reflow are convection air and infrared radiation (IR). Using the air as the medium to transfer heat, convection is ideal for heating components that "protrude" from the board, such as leads and small parts. However, in the process, a "boundary layer" between the convection air and PCB is formed, making heat transfer to the latter inefficient, as shown in Figure 2.
Figure 2. Boundary layer diagram that is formed during convention air heat transfer, making the process inefficient.
With the IR method, infrared heaters transfer energy via electromagnetic waves, which will heat components evenly when properly controlled. However, subtract such control and PCB and component overheating can occur. IR mechanisms such as lamps and heater bars have limited surface areas with most of the heat transfer concentrated on the area of the PCB directly below, hampering even coverage. For this reason, IR heaters must be larger than the targeted boards to ensure a uniform heat transfer and to have enough thermal capacity to prevent PCB cooling.
Of the three thermal transfer mechanisms conduction, radiation and convection only the latter two can be controlled by a reflow oven. Thermal transfer by radiation is efficient and powerful, as expressed in the following equation:
T(K) e = bT4
Where the thermal energy or emission power of radiation e is proportional to the fourth power of its absolute temperature, and b is the Stefan-Boltzman constant.
Because the thermal transfer power of infrared heating is very sensitive to the heat source temperature, precise control is required. While convection heating is not as powerful as radiation, it does provide good, even heating.
IR + Forced-convection HeatingToday's most advanced reflow oven technology combines the merits of both convection and IR heating. The peak temperature differential between components can be held below 8°C, while those between PCBs during continuous mass production can be stabilized to approximately 1°C.
The basic concept of IR + forced convection is to use infrared as the main heating source for optimum thermal transfer and to capture the uniform heating qualities of convection to reduce the temperature differential between components and the PCB. Convection aids in heating components with large thermal capacities, such as BGAs, while cooling components with smaller capacities.
Figure 3. Heating curve diagram. The heating disparity shown will occur if only one source of heat is used, whether IR or convection.
In Figure 3, (1) represents the heating curve of a component having a large thermal capacity and (2) of a component with a small thermal capacity. The heating disparity shown will occur if only one source of heat is used, whether IR or convection. When only IR is used as the main source, the solid line curves will result. However, the heating curves depicted by the dotted lines display the merits of a combination IR/forced convection system wherein the effect of adding forced convection will be to heat components below the set temperature while cooling those parts that have risen above the temperature of the hot air.
Figure 4. How nozzle convection action compares to that of forced convection. The latter is seen as a more efficient method of heat transfer.
A second feature of an advanced reflow oven is its ability to transfer convection heat to the PCB more effectively. Figure 4 contrasts the thermal transfer characteristics of conventional nozzle convection heating vs. forced convection heating. The latter technique can transfer heat evenly to the PCB and components three times more efficiently than nozzle convection.
Finally, unlike the bar and lamp-type IR heaters used in older reflow ovens, the newer-generation systems use an IR panel heater that is considerably larger than the PCB to ensure uniform heating (Figure 5).
PCB Heat DeviationA test sought to compare the temperature difference between a QFP140P and PCB, and a 45 mm BGA and PCB under three conditions: when reflowing solely with IR panel heaters, only with convection heat and with a combined IR/forced convection heating system.
The convection reflow yielded a temperature differential of 22°C between the QFP140P and PCB (70 seconds after insertion of the PCB during preheating). By contrast, heating via the combined system resulted in a temperature disparity of only 7°C, while convection heating of the 45 mm BGA resulted in a temperature differential of 9°C and the combined system reduced the difference to 3°C. Additionally, the peak temperature differential between the PCB and 45 mm BGA, when reflowing with the combined system, was only 12°C, using a conventional profile. This difference could be reduced to 8°C with the use of a trapezoid profile, described later. (In continuous mass production, temperature instability in the reflow oven will have a significant effect when using lead-free solder pastes. Testing has shown that the peak temperatures of PCBs that were 250 x 330 x 1.6 mm in size and inserted 5 cm apart were within approximately 1°C.)
An Optimum Reflow ProfileWith lead-free solder pastes, the temperature difference between components must be minimized as much as possible. This also can be achieved by adjusting the reflow profile. With conventional profiles, while temperature differences between components cannot be avoided when the board is spiked, they can be reduced by several methods:
Lengthening Preheating Time: This greatly minimizes the thermal difference between components before spiking to peak reflow temperatures. Most convection reflow ovens use this measure. However, since the flux can be evaporated too rapidly by this method, it may result in poor wetting because of lead and land oxidation.
Raise the Preheating Temperature: Conventional preheat temperatures generally are 140° to 160°C, which likely will be raised to 170° to 190°C for lead-free solder pastes. Raising preheat temperatures reduces the required spike to the peak temperature, which in turn reduces the temperature differential between components (lands). However, if the flux cannot accommodate the higher thermal levels, it will again evaporate and result in poor wetting because of lead and land oxidation.
Trapezoid Profile (Extended-peak Temperature): Extending the peak temperature time of components with small heat capacity will permit components with large heat capacity to reach the required reflow temperatures and avoid overheating the smaller components. With the trapezoid profile, shown in Figure 6, a modern combination reflow system can reduce the temperature difference between a 45 mm BGA and small outline package (SOP) body to 8°C.
Figure 5. New-generation reflow ovens use panel heaters larger that the target PCBs to ensure uniform heating.
The Nitrogen Reflow OvenLead-free solder pastes may exhibit diffi-culty in wetting since their melting temperatures generally are high while the differences between the peak reflow temperatures are not very large. Additionally, the metal compositions of lead-free pastes generally feature poor spreadability. Further, lead-free pastes with high melting temperatures will create problems when mounting top- and bottom-sided PCBs. During reflow of the
A-side, greater B-side land oxidation will occur at the higher temperatures. Above 200°C, the thickness of the oxidized membrane increases rapidly, which can lead to poor wetting when reflowing the B-side.
Solder pastes with a composition of Sn/Zn also may present problems (Zn oxidizes easily). If oxidation occurs, the solder will not fuse with the other metals. Accordingly, nitrogen use will be required to maintain high productivity in lead-free processing.
Figure 6. The trapezoid heating profile decreases heat deviations at peak temperatures and increases solder flow.
In a combination IR/forced convection system with IR heater panels as the main heating source (and convection as a uniform heating medium), nitrogen consumption can be reduced to less than one-half the amount required for current full convection reflow ovens. (The maximum nitrogen consumption of an oven that can accommodate 450 mm width PCBs is 2,00l per minute.) An optional internal nitrogen generator may eliminate the need for large N2 tanks.
Automated Process MonitoringIn addition to requiring next-generation oven technology, the narrow lead-free process window makes continuous process monitoring imperative because even slight process drift amounts can result in an out-of-spec soldered product. The most efficient way to monitor the reflow process is with an automated, continuous real-time thermal management system. The real-time thermal manager permits assemblers to obtain and analyze real-time live data on their soldering processes by continually monitoring process temperatures in the reflow oven. Such systems typically consist of 30 thermocouples embedded in two slim stainless steel probes permanently mounted just above or below the conveyor. The thermocouples monitor process temperatures continuously, taking readings as often as every five seconds. These temperatures are displayed as process profiles on the oven controller's PC screen (Figure 7).
Figure 7. Placement of thermal manager probes in oven. Readings may be taken as often as every five seconds.
The real-time thermal manager provides a product profile for every board processed by creating a mathematical correlation between the profile, as measured by a pass-through profiler, and process temperature, as measured by the real-time thermal manager thermocouple probes. This "virtual" product profile is calculated every 30 seconds, with profile statistics such as peak temperature also calculated and updated continually.
The real-time thermal manager sounds an alarm if the process drifts, and shuts down the feed conveyor if it goes completely out of spec. This enables users to maintain a permanent record of the thermal profile for every board produced, and can feed data to an external statistical process control package for real-time process control. The data from the real-time thermal manager also can be distributed via the Internet to remote locations, maximizing the value of scarce engineering resources.
Other benefits of real-time continuous profiling include elimination of production-stopping confirmation profiles with standard pass-through profilers and scheduling of preventive maintenance on an as-needed basis. Research has found that modern forced-convection ovens can operate efficiently for extended periods without maintenance. Use of a real-time thermal manager alerts users to deterioration in oven performance instantly and permits preventive maintenance to be scheduled when required.
Lastly, a tightly controlled thermal process can significantly reduce solder-joint defects and the associated expensive rework. In fact, the real-time thermal manager has become an industry-wide indicator of dedication to quality.
Reflow Profile OptimizationState-of-the-art software now can simplify the task of converting to lead-free assembly. Among the more recent software is an automated profile prediction tool that permits users to define optimal profiles in minutes. The tool centers the profile in a window designated by users who wish to set specific limits. An example is the trapezoid profile previously mentioned i.e., if the assembly cannot withstand temperatures above 240°C but must take a minimum of 230°C, the automated prediction tool will find an optimal profile that will be centered between the high and low limits.
ConclusionThe implementation of lead-free solder pastes will greatly reduce the reflow process window, especially with respect to required peak temperatures. The temperature difference between components must be reduced and variations in the reflow oven during continuous production must be minimized for high-quality, high-productivity manufacturing. To accomplish this, thermal transference via the reflow oven must be controlled precisely. A combination IR/forced convection system, with each heating element individually and precisely controlled, provides the means required to process lead-free assemblies reliably. When combined with an automated profile prediction tool and a continuous real-time thermal manager, this reflow technology offers prospective lead-free electronics manufacturers the potential for zero-defect production.
ACKNOWLEDGEMENTT. Ogino, K. Nakao and E. Iwasaki, Electronics Component Department, Furukawa Electric provided technical support for this article.
HIRO SUGANUMA, executive vp, and ALVIN TAMANAHA, general sales and engineering manager, may be contacted at Seika Machinery Inc.; (310) 540-7310; Fax (310) 540-7930; E-mail: hiro@seika.com or alvin@seika.com.