Wave Solder Thermal Profiling
The right methodology will go a long way toward reducing defects.
Temperature controls on different soldering processes always have to be monitored, and due to the quantity of factors that take place in wave soldering, it is in this process where thermal control becomes indispensable. Whether the assembly is simple (single-sided PCBs populated with conventional components) or complex (double-sided PCBs populated with QFPs, BGAs, etc., on both sides), it is important to monitor several parameters in order to obtain acceptable, reliable solder joints.
Before setting the profile (Figure 1), check that the potential process variables are monitored and under control. For example, all the parts to be soldered (the PCB and components to be mounted) must pass solderability requirements according to company standards. Also, check the flux shelf life, the density (for foam fluxers units), and the quantity and penetration.
The solder pot measurements must be taken and contamination levels must not exceed established standards. Likewise, the machine itself must be characterized according to established standards. These characteristics include preheat and solder bath temperatures, conveyor speed, wave heights, contact time and parallelism. Several soldering profiling devices can help extract all these technical data.
Profile considerations. As in the reflow process, every product has its own thermal profile. The thermal profile’s features will be determined by several elements:
The flux type, and the time and temperature to completely evaporate the solvents and achieve optimum activation. For Pb-free applications, due to the slower wetting and higher temperatures, the flux must offer sustained activity and a higher thermal activation point.
The wave configuration. Depending on the product, different configurations will be used. For example, for Pb-free applications or products with high thermal mass, it is recommended to raise the contact time to increase thermal transfer. To maintain the same throughput (same conveyor speed) and preheat ramp, according to manufacturer requirements, it is necessary to use special nozzles that provide a higher contact time. Preheat module configuration is another issue. Zones with top and bottom preheat areas will increase the board assembly temperature without increasing the bottom-side preheat and overstressing the flux.
The solder frame. This input is included in our thermal profile due to the thermal absorption of the material and the so-called shadow effect in areas close to the components to be soldered.
When soldering problems because of high thermal dissipation in the assembly are encountered, could the thermal profile be increased as needed to achieve these requirements? Not without compromising product reliability and adding to the potential for product failure. In such cases, review the product design or PCB substrate characteristics, and change the chemistries and solder materials, processes or equipment, if allowed.
Creating the profile. To obtain the profile, a sample coupon is needed. An assembled coupon is recommended, if possible, to most closely simulate the thermal response of the actual assembly.
Next, the thermocouples will be connected to the through-hole components to be soldered (Figure 2). If used, the solder frame will protect the components. Ceramic chip capacitors will be placed on the bottom to control for thermal shock.
To obtain data, thermocouples are connected to the profiler; the equipment is turned on, and the board and profiler are run through the wave. When the board exits the machine, the data are downloaded by the software application.
With those data, the preheating ramp, temperature prior to the solder bath, thermal shock, contact time and cooling ramp can be analyzed. At the same time, soldering requirements like wetting or solder fill will be validated.
Common and uncommon problems. During profile development, it is very common to find solder defects such as opens, shorts or insufficient solder fill. However, other harder-to-detect issues can be uncovered, such as cracks in ceramic components (Figure 3) and undesired reflow on topside components. Cracks from excessive thermal shock (Figure 4) can form and can cause variation during operation or internal shorts. Undesired reflow is due to an excessive temperature on the top of the PCB and can cause component misalignment or variation in solder joint properties.
It is necessary to understand the importance of temperature control in all the processes, mainly in reflow and wave. To find the root cause of some board failures can become very difficult and, of course, the cost of repair can become a problem too. With a good thermal profiling methodology, this cost could be saved or reduced significantly.
Francisco Anaya is a process engineer at Celestica (Valencia) (celestica.com); fjaosa@celestica.com.
