Never use high-temperature tapes, such as polyimide (kapton) or aluminum tapes, because they tend to come loose during reflow and you may measure the temperature of the air in the oven, not the temperature of the solder joints. Thicker thermocouple wires add too much heat sink. Type-k thermocouples with a wire gauge of 36 AWG should be used. When developing any profile, it is important to use the right thermocouple. If the tight reflow temperature band of 226˚ to 228˚C is difficult to achieve to solderīoth tin/lead and lead-free BGAs in a backward- compatible scenario, consider selective laser soldering, or find an alternative source for BGAs with tin/lead balls. A peak temperature of 226˚ to 228˚C with 45 to 60 seconds time above liquidus (TAL) will be sufficient to reflow lead-free BGAs without damaging all tin/lead components on the same board. Therefore, a peak temperature of 210˚ to 220˚C will be suitable for tin/lead, but inadequate for lead-free BGA balls with melting point of 217˚ to 221˚C. Keep in mind that you will use tin/lead solder paste because most of the components on the board are tin/lead. If you want to solder all tin/ lead components and some lead-free BGAs in the same oven because tin/lead versionsĪre unavailable, you must use a peak temperature that will not damage the tin/lead components, but is sufficient enough to reflow lead-free BGAs. If you do not have access to a laser selective soldering system, use option 2. The selective laser soldering system will place and solder only lead-free BGAs without impacting neighboring tin/lead components already soldered in the convection oven. After tin/lead components have been reflow-soldered, use selective soldering with a laser to place and solder all lead-free BGAs. Do not place lead-free BGAs with other tin/lead components. Use the standard tin/lead reflow profile with peak reflow of 210˚ to 220˚C for all components except lead-free BGAs. What kind of reflow profile should be used? There are two options: If the tin/lead profile with maximum peak temperature of 220˚C is used because all components are tin/lead, BGA balls will reflow partially, or not at all, creating a serious solder joint reliability issue. The real problem arises when using lead-free BGAs on a primarily tin/lead board. There is no issue when using leaded components with lead-free surface finishes. It is not economical for component suppliers to supply both tin/lead and lead-free versions of the same component. Let’s look at backward compatibility, in which some components are only available with lead-free surface finishes. Backward- compatibility issues in which some lead-free components must be used on a primarily tin/lead board can compound the problem further. The same is true for smaller, temperature sensitive components requiring lower peak temperatures for shorter durations. Temperature for longer durations, a tighter process control is required to achieve good soldering results in lead-free assemblies. As discussed in my February column, to accommodate constraints imposed by large components with high thermal mass requiring higher peak
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