![]() ![]() ![]() 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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