Developing a reflow profile for electronic assemblies is like trying to figure out the time and temperature (bake profile) to bake turkey, chicken, and shrimp for the same length of time at the same temperature in the same oven without undercooking the turkey or overcooking the shrimp. The difference, however, is that using a bad reflow profile for electronic products is much more consequential than an undercooked turkey or overcooked shrimp and some disap-pointed guests you invited for dinner.
The basic idea of developing a good profile is to make sure all solder joints reach the minimum temperature to achieve good solder joints but don’t exceed maximum temperature to prevent damage to components or to solder joints. This is not an easy task, especially when you have a board with components of different sizes and thermal masses: BGAs, sockets, finepitch, BTCs, and 0402/0201, similar to the previous example.
The good news is that we have the tools, technologies, and processes to deal with the profiling challenge. What I plan to do in this and a few follow-up columns is to provide specific guidelines and rules for developing a unique profile for each product without any damage and warpage to components and boards and with minimum possible profile-related defects. More specifically, I will try to:
- Provide an overview of various types of thermal profiles, purpose, key requirements, and challenges in developing thermal profiles.
- Explain the importance of different soldering zones—such as preheat, soak, reflow, and cooling—and their impact on solder quality.
- Discuss the details of how and where to attach thermocouples to achieve the desired soldering temperatures in various soldering zones.
- Emphasize the importance and difference between time above liquidus (TAL) and true TAL and their impact on the quality of solder joints, especially on head-on-pillow.
- Provide specific recommendations for thermal profiles for most commonly used
- Pb-free (SAC and low-temp), Sn-Pb, and mixed alloys.
- Discuss conflicting requirements of different types of packages and alloys on the same board and show some examples of profile related defects.
What Is a Thermal Profile?
A thermal profile is a unique temperature vs. time plot for each fully populated printed wiring board assembly (PWBA) using thermocou-ples attached to the solder joints with high-temperature solder, copper, or aluminum tapes to selected representative components on the board as the board travels at a given belt speed through various temperature zones of an oven or soldering system. There is a lot in that one sentence, but it is worth paying attention to some of the things I mentioned.
For example, you need to use a fully populated board and definitely not a bare board or another board that possibly looks like the board but a fully populated board that comes as close as possible to the thermal mass of the product you are profiling. You are using thermocouples that are attached with high-temperature solder or copper or aluminum tapes and not Kapton tapes used by many people. The location of attachment is generally the solder joint itself (unless you’re specifically monitoring other spots, such as temperature-sensitive body components or the bare board itself, when using low-Tg PCBs.) And even the length (about a meter) of the thermocouple wires and their gauges (about 36 gauge) is very important so that you don’t get the incorrect temperature.