Issue link: https://iconnect007.uberflip.com/i/1243344
88 SMT007 MAGAZINE I MAY 2020 d. 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-pil- low. e. Provide specific recommendations for ther- mal profiles for most commonly used Pb-free (SAC and low-temp), Sn-Pb, and mixed alloys. f. Discuss conflicting requirements of differ- ent 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 wir- ing 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 pop- ulated 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 ther- mocouples that are attached with high-tem- perature 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 thermocou- ple wires and their gauges (about 36 gauge) is very important so that you don't get the incor- rect temperature. And you need a unique profile for each product. One profile will not work for all products, even if they tend to look alike. The reason is simple. Each PWBA has a unique thermal mass. Different boards have vary- ing thermal mass because they have a vari- ous number of layers, and the number and location of ground and power planes may be different, and they certainly have different types of components. Even the same board will need two profiles if it has components on both sides. For example, a single-sided board will need different temperature and belt speed settings than the same board loaded with components on both sides. Think of a single-sided board as a chicken and the dou- ble-sided heavier board as a turkey. Chicken and turkey require very different thermal pro- files to prevent overcooking or undercooking. PWAs are no different. What Is a Misleading Profile? Profiles not correctly developed are mislead- ing profiles. For example, profiles using bare boards will give you a misleading profile. You must use a fully loaded board. Profiles using rejected bare PCBs and dummy components or pennies that simulate the thermal mass of actual components is a good place to start if real boards and components are not available or affordable. Profiles using Kapton tapes to attach ther- mocouples will give you wrong tempera- tures, especially since they tend to come off during the profiling process. You must attach TCs using high-temperature solder or copper or aluminum tapes. Thermocouples attached to the PCB surface and not attached to solder joints will give you a misleading profile. It is important to keep in mind that developing the thermal profile is a destructive operation but Profiles not correctly developed are misleading profiles. For example, profiles using bare boards will give you a misleading profile.