When you think printed circuit boards (PCBs) cannot get any smaller, they continue to do so, especially in this day and age when IoT devices are springing up everywhere. See Fig. 1 for examples of denser and smaller PCBs. The smaller the PCB, the more challenges associated with creating the correct thermal profiles.
It’s especially critical to have a one of a kind thermal profile for smaller boards since they can only withstand a lower duration peak temperature cycle than the larger PCBs when subjected to reflow because of their size.
Thermal profiles are developed depending on the size of the PCB and the amount of critical components to be installed on the board, along with other factors that come into considerations as well, such as the number of layers and thickness of the board. Profiles for both tin-lead and lead-free are different because a lead-free profile requires a higher peak temperature than a tin-lead profile.
To get more details, check out our article on thermal profiles for small PCBs.
Meanwhile, here are a few tips and hints to help you better understand thermal profiling for small PCBs.
- The most important rule is that thermal profiles aren’t created equal and each board side requires a unique profile.
- Thermocouples need to be placed on the boards in a number of distinct and correct ways. Talk to the process engineer about this.
- Thermocouples are placed in areas where critical components are located when the boards are in array or panel form.
- Thermocouples provide actual temperature reading from specific areas where they are placed on the PCB.
- Thermocouples collect data to determine if certain areas are being overheated and/or whether some aren’t getting enough heat.
- Other factors that need attention are the packaging material for the components, as well as material for the PCB, such as polyimide, FR4 or Rogers because they have different Tg or glass transition temperature.
It’s worth noting there are sensitive components that cannot handle high temperatures and may fail during testing. They are regarded as sensitive based on their susceptibility to heat. Some devices like a micro BGA, for example, cannot be exposed to temperature exceeding 230°C. OEMs normally do provide instructions regarding component reflow guidelines that help the process engineer set up his/her reflow profile.