Profiling in High Frequency PCB Design
High frequency pcb design involves selecting components with precision and placing them on the surface of the printed circuit board (PCB) in an accurate manner. This is accomplished using pick and place machines equipped with cameras, robotic arms, and vacuum nozzles. The PCB is then placed in a reflow oven where the soldering process takes place. This is where solder reflow profiling plays an important role. Ideally, the reflow profile will be optimized for the chemistry and alloy of the selected solder paste and the parts being assembled. A well-optimized reflow profile will reduce the risk of defects and enable a more efficient reflow process.
A reflow oven heats the high frequency pcb design and its components to a temperature above their melting point, known as the temperature above liquidus (TAL). As the assembly warms up in the preheating zone, the temperatures of the individual components rise at different rates due to differences in thermal inertia. This can cause a thermal soak and lead to problems like voids or solder bridging. An optimal reflow profile will feature a preheating zone, a soak zone, and a reflow zone, with the temperature rising in each of these zones at a controlled rate to ensure that the PCB and its components are heated evenly.
The temperature should increase at a maximum rate of 3 degC per second during the preheat zone. This allows the solvents in the solder paste to evaporate and the flux to become active. Then, the temperature should remain stable during the soaking zone to bring the pad and component surfaces to an even temperature. This helps prevent solder bridging and voids by allowing the applied solder to coalesce with the copper pads and component pins, which is more effective than just a rapid temperature rise.
Solder Reflow Profiling in High Frequency PCB Design
Finally, the temperature should spike during the reflow zone, which is where the solder melts and joins with the copper surface of the pads and component pins to form the joint. The ideal peak temperature for the reflow zone is 25 degC above TAL, because this is where solder wets best with the copper and tin.
Another critical aspect of the reflow profile is the cool down rate. The rate should not be greater than 4degC per second, which could damage the assembled PCB. An optimal cool down rate will help to maintain a finer grain structure for the solder, making it stronger and more resilient against vibration and stress.
Once the reflow profile parameters are established, the PCB should be wired with thermocouples to measure the temperature as it is subjected to the heating cycle in the reflow oven. At a minimum, one thermocouple should be attached to the location of a light mass part, such as a resistor or capacitor, and another to the location of a heavy mass part, such as an IC. A thermocouple should also be attached to the ground plane to measure the overall assembly temperature. This data is used to refine the reflow profile, and it can be compared with simulation predictions to validate and tune the profile.
