Solder Reflow Profiling Play in Flexible Printed Circuit Board
The solder reflow profile is a crucial part of the process of heating and melting tin-lead or lead-free solder on a flexible printed circuit boa. It is used to optimize the reflow process so that the components and pads on the PCB are heated at particular temperatures for specific periods of time to ensure proper metallurgical bonding between the component leads and the PCB pads. This minimizes drastic temperature changes that could cause cold solder joints, charred boards, tombstoning or uneven wetting between the components and the pad surface.
In the reflow process, heat is transferred to the assembled bare board and its components from the reflow oven in order to melt and solidify the solder paste into a strong metallurgical bond with each of the component and pad contacts on the flexible printed circuit board. The ideal reflow solder profile will transfer this heat in an efficient manner to minimize excessive temperature changes which can result in cold or brittle solder joints, charred or damaged components, or tombstoning.
A common reflow profile is known as the Ramp-Soak-Spike (RSS) profile. During the preheating zone, the temperature of the components and PCB are slowly ramped up to a soak or dwell temperature. This is done to prevent the components from cracking due to thermal shock during reflow and also allows the solder paste to fully saturate and activate its flux properties. Then during the soak zone, the temperature is kept at a constant value while it waits for the reflow zone to reach its spike temperature.
What Role Does Solder Reflow Profiling Play in Flexible Printed Circuit Board?
At the peak of the reflow zone, the temperature rapidly increases to its maximum value for a short period of time – this is when the solder fuses with the component leads and the pad surfaces of the PCB to form a strong metallurgical bond. The temperature is then gradually lowered back down to the soaking zone temperature.
Once the profile parameters have been established, the assembly to be run in the reflow oven should be wired with external thermocouples that measure temperature throughout the reflow process. Ideally, at least one thermocouple should be attached to the pad of a light mass component such as a resistor or capacitor, another thermocouple to a high-mass component such as an IC, and a final thermocouple to a pad of a power or ground plane component. These thermocouples should be firmly and tightly fastened to the circuit board using epoxy or Kapton tape. Using solder or aluminum tape is not suggested as it can result in trapped air and loosening of the TCs during the reflow process.
Lastly, it is important to make sure that the reflow profile that is chosen is for the actual product that will be made. This will help avoid mistakes and unnecessary rework due to a mismatch between the profile and the end-user’s design. This can be a complex task, especially when the reflow profiles are provided by solder paste or preform manufacturers who do not have any knowledge of a particular PCB design or its heat sensitive components with temperature restrictions.