Solder Reflow Profiling Optimizes PCB Assembly Quality
Profiling Optimizes PCB Assembly Quality
The reflow soldering process joins together thousands of tiny electronic components attached to a circuit board by heating the components and the board to a temperature at which the solder paste is transformed from its solid state into its molten state. The thermal profile of the reflow process is crucial to the success of the assembly and the quality of the finished product, especially with today’s smaller and more complex PCB assemblies. A poor thermal profile can lead to a variety of defects such as tombstoning, underfill, and voids. Solder reflow profiling optimizes the temperature and time of each phase of the reflow process to mitigate these issues.
A typical reflow profile has four phases, preheat, soak, reflow and cooling. The preheat phase preconditions the assembly and removes any volatile solvents from the flux before the actual reflow starts. The preheat phase also helps to avoid thermal shock by gradually exposing the board and the components to the reflow temperature. The soak phase, a fixed temperature zone, gives the circuit board assembly a chance to adjust to the new temperatures and allows the flux to activate before the reflow begins.
During the reflow phase the reflow temperature rises above the melting point of the solder alloy and the intermetallic layers begin to form. The peak reflow temperature and the time spent above liquidus (TAL) are used to define the actual reflow portion of the profile. The TAL should be shorter than the peak temperature to reduce oxide formation and prevent thermal shock. The cooler phase cools the assembly to a temperature below the CTE (coefficient of thermal expansion) of the joining surfaces and is used to help ensure that the components and solder joints are mechanically sound.
Solder Reflow Profiling Optimizes PCB Assembly Quality
The main objective of a reflow profile is to maintain a consistent, repeatable temperature change during the entire reflow process. In order to do this the reflow process needs to be well controlled. A good way to control the temperature changes during reflow is to use a Ramp-Soak-Spike (RSS) profile. The RSS profile starts with a rapid ramp up to a pre-reflow temperature, then a constant temperature or soak phase, then finally the temperature spikes to the peak reflow temperature before quickly cooling down.
The most important step in the reflow profiling process is to properly calibrate the thermocouples that are used to measure the temperature of the components during the reflow cycle. This is accomplished by attaching the TCs to locations on the component and the board and then using software to correlate the data. Ideally, this will provide an accurate model of the thermal profile.
This is critical to ensure that the reflow temperature is within the temperature limitations of the component and to eliminate the risk of solder bridging and tombstoning during the reflow process. Using the right thermal profile calibration tools will save time and money and help to reduce the risk of costly defects such as voiding. For more information on how to do this, read our blog post Using the Right Profile Calibration Tool to Eliminate Soldering Defects.