Reflow Profiling Optimize Soldering in PCB Assembly
Reflow profiling optimizes soldering in PCB assembly by controlling the amount of heat transferred into the board during the reflow process. The profile controls the temperature of the components, the reflow oven and the reflow paste to transfer enough heat to melt the solder and form the solder joints while avoiding damaging the board or its components. Generally speaking, the reflow profile can be broken down into four phases – preheat, soak, reflow and cooling.
The preheat phase heats the components, the printed circuit board assembly and the solder paste to a specified soak or dwell temperature. The soak temperature is usually a little bit higher than the reflow temperature to prevent the solder from melting or damaging the components or PCB during the reflow process. The preheat phase also preconditions the assembly, removes flux volatiles and reduces thermal shock. The preheat phase should be a little longer than the actual reflow cycle.
During the reflow cycle the assembly is heated to the desired peak temperature and then reflowed to the liquidus point (183degC for Sn63 and 179degC for Sn62) and a time above liquidus (TAL) target of 30-90 seconds. The reflow cycle should be as short as possible in order to minimize intermetallic growth and brittleness of the solder joint. However, a high TAL can result in marginal or non-wetting of the solder paste on the surface of the component resulting in poor wetting and possibly defects.
How Does Reflow Profiling Optimize Soldering in PCB Assembly?
After the reflow cycle, the assembled product is allowed to cool at a rate not to exceed 4degC/second. This allows the solder to reflow and solidify into a fine grain structure while minimizing stresses on the components and the solder joints which could cause brittleness.
There are many different reflow profiles available depending on the product design and the reflow process settings, or recipe, in the reflow oven. A common profile is the Ramp-Soak-Spike (RSS) profile, in which the temperature ramps up in the preheating zone, remains constant during the soaking zone and then spikes to its peak in the reflow zone. This profile is very effective at reducing defects such as bridging and voiding.
Another popular reflow profile is the Tent or No-Soak profile, which has the temperature remain constant from the preheat zone through to reflow, but with no distinct soak phase. This is more effective than the RSS profile for most applications and works well with most solder paste formulations but it should be used cautiously with large components or high-mass substrates. It should also be used with Pb-free reflow chemistries which require more time to reach and stay above the liquidus point.
In addition, the selective soldering process requires sophisticated machinery. This can make it costly to operate and maintain. However, using modular equipment that can be upgraded with new technology and training for staff can help to offset these costs.