Professionals who have done BGA rework station work know that "warming up" is the prerequisite for successful rework. PCB processing at high temperature (315-426°C) for a long time will bring many potential problems. Thermal damage, such as pad and lead warping, substrate delamination, white spots or blistering, and discoloration. The "invisible" damage to PCB caused by high temperature is even more serious than the problems listed above. The reason for the huge thermal stress is that when PCB components at room temperature suddenly contact a soldering iron with a heat source of about 370°C, a desoldering tool or a hot air head to stop local heating, there will be a temperature difference of about 349°C on the circuit board and its components, resulting in "Popcorn" phenomenon.
Therefore, regardless of whether the PCB assembly plant uses wave soldering, infrared vapor phase or convection reflow soldering, each method generally requires preheating or heat preservation treatment, and the temperature is generally 140-160°C. Before the implementation of reflow soldering, a simple short-term preheating of the PCB can deal with many problems during rework. This has been successful for several years in the reflow soldering process. Therefore, the benefits of stopping preheating in the prevalence of PCB components are manifold.
The benefits of BGA rework station preheating are multifaceted and comprehensive. First, preheating or "heat preservation" of the components before the start of reflow helps to activate the flux, remove oxides and surface films on the surface of the metal to be welded, and the volatiles of the flux itself. Accordingly, this cleaning of activated flux just before reflow will enhance the wetting effect. The preheating of the BGA rework machine is to heat the entire assembly below the melting point of the solder and the reflow temperature. This can greatly reduce the risk of thermal shock to the substrate and its components. Otherwise, rapid heating will increase the temperature gradient in the component and cause thermal shock. The large temperature gradient generated inside the component will constitute thermo-mechanical stress, causing the embrittlement of these materials with low thermal shrinkage rate, resulting in splitting and damage. SMT chip resistors and capacitors are particularly vulnerable to thermal shock. In addition, assuming that the entire assembly stops preheating, the reflow temperature can be reduced and the reflow time can be shortened. Assuming that there is no preheating, the only way is to further increase the reflow temperature or extend the reflow time, whichever method is not suitable and should be avoided.
As a benchmark for soldering temperature, different soldering methods are used, and the soldering temperature is also different. For example, most wave soldering temperature is about 240-260℃, vapor phase soldering temperature is about 215℃, and reflow soldering temperature is about 230℃. To be correct, the rework temperature is not higher than the reflow temperature. Although the temperature is close, it is never possible to reach the same temperature. This is because all rework processes only need to heat a partial component, and reflow needs to stop heating the entire PCB assembly, whether it is wave soldering IR or vapor phase reflow soldering. The BGA rework station can be heated from the top of the components, supplemented by large-area infrared heating, and can quickly weld various SMD surface mount devices. The upper or lower temperature zone can be freely selected by the software or the upper or lower temperature zone can be used separately, and the upper and lower heating body energy can be freely combined. It can quickly warm up to the specified temperature, and at the same time, the peripheral temperature measurement interface can complete the precise detection of the temperature, and timely stop the analysis and correction of the temperature curve of the theoretically collected and repaired device.