Pcba printed circuit board
Printed circuit boards are used in a variety of commercial, professional and household products. They have been used for decades and are a crucial part of the electronic manufacturing services industry (EMS). The EMS industry is a multibillion dollar sector designing, manufacturing and distributing electronic parts and assemblies. In addition to the high volumes of PCBAs produced, it also produces a number of other products including wired and wireless communications equipment, medical devices and automotive parts.
Solder reflow profiling is the process of determining the best temperature profile to use in the reflow oven during the assembly of printed circuit board pcba. Profiling allows the reflow process to achieve consistent and high quality solder joints. The key to good profiling is the control of the peak temperature and time above liquidus, which determines how well the copper and tin connect in a solder joint.
When profiling a reflow oven, the first step is to determine an ideal preheating zone. This is the initial heating of the assembly and is critical to preventing thermal shock to sensitive components. The preheat zone should increase the temperature of the assembly at a rate of 1 to 3 degrees Celsius per second. This is important because it prevents components from warping or solder paste drying.
What role does solder reflow profiling play in Pcba printed circuit board?
The next step is to determine an ideal soak zone. The soak zone raises the temperature of all areas of the assembly to an even level, which is difficult because the various component parts absorb heat at different rates due to differences in their thermal inertia and color. The duration of the soak zone depends on the type of solder paste being used, but it is recommended that it should not exceed 30 seconds.
During the reflow phase, the flux in the solder paste reduces surface tension between the tin and copper, which leads to a solid metallurgical bond. This is a critical step because it allows the two metals to join in a mechanically sound manner. The amount of time above liquidus during reflow is determined by the peak temperature and a TAL of 30-90 seconds is typical.
A cooling zone is then established to slowly cool the assembly to a point where it becomes a solid. This is done to avoid thermal stresses that can lead to fracturing, tearing and voids. It is common to recommend a cooling slope of 4 degC/s.
In order to accurately profile the reflow oven, thermocouples should be placed on the assembly. These should be attached to the warmest and coldest points on the board. Ideally, these should be located at interconnections (lead-to-pad junctions) where the most and least heat is applied to the board. Thermocouples are usually attached using epoxy or solder, as other methods such as Kapton tape are not suggested because they can loosen the adhesive over time.
