PCBA Tech - Regarding SMT reflow soldering process settings

At the beginning of this article, I mentioned that many users are not doing well in SMT chip processing reflow soldering. The following are some common problems and errors. Readers may wish to see whether these problems exist.

1. Set the furnace temperature exactly according to the temperature curve index provided by the solder paste supplier

At present, most users only use the information provided by the solder paste supplier as a basis for setting the soldering temperature. This raises two questions. First, the curve suggested by the solder paste supplier only considers the solderability of the solder paste, and it is impossible to know the other requirements on the user's PCBA. Therefore, the curve can only be used as a reference rather than a standard. Especially for the temperature and time of the soldering zone, the user's consideration is often not about the solder paste. In addition, solder paste suppliers are often not very accurate in the characteristics of the constant temperature zone, which is related to the characteristics of the solder paste supply industry. Therefore, the user's welding process settings cannot be optimized.

2. Lack of the concept of "craft window"

In engineering projects, we are very taboo about the lack of the concepts of ‘window’, ‘upper and lower limits’ and ‘tolerance’. Because this will make us ignore and unable to optimize and control our technical characteristic parameters. The same is true for the reflow soldering process.

Although the above figure 2 explains the principle, we only use a single curve indicator. But in actual work, we must have upper and lower limits for each process characteristic parameter. That is, there is a clear ‘process window’ to operate.

3. Misjudgment of hot and cold spots

With the process window, it is appropriate to ensure that the temperature of each point on the PCBA is within this window. In actual work, it is impossible for us to measure every solder joint. Therefore, the main point of the reflow soldering process setting is how to confirm the coldest and hottest spots on the PCBA. When we can meet these two requirements through process adjustments, other solder joints will naturally be met at the same time. In the traditional practice, users often determine where the temperature measuring thermocouple should be set by observing the size of the device. This is a very old practice. In the past infrared welding technology may be somewhat reliable., But its reliability is very small in hot air welding. If the reader has ever seen a small rectangular part such as 0603 with a temperature difference of up to 8 degrees at both ends, or a temperature difference of 13 degrees around the QFP pins, or when there is a temperature difference of up to 20 degrees at the solder joint position of the same device in different circuits on the imposition, You will believe that this method of observation and forecasting is absolutely unacceptable.

4. Unclear

When setting up and modulating the welding process, we may encounter products that are difficult to design. These products may have a huge difference in thermal capacity due to the selection and layout of the components on the board. If the capability of the reflow oven used is not very strong, or the solder paste used is not very tolerant on the soldering window, the process modulation may not be able to take into account the quality of all solder joints. In this case, we must make a trade-off on the quality of the solder joints. Many users are unable to make effective choices due to inadequate DFM/DFR (design for manufacturability/reliability design), or the production department’s lack of understanding of the life requirements of each material/solder joint on the product. Many users are completely unaware of this method of process modulation and optimization.

5. Misunderstanding the five processes as a single process

As mentioned earlier in this article, SMT chip processing reflow soldering actually includes a set of five processes including heating, constant temperature, soldering, soldering, and cooling. If this important link is ignored, it may cause confusion or wrong decisions for us to solve process problems. For example, the handling of solder ball problems, solder ball problems may occur at the time of heating, constant temperature, or improper handling of the soldering process, but the causes are different. The solder ball problems caused by the heating process are mostly caused by gas explosions, and most of them are related to material quality, inventory time and conditions, and solder paste printing process (Note 3). But if it is caused by a constant temperature process, it is mostly related to improper temperature/time setting or deterioration of the solder paste. Related to the soldering process, it is caused by a high degree of oxidation and improper temperature/time settings. The appearance of the solder balls appearing in each case is different, and the processing method is also different. If it is not analyzed as different processes and different mechanisms, it is only possible to adjust the SMT equipment indiscriminately or try blindly.