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Smd's reflow soldering process and the benefits of IR soldering

date:2021-07-07 18:05:54

1. SMD reflow soldering:

After infrared heating reflow soldering, generally called infrared soldering, it is mainly used for soldering substrates with surface mount components. Generally, the substrate is transported through a machine with a series of heating elements, such as rod-shaped radiators positioned transverse to the transport direction. Components can be placed above the transferred substrate, but in many cases there are also components below the substrate to add heating rate and improve temperature uniformity.

2. Advantages of IR welding:

1) It is a clean and environmentally friendly method

2) The heating is non-touch, no need to accurately position the product to be welded

3) The heating power is easy to control

The primary disadvantage of IR heating is the difference in heating rate, which is caused by the different absorption coefficients of the materials used and the thermal mass of different components, which is related to the surface area of IR radiation that can be touched.

The temperature in the IR furnace is a mixture of radiation and convection, which is unclear. It is simply meaningless to measure the temperature with a thermocouple hung in the furnace; the only useful way is to measure the temperature of a specific product when it is transported through the furnace. If there are heaters below and above the conveyor belt (which is generally the case), they will affect their temperature control, especially when they can "see" each other.

The primary difficulty in infrared soldering of circuit boards with surface-mounted components is that components with different heat requirements have different heating rates. This means that when multiple components are soldered together, some may have exceeded the soldering temperature, while others are still far from this temperature. When heating continues until reflow, some components will reach unbearable high temperatures. In actual furnaces, three-step heating methods are generally used: rapid heating at the beginning, equilibrium and rapid heating again. Regarding the second step, the area in the furnace can be adjusted to generate a temperature plateau in the area between 120 0 C and 1600 C, where the temperature rise is as low as about 0.50 K/s and before the sharp rise in welding temperature is recovered The temperature difference can be homogenized. Rapid heating during the welding phase is necessary to limit the duration of this phase. In addition, the most important thing is that there is no or only a small temperature difference between different components before the beginning of the rapid heating of the welding stage to avoid any such welding defects, such as cold welding and leaching. Ideally, at the end of the homogenization process, that is, before the reflow soldering, the temperature of the light component and the heavy component are practically the same. However, this is difficult to obtain in production recirculation systems, even if these systems are appropriately long. The temperature-time curve is measured in a large-scale production furnace; in the first step, the pin temperature of the SOT-23 package rises faster than the pin temperature of the PLCC-68 package; then the temperature difference is reduced. During the second stage of heating, the difference slightly increases and decreases again. After that, the welding process with the rapid increase of temperature difference began, but at this moment the difference between the two temperature curves is still very large, so the difference between the peak temperatures reached is also very large.



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