1. Reflow soldering of SMDs:
Reflow soldering by infra-red heating, often called infra-red soldering, is used mainly for the soldering of substrates with surface mounted components. Usually, the substrates are conveyed through a machine having a series of heater elements, e.g. rod-shaped radiators positioned transversely to the transport direction. The elements may be placed above the substrates being conveyed, but in many cases there are also elements below the substrates to increase the rate of heating and to improve the homogeneity of the temperature. A possible set-up of such a machine is shown in figure below.
Sketch of an IR-soldering furnace. The heating is chiefly characterized by the wavelength of the elements in the machine.
2. Benefits of IR soldering:
i) it is clean and environmental friendly method
ii) the heating is contact free and accurate positioning of the product to be soldered is not necessary
iii) the heating power is easy to control
The main disadvantage of IR heating is the difference in heating rate, resulting from the different absorption coefficients of the materials used and from the different component thermal masses, related to the surface area that is accessible to IR radiation.
The temperature in an IR furnace, with a mixture of radiation and convection, is ill-defined, and measuring the temperature with a thermo-couple hanging in the furnace has little or no meaning; the only useful method is to measure the temperature of a defined product while it is transported through the furnace. If there are heaters below and above the conveyor (which is usually the case), they mutually influence their temperature control, especially when they can ‘see’ each other.
The main difficulty with infra-red soldering of boards with surface mounted components is the different rate of heating of components of different thermal demand. This means that, when a variety of components are soldered simultaneously, some may have already passed soldering temperature, whereas others are still quite far from this temperature. When heating is continued until reflow, some components will have reached an intolerably high temperature. In practical furnaces, often a three-step heating approach is used: starting with rapid heating, equilibration and again rapid heating. For the second step the zones in the furnace may be adjusted to produce a kind of temperature plateau in the region between 120 0 C and 1600 C, where the temperature rise is as low as about 0.50K/s and where the temperature differentials can homogenize before the steep rise to the soldering temperature is resumed. Rapid heating in the soldering phase is necessary in order to limit the duration of this phase. It is furthermore most important that there are no or only small temperature differences between the various components, just before the start of the rapid heating in the soldering phase to avoid any such soldering defects as cold soldering, leaching. The ideal situation is where the temperatures of the light and heavy components are practically the same at the end of the homogenizing step, i.e. just before solder reflow. However this is difficult to obtain in production reflow systems, even if these are fairly long. The temperature-time curves have been measured in a large production furnace; in the first step the temperature of the leads of the SOT-23 package rises much faster than that of the leads of the PLCC-68 package; and subsequently the temperature-differential are reduced. During second stage heating, the differentials increase slightly and again reduced. After that, the soldering step with its rapid increase of temperature differentials has started, but at that moment the difference between the two temperature curves is still large, and hence the difference between the peak temperatures reached is also large.