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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.
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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 1200
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.
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