Infrared Furnace
Radiant Technology Corporation is a leading global supplier of precision thermal
processing systems for the electronics and technology markets
Infrared Heating
Infrared waves form part of the electromagnetic spectrum. Electromagnetic waves
with wavelengths from 0.78mm to 1000mm are called infrared waves. You are already
familiar with electromagnetic waves of different wavelengths. Microwaves, X-rays,
radio waves and visible light are all electromagnetic waves. Infrared waves produced
inside a RTC Furnace lie predominately in the near and medium infrared range with
wavelengths ranging between 0.5 and 3.0-mm.
When using infrared lamps, higher heat-lamp temperatures emit higher radiant energy.
This elevated energy translates to a shorter electromagnetic wavelength of emitted
IR radiation. While the IR waves of a heat lamp come from a continuous range of
wavelengths, the dominant wavelength (ldom)
as given by Plank’s distribution principle is the wavelength transmitted with the
highest occurrence. So for a given temperature, only one
ldom exits.
Dominant Wavelength Graph
Advantages of IR Heating
Heating via conduction and convection operates by transferring heat to object surfaces.
Heat is then transferred from the surface to the layers beneath. Heat transfer,
however, is not uniform, causing temperature differences and unequal expansion across
an object. The unequal expansion due to the uneven heating is called thermal stress
and can cause objects to fracture called thermal shock.
IR radiation heats molecules below an object’s surface and allows for more uniform
heat distribution than can be provided by conduction and convection heating alone.
Rapid heat up time is also achieved with IR technology due to the high energy-transfer
rate of IR waves. The speed of conduction and convection heating is proportional
to the temperature difference between the object and heating environment, whereas
the speed of IR heating is proportional to the difference between the fourth powers
of the object and environment temperatures.
Temperature Profiling
As discussed previously in Section 3.3 “ Thermal Process ”, products passing through
the furnace go through a set of temperatures known as a temperature profile. The
process engineer must setup the furnace to achieve the temperature profile with
the product. To do this, the engineer must have an idea of what the cycle of the
product must look like. Six zones are visible labeled Z1 – Z6. Depending upon the
setup of the furnace, more zones may be present.
Initially, temperature profiles must be recorded from inside the furnace. To get
to the point of taking a temperature profile, the following list of topics will
be covered.
- Profile Specifications
- Basic Variables
- Type of Profiles
Profile Specifications
In general, the temperature profile is defined by the following specifications:
Heating Rate: The rate of increase of temperature from room temperature.
Dwell/Hold Time: The time the product remains above a certain temperature
or a range of temperatures.
Second Heating Rate: The rate of increase of temperature from the
temperature reached during the hold time, as required.
Peak Temperature: The maximum temperature reached with a +/- range.
Second Hold Time: Same as hold time, as required.
Cooling Rate: The rate of decrease of temperature to a lower/critical
temperature.
Sometimes a conveyor belt speed range is requested for a desired product speed.
In this case, the above specifications are met within the specified speed range.
In general, the speed range depends on the size and type of furnace. Another important
consideration is to understand that many sets of belt speeds and temperature settings
will meet a given set of profile specifications. Furthermore, higher belt speeds
result in greater temperature deviations and lower consistency from the desired
temperature profile.
Basic Variables
The two most influential and basic variables in setting up a temperature profile
are:
- Conveyor Speed: The time required to pass through the process section
- Temperature Set points: The energy level in each zone.
The combination of the time-temperature exposure of the product determines the temperature
profile. The temperature settings in each zone set the heating rate and hold times
of the product.
A third and less influential factor in the temperature profile is:
- Flow Meter Settings: The rate of gas flow through the process section.
If the furnace is equipped for a controlled atmosphere, this will be an important
factor to consider.
Types of Profiles
In most processes, two kinds of temperature profiles exist:
- Equilibrium (flat) profiles:
- Hybrid thick film and PTF firing
- Glass or metal/solder sealing of IC packages
- Die-attachment processes
- Drying/Curing of polymeric product
- Non-Equilibrium (peaked/spiked) profiles:
- Solder reflow attachment
- Solar cell firing processes
- Cerdip lead-frame attachment
Most microlelectronic and semiconductor thermal processes fall into one of the above
categories, or some combination of the two. Setup the furnace according to the type
of process that will be used with the furnace.