What is a Reflow Oven?
A reflow oven is known as a PCB soldering machine. After the placement of tiny components on the surface of the Printed Circuit Board (PCB), is ready to pass through the different chambers for the soldering process.
Each chamber has its temperature zone for example a reflow oven has eight zones. In the first two zones, there is a low-temperature level for the preheating of solder paste likewise in the middle zones there is a high-temperature in which the solder paste melts down and tiny chips get attached with the PCB board, and the last two zones are cooling zones where the PCB board gets cooled down and reflow soldering process is completed.
The machines used in the process must be able to perform to their specifications. The machines should operate within the range that they’re designed. Pushing a machine beyond its operating limits will ultimately cause a drag. The machines must be reliable and service-able
The manpower must be properly trained. All machines in the line require knowledgeable operators and maintenance personnel. A machine is merely nearly as good because of the person operating it.
The methods and procedures must be defined and strictly adhered to. Inventory The methods and procedures must be defined and strictly adhered to. Inventory procedures should be implemented to make sure that components and boards don’t stay on the shelf too long. Problems should be fully documented including all data that pertains thereto.
Finally, the processing environment must be controlled. This includes both the environment inside and out of the doors of the machines. For example, if the humidity in the soldering area is too high, this can cause the solder paste to absorb moisture, which may cause problems during reflow. If the temperature within the space isn’t controlled, the thermal profile within the reflow oven could even be affected, which again may cause a defect
Producing high-quality solder joints requires that five general process variables got to be in control; the materials, machines, manpower, methods, and thus the environment.
HOW MANY HEATING ZONES ARE REQUIRED?
This question frequently arrives and can be a source of confusion. One theory is that if ten zones are good, then twenty zones are better. While in theory, this may be true, in practicality it may not be true. While it improves the flexibility of the oven, it also adds to the complexity and makes oven set-up more difficult.
Let’s review this question from the standpoint of how many zones are really needed. Reflowing solder paste typically requires two or three different heating rates to accomplish the process. In the figure below, there are three “typical” profile bands for RMA, water-soluble, and no-clean solder pastes. These profiles are rather generic, and one should consult with their solder paste vendor to determine the desired profile. All of the pastes in the figure could be successfully reflowed using three zones.
A second issue is the question of independent top and bottom heat control. A common conception in SMT reflow is that if you can control your top and bottom heaters separately, then double-sided reflow can be accomplished (without reflowing side A twice). This will not work with most SMT reflow processes, as most products are too thermally thin. Independent top-bottom heating control does help in machines with dual conveyors that are located at different heights in the chamber. Product warpage may also be minimized through proper control. The bottom line is that most machines have this feature, but most often the top and bottom heaters will be set at the same value.
Edge heating zones become significant when edge conveyors are used. Edge conveyors act as heat sinks on the edge of the product. Two zones of edge heat (one front, one back) will assist in compensating for edge conveyors. If mesh conveyors are used, these zones are unnecessary in the reflow oven unless the product width is quite wide.
WHAT IS THE PROFILE PROCESS BAND?
In reality, the profile is not a line, but rather a band or process window. The size of the band is defined by the range of temperature deviation that can occur during the reflow oven process and yield high-quality results. This is indicated by the crosshatched area shown in the figure. In the example, the profile band size is indicated as 25°C. The actual size of the band will vary depending upon solder paste, component type, and circuit board material.
REFLOW SOLDERING DEFECTS
If the product temperature profile is not maintained within the band, a defect will occur. Chip capacitors may crack if the rising rate in the preheat zone is excessive. Solder spatter and solder balls may occur if drying is excessive or insufficient. If drying is excessive, the solder paste may skin over and not allow any of the volatiles to escape. As the temperature of the solder joint continues to heat, the vapor pressure increases until the solder joint virtually explodes as the gases escape rapidly.
The result is solder spatter and solder balls. If the preheat is insufficient, solder spatter and balls may occur in the reflow zone when higher heating rates occur. It is critical that the paste is dried at low controlled heating rates. In the reflow zone, insufficient heat will result in non-reflowed or partially reflowed solder joints. The joints will have a dull, rough surface finish in this case. Excessive heating in the reflow zone may result in burnt boards, damaged components, or possibly solder dewetting. In a dewet solder joint, the solder is reflowed but it is pulled away from the pad or the component lead.
DEFINE THE BAND
The profile band needs to be defined for the given SMT assembly. To define this band, one should be aware of not only the solder paste requirements but also any specific requirements of components or the circuit board material. The processing bandwidth is defined as the total deviation in temperature that can occur and yield reliable results.
REFLOW OVEN PERFORMANCE
From a process standpoint, the reflow oven must be able to produce consistent results within the reflow profile band. The user should be aware of all variables that influence the band. These variables may be defined as product-related and reflow oven-related. By far the most critical product-related non-uniformity is that created by large mass differences on the product. Since it is easier to heat an area that has no components as compared with an area with large components, temperature differentials will exist on the product. Machine-related non-uniformities such as repeatability under product load and edge-to-center and front-to-back product heating uniformities must also be considered. To achieve high quality, low defect soldering results, the sum of all the non-uniformities must fall within the defined reflow profile band
In reality, the profile is not a line, but rather a range of temperatures all solder joints must be exposed to. The total temperature deviation from the component thermal mismatch, oven loading, and oven uniformity must be within the band.
WHAT ARE THE MODES OF HEATING in Reflow Oven?
There are three different heating modes involved with most SMT reflow processes; convection, conductor, and infrared radiation (IR). All three of these heating modes occur naturally in our daily lives. Perhaps the easiest way to understand each of these heating mode is through example.
CONVECTION in a reflow oven
Convection heat transfer occurs when a fluid (such as air, nitrogen, or water) passes over a solid (such as an SMT assembly). A cool breeze on a hot sunny day provides convective cooling. Hot air from a hairdryer provides convective heating. Convection heating or cooling requires contact of the flow with the solid par Only the layer of the flow that is in contact with the pa is actually transferring heat.
CONDUCTION in a reflow oven
Conduction heat transfer occurs when two solid masse of different temperatures are in contact with each other A good example is when a pan is placed on an electric burner. Most of the heat is transferred to the pan by that contact between the pan and the burner. Conduction also occurs within the same massif a temperature differential exists within the mass.
Referring to a system as 100% convection or 100% IR is incorrect. A system may be dominated by convection or IR, but the other heat transfer is always present in significant quantities.
RADIATION in a reflow oven
Infrared Radiation (IR) occurs when two bodies of different temperatures are in sight of each other. The best example of IR is the heating of the earth by the sun. IR is a non-contact heat transfer.
In SMT reflow oven applications (with the exception of the vapor phase), convection, conduction, and IR all play a role in the healing process. Referring to a system as 100% convection or 100% IR is incorrect. A system may be dominated by convection or IR, but the other heat transfer is always present. IR dominant systems usually range in the convection/IR ratio of 60/40 to 40/60. Convection dominant systems have ratios in the range of 70/30 to 80/20. Conduction heat transfer occurs more within the product during the heating process. Heat is transferred through the product to help minimize hot or cold spots.
HOW DOES INFRARED HEATING WORK in Reflow Oven?
Infrared (IR) heat transfer occurs when two objects at different temperatures are in sight of each other. The heat is transferred by electromagnetic waves of 0.78- 1000 micron wavelengths. All objects emit some level of infrared. The quantity of infrared energy emitted and the wavelength of the emission are both dependent upon the absolute temperature of the object. As the source temperature increases, the heat transfer output increases exponentially to the fourth power. Increasing the source temperature results in shorter wavelengths. Decreasing the source temperature results in longer wavelengths. In SMT reflow the amount of heat transfer is important, so as not to exceed the 2OC/sec rate. When operating at these low heating rates all types of radiant emitters (lamps, panels, tubes) operate in the medium to long wavelength ranges
INFRARED HEATING EQUATION IN REFLOW OVEN
In order to understand what parameters are important in infrared heating, one can consider the general equation for heat transfer between the heat source and the object being heated. Again, the purpose is not to memorize the equation, but rather to point out the significance of what is involved. The general equation for heat transfer is as follows:
Absorption characteristics of solder paste and circuit board material make IR an efficient means of heating
The geometric view factor “V” is the fraction of energy that leaves the source that hits the target. In SMTreflow, the oven chamber designs yield very high view factors in the range of 0.90 to 0.95. An important aspect of the view factor comes in product design. If two very large components are in close proximity to each other, the view factor to a solder joint between them is decreased, which makes it more difficult.
The source emissivity and the target absorptivity factors are in the range of 0.90 to 0.95 for most SMT applications. Solder paste is an excellent absorber of infrared energy. Shiny gold components may be difficult to heat as they tend to be reflective. Most often, however, the board material, solder paste, and the components all absorb quite well. The figure below shows the spectral absorptivity for an RMA solder paste.
Control of infrared is generally done by controlling the source temperature. Providing that the overall source emission can be regulated, IR can provide high levels of repeatability.
HOW DOES CONVECTION HEATING WORK?
Convection heat transfer occurs when fluid at a given temperature contacts a solid mass at a different temperature. If the fluid is hotter than the mass, the mass will be heated. If the fluid is cooler than the mass, the mass will be cooled. Perhaps the easiest way to understand convection is to look at the convection equation and note what is important and what is not important.
Note that in the parallel flow diagram, there is an area referred to as the “boundary layer”. In the boundary layer, there is no flow motion, which causes convective heat transfer to be very low. It is beneficial to break up the boundary layer to allow the flow to transfer heat freely to the object. The diagram shown below shows perpendicular flow convection in a reflow oven.