Although potential electronics manufacturing services (EMS) partners usually look into capabilities and equipment lists, carrying out a comparison can be a daunting task as most EMS partners opt for different brands of equipment, specifically for surface mount assembly (SMT) equipment. Therefore, it is more useful to compare technologies such as flying probes and ICT, and convection and vapor phase reflow.
Convection Reflow Oven
With multiple heating zones, sometimes as many as 12, followed by a cooling element, convection reflow ovens usually have individual temperature controls for each zone. After the SMT assembly process is completed, a conveyor belt carries the populated printed circuit board (PCB) into the oven, which exposes the PCB to a controlled time-temperature profile.
The Printed Circuit Board production lines usually place the convection reflow oven in-line with the SMT assembly equipment, allowing for a relatively high throughput, without additional handling. However, each product requires its own reflow profile and the engineering team has to create this before start of production.
Convection reflow ovens usually have a large footprint, and therefore, consume a large amount of floor space in the factory. Although each heating zone has its own temperature control, engineers usually face a challenge when reflowing densely populated circuit boards in a convection reflow oven, as it is not possible to control the temperature at individual component level.
Vapor Phase Reflow Oven
Unlike convection, this type of ovens uses condensation or vapor phase for soldering. The vapor comes from boiling perfluoropolyether, an inert heat transfer liquid. In contrast with the convection reflow oven, the vapor phase reflow oven has a much smaller footprint, and the PCB assembly moves vertically up and down instead of sideways.
The vapor layer transfers heat to the PCB and associated components as the assembly sits within it. The heat transfer rate is high, achieves good wetting, and requires much less power input. The process produces very little temperature difference between components of different thermal mass on the PCB. This makes the process very suitable for densely populated PCBs.
In a vapor phase oven, limitations of the physical temperature reliably prevent overheating of any part in the soldering process. As the vapor has a higher density, it is heavier than the surrounding air. This allows the soldered parts to remain sealed inside a neutral atmosphere. For instance, the inert fluid boils at 230°C and creates the vapor layer above it at 230°C. The air over the vapor phase however, does not heat up more than 50-80°C.
The heat transfer fluids used in modern vapor phase reflow ovens, such as perfluropolyether do not contain any CFC or other harmful ingredients that could place limitations for transportation and storage of these liquids. Its main properties are is excellent chemical and thermal resistance, very high electric insulation properties, non-toxicity, low viscosity, and no flash or fire point.
Control of Heat Transfer Through Adjustment of Heat
Engineers adjusted the temperature gradients in earlier vapor phase machines by regulating the power to the heating elements. Greater the power transferred to the heaters, more are the vapors produced, and more the heat transferred to the PCB assembly.
As the rising vapors create an inert atmosphere, the process heats up the boards and the soldering process takes place in an oxygen-free atmosphere, which reduces the oxide formation and improves wetting. However, the slight time delay in the creation and subsistence of vapors with the heater controls prevents the creation of sophisticated temperature profiles. This has led to Soft Vapor Phase (SVP) type of reflow machines.
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Control of Heat Transfer through Adjustment of Level
To realize any temperature gradient with a vapor phase reflow machine, engineers now follow the patented process of the soft vapor phase mode. The benefit of the SVP process is engineers can control the immediate temperature gradient as a function of the height level of the boards above the liquid surface.
In the SVP process, as the PCB moves into the vapor, its temperature increases. Holding the board at a certain depth realizes the pre-heating of the board. As the depth increases, the board reaches the liquidus temperature. By preselecting and controlling the soldering time automatically, engineers can create any thermal profile necessary. Once soldered, the process moves the board up to the vapor boundary to lower temperatures and finally out of the vapor to cool down. The SVP process does not require additional mechanisms to control overheating.
However, void formation is an unavoidable risk diminishing the electrical and thermal conductivity of the solder joint. For countering this, engineers prefer a vacuum controlled process, capable of outgassing such voids.
Reduction of Void Formation through Control of Vacuum
By adjusting the pressure over the liquid, engineers assure stable conditions, as a reduction of pressure lowers the boiling point of the liquid and vice-versa. The use of vacuum extends the time above the liquidus by about 30 seconds and this effectively reduces the number and formation of voids, as it is necessary to conduct the void reduction process in the molten state of the solder.
Advantages of Vapor Phase Reflow Soldering
Vapor phase soldering transfers heat at about 100-400 W/m2 K, which is considerably higher than 10-60 W/m2 K of heat transferred by convection reflow soldering. Moreover, the heat transferred by vapor phase is uniform on all components of the PCB, reducing stresses during soldering. The ability to optimize the thermal profile with extended peak times makes the process suitable for all types of electronics. Additionally, the vacuum controlled process takes care of outgassing of the voids, leading to more uniform and reliable joints.
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