Radiation: Heat transfer through radiation takes place in form of electromagnetic waves mainly in the infrared region.Heat flows from the warmer to the cooler object until they are both at the same temperature. It occurs when two objects at different temperatures are in contact with one another other. Conduction: Thermal conduction directly transmits through a substance when there is a difference of temperature between adjoining regions, without movement of the material.There are three ways to transfer heat from heat source to heated objects as following: A solder joint will be formed after it cools down below molten point. The liquid alloy will merge with PCB copper pads and become a eutectic alloy mixture. In the electronics manufacturing industry, SPC (statistical process control) helps determine if the process is in control, measured against the reflow parameters defined by the soldering technologies and component requirements.įor ensuring soldering SMDs to a circuit board, the heat should be transferred to the solder alloy past until its temperature reaches to its molten point. Thermal profiling is the act of measuring several points on a circuit board with a thermal profiler to determine the thermal excursion it takes through the soldering process. The temperature that the board passes as it is processed through the oven is called Profile. The set-up of temperatures for all of the zones and the belt speed is referred to as SMT Program. This set point corresponds to the temperature the circuit board should be exposed to as it passes through that zone. During the SMT reflow process, the zones are programmed with a thermal set point. Depending on the length and throughput of the oven there may be few zones, 8 or less, or many zones, 12 or more. Inline Reflow Ovens are configured with a number of heating zones followed by cooling section(s). The maximum temperature assemblies can reach is limited by the boiling point of the medium. There is no need for any protective gas (e.g. High energy efficiency due to the high heat transfer coefficient of vapor phase media.Some advantages of vapor phase soldering are: The liquid used is chosen with a desired boiling point in mind to suit the solder alloy to be reflowed. The heating of the PCBs is sourced by thermal energy emitted by the phase transition of a heat transfer liquid (e. Vapor Phase Oven: Also known as Condensation Soldering.Thus, nitrogen ovens typically have nitrogen injection in at all times which decreases defect rates. The nitrogen reflow oven takes a few minutes to reduce Oxygen concentration to acceptable levels within the chamber. This minimizes oxidation of the surfaces to be soldered. Nitrogen (N 2) is a common gas used for this purpose. Some ovens are designed to reflow PCBs in an oxygen-free atmosphere. Ovens which also use fans to force heated air towards the assemblies are called infrared convection ovens. The heat source is normally from ceramic infrared heaters, which transfers the heat to the assemblies by means of radiation. The PCB moves through the oven on a conveyor belt, and is therefore subjected to a controlled time-temperature profile. Generally, there are several heating zones followed by one or more cooling zones. Infrared and Convection Ovens: The oven contains multiple zones, which can be individually controlled for temperature.There are two types of reflow ovens as following: Later on, the industry switched to convection heat transfer. In the early days of electronics PCB assembly, infrared (IR) heating technology was employed.
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