Another important effect of this stage is that the flux in the solder paste begins to react aggressively, increasing the wettability (and surface energy) of the weldment surface. This helps reduce the effects of thermal stress in the reflow area and limits other soldering defects such as larger volume component lift. The first is to achieve a uniform temperature for the entire PCB board. The setting of the constant temperature zone is mainly controlled within the parameters of the solder paste supplier and the heat capacity of the PCB. This is especially true for solder pastes with high flux content. Another side effect of too high a heating rate is that the solder paste can collapse and cause short circuits. ![]() Excessive heating rates can thermally stress the component, which can damage the component or reduce its performance/lifetime. The preheating zone needs to volatilize the solvent, but the temperature rising slope must be controlled. The role of the solvent is mainly as a carrier for the resin, with the additional function of ensuring sufficient storage of the solder paste. The main components of flux in solder paste include resins, activators, viscosity modifiers and solvents. If using a reflow oven recipe generator check that it also considers the convection rate.The purpose of the preheat zone is to volatilize the low melting point solvents in the solder paste. Many reflow ovens employ automated recipe generator software tools to facilitate the development of oven recipes. The pressure is adjusted by the control of the fan speed in each heated plenum. Convection rates are adjusted by setting the pressure in the heated air plenum in each zone. In a convection solder reflow oven convection is the dominant component of heat transfer. Modern reflow ovens used for mass production also have the ability to control convection rate – the speed at which the hot gases hit the product. Solder reflow ovens control the temperature profile by adjustments to the zone temperature set points and the belt speed. Controlling the Solder Reflow Oven Temperature Profile The reflow oven profile will be compared to solder specifications and board/component limitations including: ramp rates, peak temperature, TAL (time above liquidus) and other factors. time data can also be downloaded as a table and can be manipulated by MS Excel or other statistical programs. The measured reflow oven profile is often displayed visually, as a graph, but the raw temperature vs. The profiles can be measured in a loaded or unloaded state by either processing boards ahead of and behind the thermocoupled board to simulate the load or by sending the thermocouple board through alone for an unloaded test. The attached thermocouples can be measured by trailing the wires through the oven while processing or using a recording device that can be passed through the reflow oven by use of an insulating cover. Placing multiple thermocouples in differing locations and on different types of components on the board will give you the most comprehensive understanding of the thermal profile. Best practices for attaching thermocouples also suggest using strain relief to prevent the TC’s from being pulled off the board when the wires are tugged. Aluminum or Kapton tape can be used to attach thermocouples, but is not suggested due to trapped air and loosening of the tape’s adhesive. These thermocouples are attached using solder or epoxy. How to Profile a Solder Reflow OvenĪ solder reflow oven is profiled by attaching thermocouples (TC’s) to the board that is being processed. The difference between the reflow oven set points, or recipe, and the thermal profile is governed by the laws of heat transfer and is affected by mass (weight of the product), surface area, thickness, and the heat capacity (the ability of a material to absorb or give up heat). The temperature settings combined with the belt speed, and the convection rate settings, will produce the thermal profile on the board or product. The recipe is the temperature settings of the reflow oven in each zone. ![]() A thermal profile is a measurement of the temperature, of the product, during thermal processing. The thermal profile is an critical consideration in the solder reflow process.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |