![]() Experts usually recommend the shortest TAL possible, however, most pastes specify a minimum TAL of 30 seconds, although there appears to be no clear reason for that specific time. An insufficient time/temperature relationship causes a decrease in the flux’s cleaning action, resulting in poor wetting, inadequate removal of the solvent and flux, and possibly defective solder joints. If the profile time exceeds the manufacturer’s specification, the result may be premature flux activation or consumption, effectively “drying” the paste before formation of the solder joint. The flux reduces surface tension at the juncture of the metals to accomplish metallurgical bonding, allowing the individual solder powder spheres to combine. Time above liquidus (TAL), or time above reflow, measures how long the solder is a liquid. Conversely, a temperature that isn’t hot enough may prevent the paste from reflowing adequately. Additionally, high temperatures (beyond 260 ☌) may cause damage to the internal dies of SMT components as well as foster intermetallic growth. It is important to monitor the process temperature to keep it from exceeding this limit. A standard guideline is to subtract 5 ☌ from the maximum temperature that the most vulnerable component can sustain to arrive at the maximum temperature for process. A common peak temperature is 20–40 ☌ above liquidus.This limit is determined by the component on the assembly with the lowest tolerance for high temperatures (The component most susceptible to thermal damage). An important consideration is peak temperature, which is the maximum allowable temperature of the entire process. The third section, the reflow zone, is also referred to as the “time above reflow” or “time above liquidus” (TAL), and is the part of the process where the maximum temperature is reached. A soak profile is also recommended to diminish voiding in area array type packages. A soak profile is suggested to decrease any delta T between components of varying sizes or if the PCB assembly is very large. At the end of the soak zone a thermal equilibrium of the entire assembly is desired just before the reflow zone. Similarly, fluxes may not fully activate if the temperature is too low. Too high a temperature can lead to solder spattering or balling as well as oxidation of the paste, the attachment pads and the component terminations. The second section, thermal soak, is typically a 60 to 120 second exposure for removal of solder paste volatiles and activation of the fluxes (see flux), where the flux components begin oxidereduction on component leads and pads. The preheat section is where the solvent in the paste begins to evaporate, and if the rise rate (or temperature level) is too low, evaporation of flux volatiles is incomplete. Solder paste can also have a spattering effect. If the rate exceeds the maximum slope, damage to components from thermal shock or cracking can occur. The ramp–up rate is usually somewhere between 1.0 ☌ and 3.0 ☌ per second, often falling between 2.0 ☌ and 3.0 ☌ (4 ☏ to 5 ☏) per second. The preheat zone is often the lengthiest of the zones and often establishes the ramp-rate. Maximum slope is a temperature/time relationship that measures how fast the temperature on the printed circuit board changes. In the conventional reflow soldering process, there are usually four stages, called "zones", each having a distinct thermal profile: preheat, thermal soak (often shortened to just soak), reflow, and cooling. The goal of the reflow process is to melt the solder and heat the adjoining surfaces, without overheating and damaging the electrical components. When used on boards containing a mix of SMT and THT components, through-hole reflow allows the wave soldering step to be eliminated from the assembly process, potentially reducing assembly costs. Because wave soldering can be simpler and cheaper, reflow is not generally used on pure through-hole boards. Reflow soldering is the most common method of attaching surface mount components to a circuit board, although it can also be used for through-hole components by filling the holes with solder paste and inserting the component leads through the paste. Heating may be accomplished by passing the assembly through a reflow oven or under an infrared lamp or by soldering individual joints with a hot air pencil. Reflow soldering is a process in which a solder paste (a sticky mixture of powdered solder and flux) is used to temporarily attach one or several electrical components to their contact pads, after which the entire assembly is subjected to controlled heat, which melts the solder, permanently connecting the joint. Shenzhen Jaguar Automation Equipment Co., Ltd Reflow soldering process-Technical Data-Shenzhen Jaguar Automation Equipment Co., Ltd-reflow oven wave soldering machine manufacturer
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