Reflow soldering
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Reflow soldering is the most common means to attach a surface mounted component to a circuit board, and typically consists of applying solder paste, positioning the devices, and reflowing the solder in a conveyorized oven. The goal of the reflow process is to melt the powder particles in the solder paste, with the surfaces being joined together, and solidify the solder to create a strong metallurgical bond. There are usually four process zones in conventional reflow process, consisting of preheat, thermal soak (often shortened to just soak), reflow and cooling.
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[edit] Preheat zone
Maximum slope is a time/temperature relationship that measures how fast the temperature on the printed circuit board changes. The ramp–up rate is usually somewhere between 1.0 °C and 3.0 °C per second, often falling between 2.0 °C and 3.0 °C (4 °F to 5 °F) per second. If the rate exceeds the maximum slope, potential damage to components from thermal shock or cracking can occur. Solder paste can also have a spattering effect. 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.
[edit] Thermal soak zone
The second section, thermal soak, is typically a 60 to 120 second exposure for removal of solder paste volatiles and activation of the fluxes, where the flux components begin oxide reduction on component leads and pads. Too high or too low a temperature can lead to solder spattering or balling as well as oxidation of the paste, the attachment pads and the component terminations. Similarly, fluxes may not fully activate if the temperature is too low. At the end of the soak zone a thermal equilibrium of the entire assembly is desired just before the reflow zone.
[edit] Reflow zone
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. An important consideration is peak temperature, which is the maximum allowable temperature of the entire process. This limit is determined by the component on the assembly with the lowest tolerance for high temperatures (the component with the lowest “maximum possible” temperature determines the component most susceptible to damage), and should never be exceeded. A standard guideline is to subtract 5 °C from the maximum temperature the most vulnerable component can sustain to arrive at the maximum temperature for process. It is important to monitor the process temperature to keep it from exceeding this limit. Additionally, temperatures above 230 °C may cause damage to the internal dies of SMT components as well as foster intermetallic growth. Conversely, a temperature that isn’t hot enough may prevent the paste from reflowing adequately.
Time above liquidus (TAL), or time above reflow, measures how long the solder is a liquid. The flux reduces surface tension at the juncture of the metals to accomplish metallurgical bonding, allowing the individual solder powder spheres to combine. 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. 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. 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. One possibility is that there are places on the PCB that are not measured during profiling, and therefore, setting the minimum allowable time to 30 seconds reduces the chances of an unmeasured area not reflowing. A high minimum reflow time also provides a margin of safety against oven temperature changes. The wetting time ideally stays below 60 seconds above liquidus. Additional time above liquidus may cause excessive intermetallic growth, which can lead to joint brittleness. The board and components may also be damaged at extended times over liquidus, and most components have a well-defined time limit for how long they may be exposed to temperatures over a given maximum. Too little time above liquidus may trap solvents and flux and create the potential for cold or dull joints as well as solder voids.
[edit] Cooling zone
The last zone is a cooling zone to gradually cool the processed board and solidify the solder joints. Proper cooling inhibits excess intermetallic formation or thermal shock to the components. Typical temperatures in the cooling zone range from 30–100 °C (86–212 °F). Unlike the maximum ramp-up rate, the ramp–down rate is often ignored. It may be that the ramp rate is less critical above certain temperatures, however, the maximum allowable slope for any component should apply whether the component is heating up or cooling down. It is a parameter to consider when analyzing process results.
