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Encyclopedia > Reflow soldering

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, wet 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. Surface-mount components on a keydrives circuit board Surface mount technology (SMT) is a method for constructing electronic circuits in which the components are mounted directly onto the surface of printed circuit boards (PCBs). ... Close-up photo of one side of a motherboard PCB, showing conductive traces, vias and solder points for through-hole components on the opposite side. ... A solder is a fusible metal alloy (often of tin and lead, although lead-based solders were outlawed in many parts of the world in the 1980s), with a melting point or melting range below 450 °C (840 °F) and is melted to join metallic surfaces, especially in the fields... Oven depicted in a painting by Millet An oven is an enclosed compartment for heating, baking or drying. ... Solder paste (or solder cream)is a mix of small solder particles and flux. ...

Contents


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. Thermal shock is the name given to cracking as a result of rapid temperature change. ...


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. In the various subfields of physics, there exist two common usages of the term flux, both with rigorous mathematical frameworks. ... An oxide is a chemical compound of oxygen with other chemical elements. ... In thermodynamics, a thermodynamic system is in thermodynamic equilibrium if its energy distribution equals a Maxwell-Boltzmann-distribution. ...


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. Look up die in Wiktionary, the free dictionary. ... Intermetallics is concerned with all aspects of ordered chemical compounds between two or more metals and notably with their applications. ...


Time above reflow (TAL), or liquideous, 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. Wetting of different fluids. ...


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.


See also


  Results from FactBites:
 
Reflow soldering device - Patent 6120585 (3498 words)
The reflow soldering device according to claim 1, further comprising means for maintaining a temperature of the outdoor air at a predetermined level prior to said flux removing means mixing the outdoor air with the exhaust gas.
The reflow soldering device according to claim 7, further comprising a cooling device which cools the ambient air to a temperature that is lower than room temperature prior to said flux extractor mixing the ambient air with the exhaust gas.
The reflow soldering device according to claim 7, further comprising a temperature controller for maintaining a temperature of the ambient air at a predetermined level prior to said flux extractor mixing the ambient air with the exhaust gas.
Reflow soldering - Wikipedia, the free encyclopedia (837 words)
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, wet the surfaces being joined together, and solidify the solder to create a strong metallurgical bond.
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.
  More results at FactBites »

 
 

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