Successful profiles range in temperature up to 30 ☌ higher than liquidus, which is approximately 183 ☌ for eutectic and approximately 217 ☌ for lead-free. : 16Īfter the soak segment, the profile enters the ramp-to-peak segment of the profile, which is a given temperature range and time exceeding the melting temperature of the alloy. If the heat transfer is too slow, the flux concentration may remain high and result in cold solder joints, voids and incomplete reflow. An over-rapid heat transfer can cause solder splattering and the production of solder balls, bridging and other defects. The mass of the PCB is another factor that must be considered for the soak duration. The duration of the soak varies for different pastes. In this stage, most of the flux evaporates out of the solder paste. The amount of energy introduced to both the component and the PCB approaches equilibrium. In the soak segment of the profile, the solder paste approaches a phase change. Additionally, rapid heat can lead to steam generation within the component if the moisture content is high, resulting in the formation of microcracks. Rapid evaporation of the flux contained in the solder paste can lead to defects, such as lead lift, tombstoning, and solder balls. Many components have a specification where the rise in temperature should not exceed a specified temperature per second, such as 2 ☌/s. : 14 The most commonly used process limit is 4 ☌/s, though many component and solder paste manufacturers specify the value as 2 ☌/s. Ramp is defined as the rate of change in temperature over time, expressed in degrees per second. There are two main profile types used today in soldering: Convection Reflow Oven Detailed Description For an effective soldering process, soldering must be carried out under carefully calibrated conditions in a reflow oven. Solder paste contains a mix of metal, flux, and solvents that aid in the phase change of the paste from semi-solid, to liquid to vapor and the metal from solid to liquid. In soldering, a thermal profile is a complex set of time-temperature values for a variety of process dimensions such as slope, soak, TAL, and peak. By using algorithms such as PWI, engineers can calibrate and customize parameters to achieve minimum process variance and a near zero defect rate. In modern systems, quality management practices in manufacturing industries have produced automatic process algorithms such as PWI, where soldering ovens come preloaded with extensive electronics and programmable inputs to define and refine process specifications. There are two main types of profiles used today: The Ramp-Soak-Spike (RSS) and the Ramp to Spike (RTS). One of the major uses of this method is soldering of electronic assemblies. The method is used in a variety of industrial and laboratory processes, including electronic component assembly, optoelectronics, optics, biochemical engineering, food science, decontamination of hazardous wastes, and geochemical analysis. For example, if the process mean is set at 200 ☌ with the process window calibrated at 180 ☌ and 220 ☌ respectively, then a measured value of 188 ☌ translates to a process window index of −60%. A PWI of 99% indicates that the profile is within process limitations, but runs at the edge of the process window. A Process Window Index (PWI) greater than or equal to 100% indicates the profile is outside of the process limitations.

The center of the process window is defined as zero, and the extreme edges of the process window are ☙9%.

Raw temperature values are normalized in terms of a percentage relative to both the process mean and the window limits.

The thermal profile is often measured along a variety of dimensions such as slope, soak, time above liquidus (TAL), and peak.Ī thermal profile can be ranked on how it fits in a process window (the specification or tolerance limit). A graphical representation of the Process Window Index for a thermal profileĪ thermal profile is a complex set of time-temperature data typically associated with the measurement of thermal temperatures in an oven (ex: reflow oven).

Reflow profile