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EMS Assembly

Reflow Profiles

Solder paste composition defines the time/temperature or thermal profile required from a reflow process system. Therefore, as long as a specific solder paste is used, the required thermal profile will remain essentially the same. The EMS Assembly industry typically uses two types of standard reflow profiles—one for leaded solder assembly and the other for lead-free solder.

For an oven to achieve the required reflow profile suitable for a specific solder paste, it may be necessary to vary controls based on the PCB assembly being soldered. Factors causing variations include:

  • Thermal conductivity of components
  • Temperature thresholds of components
  • Component density
  • Size/ area of the assembly
  • Thickness of the PCB
  • Thermal mass of the assembly
  • Load capacity of the oven

So, if a PCB assembly has unique characteristics, such as thickness, materials, components, heavy copper, etc., it may be necessary to develop a custom reflow profile for quality assembly.

Generally the melting temperature of solder exceeds the maximum operating temperature of any SMD device. To avoid damage, soldering must be completed in as small a time as possible, so the SMD devices are only exposed to the reflow temperature for a minimum time period.

Major differences between leaded and lead free processes are melting temperature (183 vs 217C) and minimum peak reflow temperature (215 vs 235C). Melting temperature is the most important step in the Reflow Soldering Process. At this temperature, solder paste deposits on the PCB assembly melts and forms the joint. This happens as the assembly rides a conveyor through an oven exposing them to a temperature profile which varies in time.

Atypical reflow solder profile will have three phases:

  • PREHEAT PHASE: the PCB assembly warms up to a temperature lower than the melting point of the solder paste.
  • REFLOW PHASE: the assembly heats up to a peak temperature well above the solder melt point but remains below the temperature at which any parts of the product may be damaged.
  • COOLING PHASE: The PCB assembly cools down under control and the soldered joints solidify before exiting the oven.

A reflow phase has a lower and an upper limit for peak temperature. Measured at the solder joint, the lower limit of the peak temperature must be high enough to make reliable joints. The solder paste characteristics determine this temperature. The upper limit for the peak temperature must be lower than what the SMD parts or PCB can withstand.

Some areas on a PCB assembly will become hotter than others resulting in hot and cold spots. Hot areas may be ones with only a few components, or the smallest components, and with little copper. Cold spots will be the sections with a high density of large components, as they absorb a lot of heat. Areas on the board with large areas of copper will also allow the local temperature to remain low. Hot and cold spots also depend on dimensions of a board and orientation when passing through an oven.

It’s necessary to keep the temperature of hot spots lower than the upper limit of the peak temperature. In the same way, the temperature of the cold spots must be higher than the lower limit of the peak temperature.

The temperature profile is a temperature/time graph, representing the component body temperature. In the preheat phase, the maximum temperature is lower than the melting point of the solder paste. It is essential that small solder paste deposits not remain at the intermediate temperature for long, as their activator or flux may evaporate. Cold spots on the board warm up slowly, and therefore, solder paste deposits in cold spots retain their activator longer.

Conveyor speed in the oven should allow both the hot and the cold spots to reach equilibrium by the time the Printed Circuit Board Assembly reaches the end of the preheat phase. In the reflow phase, the solder melts to form soldered joints. The solder paste characteristics determine the minimum peak temperature that all solder joints in both the hot and cold spots must reach to form a reliable joint. To prevent component or board damage, no region may exceed a maximum peak temperature. Even if the hot and cold spots enter the reflow phase at the same temperature, the hot spots will reach the highest peak temperature faster than the cold spots can. For reliable soldering, both the hot and the cold spots must be within the allowed range of the peak temperature. To achieve requires manipulation of process parameters such as belt speed and the temperature settings.

It’s critical to limit the peak temperature below the level at which damage to the board or components can occur. Moisture sensitivity of components determines their maximum peak temperature tolerance. For PCBs it depends on the Tg characteristic. When using SnPb (with lead) solder paste, the typical peak temperature must be above 215 C, and above 235 C for lead-free solder. 260C is generally considered as the maximum allowable temperature which leaves a small process window for lead-free soldering.

Achieving a suitable reflow profile for the solder paste composition presently under use depends on several factors:

  • Type of oven used, infrared (IR) or vapor phase (VP)
  • Heat transfer method employed by the oven (typically a combination of radiation, conduction, convection, and condensation.

Electronic Manufacturing Services industry generally uses IR reflow ovens, and thermal profiling is a critical aspect to ensure proper assembly. Apart from the physical construction and characteristics of the oven which influences the transfer of heat, the PCB Assembly itself is also a dependent factor. Primarily the

  • Layout
  • Component size
  • Device package types
  • Thermal mass of the devices

Soldering profiles that do not adequately consider all of the above could result in poor assemblies, solder defects, and/or failed components.

Engineers use a thermal profiler, an instrument for generating custom reflow profiles. These measured the minimum and maximum temperatures and slopes in the oven and on the PCB through thermocouples attached to various points on the PCB as the assembly traveled through the oven. Process engineers also use software with predictive algorithms to provide highly accurate thermal profiles. Modern reflow ovens often run under computerized control with software governing the numerous functions allowing the user to operate the system free of errors. Such systems help with any mass soldering process such as reflow, while evaluating the parameters in real-time.

The availability of advanced profile evaluators offers engineers several capabilities for generating custom reflow profiles. These include the ability to evaluate temperature at any moment, review the maximum temperatures and gradients, solder wetting times, and conveyor speeds.