SMD Assembly - Reflow/IR


:: Know-How :: Soldering :: Reflow/IR
Soldering Reflow/IR Solder Paste Lead Free Development CAD Design SMT/PTH	Reflow/IR
Reflow soldering is the most commonly used form of joining SMD components to printed circuit boards. The process includes placing solder paste on the board, positioning the components and melting the solder in a reflow oven, which joins the surfaces with a metallurgical bond. The phases of the process itself occur in four zones: Pre-heating Zone: phase in which the solder paste is heated at a constant rate high enough so that impurities in the solder paste begin to evaporate but low enough to avoid damage to the components due to thermal shock. Thermal Soak Zone: Lasts between 60 and 120 seconds to remove impurities in the solder paste and to activate the flux. Too high a temperature may lead to the formation of bubbles in the paste while a temperature too low may make the paste burst soiling the component and the board. At the end of this phase it is ideal to have thermal equilibrium before proceding on to the next one. Reflow Zone The period of time during which the solder is in the liquid state, or time above liquidus. In this phase, the solder is liquified in order to join the surfaces. The highest temperature is limited by the thermal tolerance of the most fragile component in the circuit. If this phase takes too long the flux may dry out before a solder joint is formed. An insufficient amount of time in this phase may cause the flux to perform a less than optimal cleaning, causing less distribution of the solder over the surface and inferior bonds. This phase usually lasts less than 60 seconds with a minimum time of 30 seconds. Higher reflow times may cause damage to the components and inferior quality joints which may then be the cause of component malfunctions in the future. Cooling Zone: Phase during which the solder resolidifies creating joints between the components and the board. The temperature in this phase is somewhere in the range of 30-100°C going down at a constant rate in order to avoid damage due to thermal shock and excessive intermetallic excessive.

Reflow soldering is the most commonly used form of joining SMD components to printed circuit boards. The process includes placing solder paste on the board, positioning the components and melting the solder in a reflow oven, which joins the surfaces with a metallurgical bond. The phases of the process itself occur in four zones:

Pre-heating Zone: phase in which the solder paste is heated at a constant rate high enough so that impurities in the solder paste begin to evaporate but low enough to avoid damage to the components due to thermal shock.

Thermal Soak Zone: Lasts between 60 and 120 seconds to remove impurities in the solder paste and to activate the flux. Too high a temperature may lead to the formation of bubbles in the paste while a temperature too low may make the paste burst soiling the component and the board. At the end of this phase it is ideal to have thermal equilibrium before proceding on to the next one.

Reflow Zone The period of time during which the solder is in the liquid state, or time above liquidus. In this phase, the solder is liquified in order to join the surfaces. The highest temperature is limited by the thermal tolerance of the most fragile component in the circuit. If this phase takes too long the flux may dry out before a solder joint is formed. An insufficient amount of time in this phase may cause the flux to perform a less than optimal cleaning, causing less distribution of the solder over the surface and inferior bonds. This phase usually lasts less than 60 seconds with a minimum time of 30 seconds. Higher reflow times may cause damage to the components and inferior quality joints which may then be the cause of component malfunctions in the future.

Cooling Zone: Phase during which the solder resolidifies creating joints between the components and the board. The temperature in this phase is somewhere in the range of 30-100°C going down at a constant rate in order to avoid damage due to thermal shock and excessive intermetallic excessive.