Understanding the Operation of a Solder Reflow Oven for Perfect Results

2023-10-12

In a hot reflow oven, solder is heated for making electronic components. Small labs might use a mini oven with an LED reflow burner, while bigger companies prefer conveyor belt reflow furnaces.

Early in electronics manufacturing, they used infrared (IR) heating. Later, companies like BTU International introduced convection heating, which greatly improved heat consistency. Modern reflow ovens now use closed-loop convection for better results.

How Does Solder Reflow Oven Configure?

Inline hot reflow ovens are configured with various heating sections and a cooling section. The number of zones, whether it's a few (eight or fewer) or many (twelve or more), depends on the reflow oven's size, capacity, and length. Each zone is set to a specific temperature to be used during the soldering process, and as the printed circuit board (PCB) passes through these zones, it is exposed to those temperatures.

The recipe, a software program, contains the temperature settings for each zone and the conveyor belt speed. The profile represents the temperature the PCB experiences as it moves through the reflow oven. This recipe is customized to create the solder paste profile defined by the reflow machine's manufacturer, considering factors like maximum and minimum temperatures, Flux Duration (FAT), Time Over Liquids (TAL), and heating and cooling rates.

PCBs can go through a reflow oven in regular air, a controlled forming gas or nitrogen environment, or a combination of both. Forming gas or nitrogen is used to reduce or prevent oxidation during the heating process.

In certain less critical operations, the nitrogen soldering reflow process may have varying oxygen concentrations, ranging from over 500ppm to as low as 10ppm. SMT reflow ovens designed for nitrogen use often feature an oxygen monitor and an automated gas sampling system.

Working of a Modern Solder Reflow Oven

To solder surface-mounted electronic components effectively onto a PCB, it is necessary to apply heat to the solder paste until it reaches its molten point, which is 217°C for lead-free solder like SAC305. When this molten alloy combines with the copper pads on the PCB, it forms a eutectic mixture. As it cools back down to its melting point, a strong solder joint is created.

There are three methods for transferring heat from a heat source to the items being heated:

1.Conduction:

Conduction is the transfer of heat through a material without the material itself moving. When there is a temperature difference between adjacent areas, heat flows from the warmer area to the cooler one. This process continues until both areas reach the same temperature.

2.Radiation:

Heat transmission by radiation occurs in the form of electromagnetic waves, often in the infrared (IR) region. Radiation doesn't require a physical connection between the heat source and the warmer object. It's important to note that black bodies absorb more heat energy than white bodies, impacting the efficiency of radiation.

3.Convection:

Convection involves the transfer of heat through the movement of fluids such as air or gas vapor. It provides a contactless means of heat transfer.

In the context of heat transfer in soldering, ceramic heat elements emit heat through infrared radiation, but they don't directly transfer this heat to the circuit. To ensure uniform heat distribution, the heat energy first passes through a heat adjuster. A convection blower or fan then propels the heated air into an inner compartment, ensuring consistent heat is experienced on any part of the target circuit board.

How Solder Reflow Ovens Work？

The working principle of a solder reflow oven involves heating solder paste to its melting point to create strong solder joints between surface-mounted electronic components and a printed circuit board (PCB). Here's a simplified explanation of the key steps:

Application of Solder Paste: Before the components are placed on the PCB, a solder paste, which is a mixture of solder particles and flux, is applied to the soldering pads on the PCB. This paste serves as the adhesive that will create the solder joints.
Component Placement: Surface-mounted electronic components are carefully placed on the solder paste-coated pads of the PCB, positioning them precisely.
Preheating Zone: The PCB with the components is then transported into the reflow oven. The first part of the oven is the preheating zone. In this zone, the temperature gradually increases to a level where the flux in the solder paste becomes active. This helps remove any oxides from the soldering pads and the components.
Soldering Zone: After preheating, the PCB enters the soldering zone. In this section, the temperature quickly rises to the melting point of the solder paste (e.g., 217°C for SAC305 lead-free solder). The solder paste transitions from a solid to a molten state. The molten solder alloy bonds with the component leads and the PCB pads, forming strong solder joints.
Cooling Zone: Once the solder joints have formed, the PCB moves into the cooling zone, where the temperature decreases gradually. This controlled cooling process ensures that the solder joints solidify and create secure connections between the components and the PCB.

The reflow oven typically has multiple heating zones with precise temperature control to achieve uniform heating and cooling across the PCB. The entire process is carefully monitored and controlled to ensure that the components are soldered properly, and the solder joints are of high quality.

By following this reflow soldering process, manufacturers can achieve consistent and reliable connections in the production of electronic devices.

What Is The Role Of A SMT Reflow Oven In Soldering?

The central element of the entire reflow soldering process is the reflow oven. For Surface-Mounted components, the reflow soldering process typically utilizes infrared or convection ovens, which consist of various zones with adjustable temperatures. These ovens are linked in a conveyor-like line, allowing different stages of heating and cooling.

The size of the oven must match the production load and rate, as it determines both the minimum speed of the production line and the process's repeatability. If the production processing speed exceeds the line's speed, a larger reflow soldering furnace may be needed for larger-scale operations.

Parts and Stages of an SMT Reflow Oven:

1.Preheating Stage:

The preheating stage gradually raises the temperature of the PCB to a point where the solder can reflow. Careful monitoring is essential during this phase to prevent damage to the PCB and electronic components. A gradual temperature increase ensures uniform heating.

2.Thermal Soak:

During preheating, the PCB undergoes a thermal soak process where it is held at a specific temperature to equalize component temperatures. The duration of the thermal soak varies depending on the PCB and its components. This step ensures that all elements are at a consistent temperature before reflow soldering.

3.Reflow Stage:

In the reflow stage, the oven's temperature exceeds the solder paste's melting point. As the paste liquefies, it forms solder joints on the PCB. The flux in the solder paste plays a key role by reducing surface tension and promoting metallurgical bonding. Maintaining the right temperature and duration is critical for efficient, high-quality soldering.

4.Cooling Stage:

After the solder has cooled to form joints, the PCB goes through a cooling process to solidify the molten solder and secure the components. Gradual cooling is vital to prevent PCB stress and damage. The cooling rate typically ranges from 30 to 100 degrees, with a cooling rate of about 3 degrees per second.

Properly executed reflow soldering, including gradual preheating, thermal soak, reflow, and cooling, ensures the production of mechanically secure and sound connections.

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