Picking the Best Reflow Oven for PCB Assembly Process


A PCB reflow oven is an essential piece of equipment for efficiently soldering surface mount devices (SMDs) onto printed circuit boards (PCBs) during the assembly process. Reflow soldering involves applying solder paste to PCB pads, placing components onto the solder paste deposits, then heating the entire assembly to melt or “reflow” the solder to form solder joints that electrically and mechanically connect the components.

Reflow ovens allow for quickly and reliably soldering large volumes of SMDs onto PCBs by heating up the entire board assembly at once. This is far more efficient than hand soldering each individual joint. Using a programmable reflow oven with different temperature zones and custom profiles also provides precise control over the reflow process for optimized soldering results.

Choosing the right reflow oven is critical for any PCB assembly operation. The type of oven, available features, and size capacity should all be considered based on the needs of the application and types of boards being assembled. This guide covers the key factors in selecting an effective PCB reflow oven that improves quality and throughput.

Types of Reflow Ovens

There are three main types of reflow ovens used for PCB soldering:

Convection Ovens

Convection reflow ovens use fans to circulate hot air and heat the PCB assembly. The temperature is precisely controlled as hot air is blown across the assembly. Convection ovens heat up and cool down relatively quickly, making them efficient and allowing fast processing. They provide even heating across the PCB. However, convection can blow delicate components off the board if not properly managed.

Infrared Ovens

Infrared reflow ovens use IR emitters to directly heat the PCB assembly. IR rapidly transfers heat for quick soldering. The emitters can be arranged in different zones to customize heating. Infrared provides highly controllable and repeatable heating profiles. However, IR tends to heat the board surface more than convection, so balancing is required to avoid damaging sensitive components. Reflectors help focus the IR energy.

Vapor Phase Ovens

Vapor phase reflow ovens immerse the PCB assembly in an inert vapor. The vapor condenses on the cooler board and components, transferring heat. This provides uniform heating across the assembly with minimal temperature differences. Vapor phase ovens efficiently achieve repeatable reflow soldering. However, condensation must be managed properly to avoid damaging the assembly. The ovens also require specific handling precautions due to the high temperatures involved.

Convection Ovens

Convection ovens use fans to circulate hot air inside the oven chamber to heat up the PCBs. The hot air helps transfer heat evenly across the boards for uniform heating and soldering.

How They Work

Convection reflow ovens contain heating elements and powerful fans. The heating elements heat up the air while the fans distribute the hot air around the oven chamber. This allows the convection ovens to quickly bring the PCBs up to soldering temperatures.

Pros

  • Even heat distribution for uniform soldering.
  • Flexible and can accommodate different board sizes.
  • Relatively affordable compared to other oven types.
  • Easy to operate and maintain.

Cons

  • Air pockets can form leading to uneven heating.
  • Limited heat capacity restricting the number of boards.
  • Slow process compared to other ovens.
  • Airflow can disturb lightweight components.

Convection ovens are a good choice for low-volume PCB production thanks to their affordable price and simple operation. But for soldering high quantities, the slow process and airflow may become limiting factors.

Infrared Ovens

Infrared ovens use infrared energy, such as short-wave or medium-wave infrared, to heat and reflow circuit boards. Infrared is directed at the circuit boards to rapidly heat solder and component surfaces through electromagnetic radiation.

How Infrared Ovens Work

Infrared waves are generated by electric coils or gas flame heaters and directed downward at the PCBs as they pass through the oven on a conveyor. Different zones of infrared lamps or heaters are controlled independently to generate the desired thermal profile. The infrared energy is absorbed by the board and components, causing rapid heating of surfaces through radiation.

Pros of Infrared Ovens

  • Very fast heating, allowing quick process times
  • Energy is directed at the boards, avoiding heating empty oven space
  • Ability to individually control different heating zones
  • Lower operating costs compared to convection ovens

Cons of Infrared Ovens

  • Can be difficult to evenly heat all components on complex boards
  • Sensitive to differences in board emissivity which affects heat absorption
  • May require shields or reflectors to prevent heating of sensitive components
  • May produce shadow effects based on component location
  • Limited max board size due to focused energy requirements

In summary, infrared reflow ovens offer fast heating for high volume production, but require tuning for even heating across diverse board designs. Their selective heating makes them best for less complex boards.

Vapor Phase Ovens

Vapor phase ovens utilize a vaporized inert liquid, usually a fluorinated fluid, to evenly transfer heat to the PCB during the reflow process. The oven chamber is filled with an inert vapor that condenses onto the PCB, releasing its latent heat. This provides very even heating across the entire PCB surface.

How Vapor Phase Ovens Work

  • The oven has a reservoir filled with an inert liquid with a low boiling point, usually around 200°C. This is often a fluorinated fluid.
  • The liquid is boiled, creating an inert vapor that fills the oven chamber.
  • When the PCB enters the vapor-filled chamber, the vapor condenses onto its surface, releasing the vapor’s latent heat and heating the PCB.
  • The PCB reaches reflow temperature when enough vapor has condensed and transferred its heat to the board.
  • The vapor insulates the board and components, ensuring even heating across its surface.
  • Once reflow is complete, the condensed vapor evaporates off the PCB and returns to the reservoir for reuse.

Pros of Vapor Phase Ovens

  • Extremely even heating across the entire PCB surface. Vapor condensation prevents hot or cold spots.
  • Gently heats all solder joints at the same rate, preventing damage.
  • Energy-efficient since the vapor transfers heat rapidly. Minimal heating of the air or chamber walls.
  • Very little maintenance required compared to convection ovens.

Cons of Vapor Phase Ovens

  • Limited peak temperature, usually only able to reach 215-230°C. Not suitable for high temperature alloys.
  • Condensation can get trapped under BGAs and QFNs if not dried properly after reflow.
  • Long process time as the board has to heat gradually via vapor condensation.
  • Fluorinated fluids are expensive and must be handled carefully.

Heating Zones

One of the most important factors when selecting a reflow oven is understanding the heating zones. A reflow oven has different zones that heat the PCB assembly to gradually reach soldering temperatures. There are typically three main zones:

Preheating Zone

The first zone is the preheating zone. This stage slowly heats up the PCB from room temperature to 150°C – 200°C. The purpose is to gradually warm all components on the board to prevent thermal shock. Slowly preheating also prevents issues like moisture evaporation from the PCB laminate or cracking of components.

Reflow Zone

The reflow zone rapidly heats the PCB assembly from 150°C to the peak reflow temperature, usually between 230°C – 260°C. The sudden increase softens the solder paste so it melts and wets the contact pads on the components and PCB. This zone needs consistent, even heating so the solder joints form correctly.

Cooling Zone

After reaching peak temperature, the assembly enters the cooling zone to be lowered to a handling temperature around 50°C. The cooling must be gradual to prevent damage from rapid contraction of components as the solder joints harden. Slow cooling also prevents defects like solder balls from turbulence.

The heating zones work together to deliver optimized heating and cooling cycles. The ability to accurately control the zone temperatures and conveyor speed is critical for a high-quality soldering process. When selecting a reflow oven, look for options that give you maximum control over the heating zones.

Temperature Profiles

The temperature profile is one of the most important factors when selecting a reflow oven. The profile refers to the programmed heating pattern that the PCB assembly undergoes during the soldering process. A precisely controlled profile is critical for proper solder joint formation and minimizing defects.

There are typically four zones in the profile – preheat, thermal soak, reflow, and cooling. Each zone serves a purpose in heating the assembly to the reflow temperature and then cooling it in a controlled manner. The ramp rates between zones and time spent at peak temperatures are carefully calibrated.

A quality reflow oven allows you to create custom profiles to suit your specific PCB designs and paste formulations. Look for ovens that provide software for easy profile creation and fine tuning. The best systems allow tracking and logging of profiles for quality control.

Precision control over the profile enables optimizing the reflow process for reliably soldering complex, multi-layer boards without damaging sensitive components. Poor profile control can lead to defects such as cold solder joints, tombstoning, solder balls, and more. Investing in an oven that provides precision profiling capabilities is crucial.

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