What Is The Core Process of Reflow Oven in SMT Assembly?

Introduction

In the SMT production line, reflow oven is the “crucial factor” determining the final quality of solder joints. Simply put, it involves a precisely controlled thermal process that melts solder paste, wets the pads and component leads, and then cools and solidifies to form reliable electrical connections. The success or failure of thousands of solder joints on a printed circuit board assembly (PCBA) depends almost entirely on the performance of the reflow oven and whether the “temperature profile” set by the engineer is perfect.

I. The Evolution of Reflow Soldering Machine: From Simple Heating to Intelligent Temperature Control

Reflow soldering technology is not static; it has undergone continuous iteration and upgrades:

• Early stage: Plate-type radiation heating, quartz infrared tube heating → Low thermal efficiency, poor uniformity.
• Mainstream development: Infrared hot air heating → More uniform heating, but still has limitations.
• Modern mainstream: Forced hot air heating → Achieves more precise and uniform temperature control through powerful hot air circulation.
• High-end advancement: Forced hot air heating + nitrogen (N₂) protection → Welding in an inert gas environment significantly reduces oxidation, enhancing solder joint brightness and reliability (especially for lead-free solder paste and high-reliability applications such as automotive electronics).
• Cooling revolution: With the increasing demand for cooling speed due to lead-free soldering (higher melting point) and high-density assembly, the cooling zone has evolved from simple “room temperature cooling” or “air cooling” to more efficient water cooling systems, which can lower solder joint temperatures faster and more controllably, optimizing microstructure.

To meet increasingly stringent process requirements, the number of “temperature zones” in reflow soldering equipment has also been increasing: from the early three-zone systems, it has evolved to the current five-zone, six-zone, seven-zone, eight-zone, and even ten-zone systems, providing engineers with more precise temperature control capabilities.

II. Choosing the Right Reflow Oven? Focus on These Eight Core Parameters!

• Zone configuration: Number, length, and width.
• Heating symmetry: Whether the upper and lower heaters are balanced.
• Zone uniformity: Whether the temperature distribution within a single zone is consistent.
• Independent conveying control: Whether the conveying speed between zones can be independently and precisely adjusted.
• Nitrogen capability: The effectiveness of inert gas protection during soldering and gas consumption.
• Cooling gradient: The cooling speed and controllability in the cooling zone.
• Maximum Temperature: The highest temperature the heaters can reach.
• Temperature Control Precision: The stability and accuracy of temperature control.

III. Mastering the 4 Key Technical Points of Reflow oven Process

Equipment is the foundation, but process is the soul. When setting the reflow soldering program, these parameters are critical to success.

1. Temperature Curve

This is the core of the core! It must be tailored to the specific characteristics of the product:

• PCB size, number of layers, thickness?
• Component size, density, thermal capacity?
• PCB copper foil area and thickness?

Engineers must use a furnace temperature tester to measure the temperature at critical solder joints on the PCB, then repeatedly adjust equipment parameters (temperature zones, conveyor speed) to plot the optimal “temperature-time” curve for the current product. This curve typically includes four stages: preheating, holding, reflow (peak), and cooling.

2. Precise time control

By adjusting the conveyor belt speed, strictly control the dwell time of the PCB in the preheating zone, reflow zone, and cooling zone. The cooling rate in the cooling zone also requires fine-tuning in conjunction with the airflow/water flow in the cooling zone.

3. Refer to the solder paste “manual”

The ideal temperature curve should closely follow the reference curve provided by the solder paste supplier. Solder pastes with different compositions (lead-containing/lead-free), alloy ratios, and particle sizes have different melting characteristics and optimal soldering temperature windows.

4. Secrets of double-sided soldering

• Standard procedure: First solder the side with smaller components and lower thermal capacity, then solder the other side. During the second pass through the reflow oven, the small components already soldered on the bottom side can typically withstand secondary heating.
• Special cases (large components on both sides or large components must be soldered first): When soldering the second side, it is essential to protect the large, heavy components already soldered on the bottom side! Common methods include using specialized high-temperature-resistant Fixtures or applying high-temperature-resistant adhesive locally to prevent them from detaching due to solder remelting during the second reflow.

Conclusion

Driven by the wave of intelligent and electrified new energy vehicles, SMT assembly line, especially reflow oven technology, is playing an increasingly important role. Reflow oven has evolved from simple heating ovens to high-tech systems featuring multi-temperature zones, nitrogen protection, and intelligent cooling. Reflow soldering processes have also transitioned from trial-and-error based on experience to precise control based on precise testing and scientific analysis. Mastering the core parameters of the equipment and the key technical points of the process is a solid foundation for ensuring the high reliability of automotive electronic products and supporting the intelligent manufacturing upgrade of China’s new energy vehicles!

Company Profile

Zhejiang NeoDen Technology Co., Ltd. has been manufacturing and exporting various small pick and place machines since 2010. Taking advantage of our own rich experienced R&D, well trained production, NeoDen wins great reputation from the world wide customers. 

In our global Ecosystem, we collaborate with our best partners to deliver a more closing sales service, high professional and efficient technical support.

We believe that great people and partners make NeoDen a great company and that our commitment to Innovation, Diversity and Sustainability ensures that SMT automation is accessible to every hobbyist everywhere.