What is Reflow Soldering? A Complete Overview

Reflow soldering is an essential process in electronics. It attaches components to printed circuit boards, making them functional for different applications. Here, you’ll learn how the reflow process works and its benefits. More importantly, we’ll cover its various stages, step by step, and see what happens in each step.

What is Reflow Soldering?

Reflow soldering is a technology PCB manufacturers use to attach electronic components and wires or connectors. Its name derives from “reflowing” previously heated solder paste to form joints.

We mainly use the procedure to assemble surface mount (SMT) components and sometimes through-hole types. The process contrasts with wave soldering, which typically attaches through-hole parts.

Reflow Soldering Equipment

A reflow soldering system combines several parts. The parts include a solder paste application machine, a pick-and-place machine, and a reflow oven. The system also consists of an array of inspection equipment.

Solder Paste Applicator

It applies solder paste to soldering surfaces. The paste is a mix of solder and flux. The solder flows into joints and solidifies after cooling, forming robust joints. The applicator uses a stencil to apply paste selectively using controlled pressure and speed.

Pick and Place Machine

The pick-and-place machine is a robotic device that places parts on the circuit board’s surface. Pick-and-place machines use vacuum suction to hold components, releasing the pressure to lower them onto soldering surfaces.

Reflow Oven

The reflow soldering oven is the most significant part of the system. It provides conditions for melting and cooling. It also features several temperature zones for the preheating and reflow stages. Heating methods include infrared, convection, or vapor phase.

• Infrared Oven – it uses infrared heaters, just as the name indicates
  
• Convection Oven – this type blows hot air to heat the circuit board and solder paste
  
• Vapor Phase Oven – the vapor phase type works by heating an inert liquid to produce vapor and create a temperature rise
  

Inspection Tools

These help inspect reflow solder joints to detect flaws like tombstoning. They can be simple manual tools or complex, automated inspection machines. They detect

• Manual Tools – Manual tools include magnifiers that operators hold to examine formed joints
  
• Automated Optical Tools – These use sophisticated cameras and computer programs to look into joints and automatically detect errors. They usually project the image onto screens
  
• X-Ray Imaging – x-rays examine the board for defects that visual or AOI inspections alone cannot visualize
  

How Does Reflow Soldering Work?

It involves melting pre-applied solder paste. The solder paste is a sticky substance and a mixture of solder powder and flux. The solder alloy melts at low temperatures while the flux removes contamination and prevents oxidation.

The sticky nature of the paste helps secure components before the solder can melt. After melting, the solder firmly mounts parts to the circuit board. The process occurs in a highly controlled environment, allowing the reflow soldering machine operator to ensure precision and minimize defects.

Reflow Soldering Process Step by Step

The process comprises several main steps. A significant part of the process occurs in the heat and cooling stations. Here, we trace the stages, starting from solder paste application to cooling or when the solder solidifies.

Solder Paste Application

The process begins with the application of solder paste. The paste contains the low-melting alloy that will form the solder joint. In this case, the joint will attach parts to a PCB.

• The solder paste machine applies solder to contact pads and other surfaces
  
• It uses a specially designed stencil to ensure precision
  
• The reflow soldering stencil is specific to the circuit board’s circuitry
  

Placing Components

The second phase is where a robotic arm places components in designated locations on the circuit board. This part requires utmost precision, usually achieved by automating placement.

• The pick-and-place machine selects the parts to be mounted
  
• It then picks and lands them on the PCB surface
  
• The parts include ICs, resistors, capacitors, and other SMDs
  

Preheating Stage

The preheating stage prepares the board for the higher soldering temperatures. It prevents thermal shock by gradually warming the board and its components to avoid sudden temperature changes.

• The circuit board goes into the preheating station, where it warms up
  
• The temperature increase is gradual at 2° Celsius per second
  
• The reflow soldering temperature must stay within limits to prevent damage
  

Thermal Soak

Thermal soak distributes heat evenly across the circuit board and its components. It also ensures the heat is optimal for reflow and within desired limits.

• The board stays in the oven for 60-120 seconds more
  
• The duration depends on the type of board, its thickness, and other factors like material
  
• The period also depends on the kind of components assembled on the PCB
  

Reflow Process

The solder finally melts under controlled temperatures. Precise temperature control prevents the components from falling off. It also ensures its efficient flow.

• The temperature increases to a level above that of the solder alloy, causing it to melt
  
• Molten solder sticks to pads, holding the components to the board
  
• The melting/reflow period lasts between 30 seconds and one minute
  

Cooling Stage

Here is where the solder cools to solidify and permanently attach parts to the circuit board. Let’s see what happens, step by step.

• The board goes to the cooling stage/station
  
• The temperature drops gradually, usually by 3° Celsius a second
  
• The solder solidifies, completing the PCB assembly process
  

Inspection

After cooling, the assembled board must undergo a vigorous inspection process. Procedures include visual checks, automated optical inspections, and X-ray imaging. There are also further tests, such as electrical testing, to check for opens and shorts, among other flaws.

Reflow Soldering Advantages and Disadvantages

Reflow has many benefits, especially when compared to a surface mount technology like wave soldering. It also has drawbacks that make it less usable. Here is a brief look at the advantages and disadvantages.

Advantages

• It has a higher accuracy level, making it suitable for high-density assembly needs
  
• Reflow soldering defects are fewer compared to other methods, such as reflow or manual soldering
  
• You can use it to assemble double-sided circuit boards, making it a more versatile process
  
• The closed nature of its ovens allows precise control of conditions, such as temperatures
  
• The high temperatures involved favor lead-free solder
  

Disadvantages

• It’s only best for small-sized and low-profile SMD or surface-mount devices
  
• Reflow solder joints are usually not sturdy enough to hold bulky parts
  
• Issues like shadowing limit the flow of solder and affect the joint quality
  

Reflow Soldering Application in Electronics

The reflow process has widespread application in electronics. Products that benefit from the process are typically compact with complex electronics. They also use high-density printed circuit boards and ICs with 0.5 mm pitches or less. Examples include:

• Consumer electronics
  
• Communication gadgets like mobile phones
  
• Portable devices, such as cameras and laptops
  
• HDI computing systems and boards
  
• Advanced electronic circuit boards for automotive systems
  
• Medical device electronics, including wearable tech and implantable devices
  

Conclusion

Reflow soldering is one of the most used soldering technologies today. It attaches surface mount devices and other components to printed circuit boards but can also mount through-hole parts. It also best suits modern electronics where the circuit is compact and packed with components.