What Is Reflow Soldering?

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May 10, 2021|Manufacturing Excellence, Technological Advancements and Materials

A variety of soldered joints connecting mechanical and electrical components make up the body of a circuit board. Attaching these joints requires soldering. Reflow soldering is a process used on circuit boards. You may wonder, how do you reflow solder? We will discuss what temperature solder reflows and how you can solder with a heat gun.

What Is Reflow Soldering?

Reflow soldering is a process that involves attaching surface mount components to a circuit board (PCB). Solder paste, a mixture of powdered solder and flux, attaches many small electrical elements to a contact pad. Use controlled heat to reflow the solder paste into a molten state; this creates permanent solder joints. You can then use reflow ovens or infrared lamps to produce this high heat—solder individualized solder joints with a hot air pencil.

The Reflow Soldering Process

Once the solder paste is applied, place the components on the board. The board is put through a tunnel with controlled heat to melt the solder paste. This secures the electrical components to the circuit board. Reflow soldering is a reliable method with very high pitch leads.

For an Effective Reflow Soldering Process

You want to make sure that the reflow soldering process is a success, consider the following:

• You will require a suitable machine.
• You will require suitable reflow profile conditions.
• Be sure to have a PCB element footprint design.
• Get a detailed and well-designed printed PCB.
• Have options for repeatable placement for surface mount components.
• Ensure that your PCB is high-quality.
• Select high-quality solder paste.

Wave Soldering vs. Reflow Soldering

What is the difference between reflow soldering and wave soldering?

Reflow Soldering

Soldering is very important during the PCB design process; it’s the only way to adhere circuits to your board. There are two ways of soldering: wave soldering and reflow soldering. Both have benefits depending on your project.

When you require bulk soldering, which involves many circuit boards in a short period, wave soldering is the most efficient method.

Reflow soldering is done on a smaller scale. A heat gun is a relatively inexpensive method to heat solder paste onto your components and the board; however, it still can get quite hot, and thus you need to be careful while handling it. Solder also contains toxins if heated too heavily, so you want to ensure your heat gun is at the correct temperature before starting.

The range for reflow temperature is between 240 and 250 degrees Celsius for a Pb-Free (Sn/Ag) solder.

Wave Soldering

Wave soldering is done by passing circuit boards over a pan, with a pump, of molten solder. This pump creates a solder that can almost be described as a “wave” over the board; this sells the board’s components. Water is then sprayed over the PCB board, cooling the solder and affixing parts in place. Use reflow soldering for smaller projects, and wave soldering tackles the larger tasks.

Reflow Soldering for PCB Manufacturing

Reflow soldering is commonly used in PCB manufacturing. Pad sizes and varieties of components are easily monitored and controlled during this process. In today’s electronic needs, this high-quality style of soldering is reliable and efficient. 

Preparation

The stages of preparation for reflow soldering include a variety of important steps.

Solder Paste

Apply solder paste to the board that requires soldering. Only add solder paste to those areas required for soldering. You can use a solder mask and a solder paste machine to apply only to the necessary areas.

Pick and Place

Apply the solder paste on the board and then place the components you require. If you need accuracy and manual placement is not an option, use a pick and place machine.

Reflow Soldering Stage

The reflow soldering stage consists of several individual steps due to the temperature changes. To properly control the temperature of the reflow tunnel will ensure the solder joints are properly attached. There are four stages used, as follows:

Preheat

Bring the board to the required temperature. If you apply too much heat, the board can be compromised by thermal stress. When using infra-red soldering, the temperature rise rate is between 2 and 3 degrees Celsius. On some occasions, temperature rise rates, down to one degree per Celsius, can be used.

Solder reflow temperature should be higher than its melting temperature, at a peak temperature window of 233 – 255°C. It is important to ensure that “wetting” occurs between the PCB and its components, with a time frame between 30 and 60 seconds.

Thermal Soak

This next stage is when the board temperature enters into the thermal soak area. Temperature is maintained here so that all areas are heated equally, and the other is to remove the solder paste. Shadowing effects may come up if the heat is not maintained.

Reflow

Reflow happens when you achieve the highest temperature of soldering, which is between 240 and 250 degrees Celsius for a Pb-Free (Sn/Ag) solder. The solder then melts to create solder joints. This reflow process involves the flux reducing a surface tension where the metal joins. This accomplishes a metallurgical bond and allows the individual solder powder to combine and melt.

Cooling

Cooling happens just after the reflow process in a way that does not damage or put stress on the board and the components. Upon completion, proper cooling prevents excess intermetallic formation or thermal shock. These typical temperatures should range from 30-100 degrees Celsius. These temperatures create a fast cooling rate to create a secure solder and provide a mechanically secure joint.

Reflow Ovens

Reflow ovens are large machines for use with PCB assembly production. Many varieties of reflow ovens provide the soldering capabilities for both large and small assemblies. For prototype and rework areas, smaller reflow machines can be used. The ovens work well in smaller work areas due to their design.

PCB/Components Footprint Design

The PCB/Component Footprint Design influences how well the assembly reflows. The size of the tracks connects to a component footprint. Thermal imbalance is created when one side of the component footprint is a different size. Copper balancing can happen when PCB designs use larger copper areas. To aid the manufacturing process, the PCB can be put into a panel, but this can lead to an imbalance in copper.

Conclusion

The reflow soldering process involves a series of important steps and high-quality materials. If you are soldering on a larger scale, you may want to consider wave soldering. Be sure to use the properly sized reflow oven in alignment with your PCB. If you require individualized soldering for joints, you may use a hot air pencil. Be sure to carefully follow guidelines and recommendations depending on the size of your PCB.