What is Reflow Soldering?

When it involves the PCB assembly process, reflow soldering is the most generally used method of soldering, which is widely used for PCB assembly.
Reflow Soldering

When it involves the PCB assembly process, reflow soldering is the most generally used method of soldering, which is widely used for PCB assembly. It offers credible soldering for every kind of the size of components and pads required, furthermore making it easy to regulate. the aim of reflow soldering is to be acceptable soldering joints with pre-heating the components, PCB, and solder paste first so melting the solder, rather than overheating without damage.

During the pick and place operation, parts are connected to the board by the solder paste. A reliable connection is formed during the Reflow Soldering process. The solder pastes melts during this process and cools down again to make an honest solder joint. This article will explain all the methods of reflow soldering, its advantages, and the different stages involved in it.

What is Reflow Soldering?

Reflow soldering is a process in which solder paste is applied to the contact pads of a PCB and melted to affix the electrical parts. The method begins by applying solder paste to the PCB in a very specific pattern employing a purpose-built stencil printer. The paste consists of a metal alloy suspended in a mixture of solvents and other materials. The board is then heated by use of a reflow oven in line with the specification for the solder paste getting used, including heating, cooling ramp rates, time above liquidus, and peak temperature (max/min). The reflow soldering technique resembles the standard soldering process in some ways. But, the one major difference is that it uses heated air to melt the solder rather than a hand tool.

 The complete assembly is had a reflow oven that has a heat lamp. The lamp warms the air that melts the solder and binds the electrical parts. Besides, you’ll be able to control the temperature of the electric fire. So, you’ll be able to heat the board gradually to make sure that it doesn’t break thanks to thermal shock. A sudden rise in temperature is understood as thermal shock. Thermal shock weakens the joints and causes them to interrupt easily. This is prevented by using the reflow soldering technique.

As mentioned already, a heat lamp is used to warm the air and melt the solder within the reflow soldering technique. The board is knowledgeable about a tunnel or reflow oven where it’s heated in a very controlled manner. In this, you apply the solder paste only over the areas (contact pads) where you would like to assemble the electronic components. The reflow soldering technique makes reliable joints that are solid. Since they are doing not undergo any thermal shock, the joints are strong. Besides, they require minimal monitoring, and also the process is conducted in controlled conditions.

Reflow Soldering in PCB Assembly Factory

Advantages of Reflow Soldering

The reflow soldering procedure is perfect for soldering surface mount parts. It is generally used for assembling PCBs in mass production. Here are its advantages:

  • It is more suitable for SMT assembly
  • It creates only a less thermal shock
  • It is one of the most dedicated soldering procedures
  • It has choices for restricted soldering
  • It needs only the lowest level of monitoring
  • It guides to smaller wastage
  • It can be utilized to solder precise components of a printed circuit boards


There are two major steps involved in the reflow soldering procedure of printed circuit boards. The primary is to use the solder paste and therefore the second is to assemble the components.

  1. Solder paste

Substantively using the solder paste to the board and also the solder paste is hardly used for the areas that need soldering. There are the solder resist layers on the board, but it’s essential to only add solder paste to the areas that need soldering, which is completed by using the solder mask and solder paste machine. And it only permits the solder paste to be attached to the PCB places where it’s needed. Then having added the solder paste to the board after adding, it can last to the following stage.

  1. Pick and place

It can fix the components in situ with the solder paste on the board. As was common to use the automated pick and place machine because it can’t work when using the manual placement to the numbers of components and necessary accuracy.

The pick and place machine places the components onto the board, and it can remain in situ with the physical phenomenon of solder paste. In fact, it’s enough for normal handing, but there are some tips you would like to grasp. it’ll be difficult to remodel if the components glue to the board, then what’s worse, all the components in situ will move to the reflow soldering machine.

Reflow soldering stage

The reflow process is formed of lots of individual processes, which ensure there’ll not be any acceptable levels of thermal shock since the board is up to the right temperature for reflow soldering. It can confirm the resultant solder joints have the very best quality if correctly analyzing the temperature of the reflow tunnel or chamber. There are four stages of the reflow soldering process these are:

  1. Preheat

The board has to be steadily up to the mandatory temperature. However, it should be damaged by the thermal stress if the speed is just too high. Additionally, some areas might not get to the required temperature because of the thermal mass if the temperature of PCB is simply too quick. However, it should not achieve the mandatory temperature if it’s too slow. In general, the temperature rises for infrared reflow soldering is between 2 and 3℃ per second, but it’s going to be all the way down to 1℃ per second on some occasions.

  1. Thermal soak

After the board is rising in temperature, it’ll get into the thermal soak area. There are two reasons why the cardboard is maintained at temperature, one is to create sure any areas that aren’t fully heated thanks to shadowing effects come up to the required temperature. The opposite is to activate the flux and take away the solder paste solvents or volatiles.

  1. Reflow

The realm of reflow is the area where get the very best temperatures within the soldering process. The solder is melting here and creates the mandatory solder joints. And also, the practical reflow process refers to the flux to scale back the physical phenomenon at the metal joint so as that achieve metallurgical bonding, and permit the individual solder powder spheres to mix and melt.

  1. Cooling

It has to be wiped out the simplest way with no stress to the components for the cooling boards after reflow. Proper cooling can control redundant intermetallic compound formation or thermal shock to the parts. The range of typical temperatures within the cooling zone is 30-100℃ (86-212°F), where is creating a comparatively quick cooling rate for the temperature, and it is often an excellent grain structure within the solder with this method that provides the foremost mechanically sound joint.

Reflow Ovens

Reflow ovens are large machines that are accustomed assemble PCBs. These machines provide soldering capability to both small and enormous PCB assembly areas. If you’re soldering during a small PCB assembly area, then you’ll be able to buy smaller reflow ovens per your rework and assembly area. These machines are tailored to suit the wants of small PCB assembly areas.

PCB/Components Footprint Design

The PCB/part Footprint Design impacts how nicely the assembly reflows. The dimensions of the tracks connect to a component footprint. Thermal imbalance is made when one side of the component footprint could be a different size. Copper balancing can occur when printed circuit board designs use more extensive copper areas. To assist the manufacturing process, the PCB may be put into a panel, but this will cause an imbalance in copper.


The reflow soldering process involves a series of important steps and high-quality materials. If you’re soldering on a bigger scale, you will want to contemplate wave soldering. make certain to use the properly sized reflow oven in alignment along with your PCB. If you need individualized soldering for joints, you’ll use a hot air pencil. Make certain to carefully follow guidelines and suggestions reckoning the dimensions of your PCB.

The ideal reflow solder profile for every assembly does exist. The reflow soldering process is often time-consuming to set up but is important to make sure all components are fully soldered without being damaged. It’s even more important when profiling a lead-free assembly thanks to the appropriate temperature range being reduced to it of a tin-lead assembly. Employing a carefully designed profile will lead to a repeatable process which can consistently deliver the specified results – it’s well worth the overtime and energy.

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