The reflow soldering process is a little bit different than wave soldering, but it’s the most common way to attach surface mount components to a circuit board. Wave soldering is more frequently used for soldering through-hole components. Although it’s possible to use reflow soldering for this purpose, it rarely is since wave soldering is more cost-effective.

In reflow soldering, we make a solder paste out of powdered solder and flux, then use that paste to attach components to contact pads. We then heat the entire assembly in a reflow oven or under an infrared lamp to melt the solder and connect the joint. You can solder individual joints with a hot air pencil if necessary.

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What are the advantages of reflow soldering?

As mentioned already, an infrared lamp is used to heat the air and melt the solder in the reflow soldering technique. The board is passed through a tunnel or reflow oven where it is heated in a controlled manner. In this, you apply the solder paste only over the areas (contact pads) where you want to assemble the electronic components.

The reflow soldering process produces reliable joints that are solid. Since they do not undergo any thermal shock, the joints are strong. Besides, they require minimal monitoring, and the process is conducted in controlled conditions. The reflow soldering technique is ideal for soldering surface mount components. It is most commonly used for assembling PCBs in mass production.

Pros of Reflow Soldering

·It is preferable for SMT assembly

·It produces only a less thermal shock

·It is one of the most reliable soldering technique

·It has options for limited soldering

·It requires only a minimum level of monitoring

·It leads to less wastage

·It can be used to solder specific parts of a PCB

What are the preparations to do reflow soldering?

There are two main steps involved in the reflow soldering process of PCB. The first is to apply the solder paste and the second is to assemble the components.

1. Solder Paste

Before applying the solder paste, you have to use a solder mask to ensure the solder is applied only to the areas where the copper pads will be installed. If you do not apply the solder mask properly, it will lead to the formation of solder bridges.

By using the solder paste, you can apply the solder paste only on the areas in PCB where soldering is needed.

2. Pick and Place

Once you have applied the solder, then you can assemble the components on the board. If the number of components is less, you can manually assemble the components.

But if the components are more or if you are soldering on a commercial level, then it is best to use a pick and place machine. With the pick and place machine, you can place the components on the board automatically. The components will be held onto the board by the surface tension of the solder paste.

But in some cases, you have to apply glue to ensure the components do not fall off from the board. If you apply glue, it will be difficult to rework the PCB. Once you have placed the components on the board, then you can move them to the reflow soldering machine.

Reflow Soldering Stage

The reflow soldering process consists of several stages to ensure the joints are strong. The temperature of the board is gradually increased before the soldering process to prevent thermal shocks.

The four different stages of the reflow soldering process are as follows,

Preheat

You have to heat the board slowly to bring it up to the required temperature. If you subject the board to rapid heating, it will lead to thermal shock and break the board.

Besides, if the heating is not stable, it will cause hot and cool spots on the board. Some parts of the board will have a higher temperature. And, some parts of the board will have a lower temperature. For infra-red reflow soldering, you have to raise the temperature of the board by 2 to 30C. In some cases, you can also increase the temperature by 10C.

Thermal Soak

Once you have brought the board to the required temperature, you have to move it to the thermal soak area. In this, you have to maintain the board at a steady temperature. This will ensure that all areas of the board are brought up to the required temperature. Additionally, it will also remove the solder paste and activate the flux.

Reflow

In the reflow area, the temperature of the board reaches the highest point. The reflow process melts the solder and fuses the solder joints. The high temperature causes the flux to reduce the surface tension at the joints of the metals causing metallurgical bonding. This causes the solder powder to combine and melt.

Cooling

Once the solder melts and fuses the joints, you have to allow the board to cool down. You have to make sure that the cooling effect does not cause stress to the components.

Cooling the board prevents thermal shock of the components and excess intermetallic formation. Proper cooling temperature should be in the range of 30 – 1000C. This temperature range creates a fast cooling effect and provides strong joints.

Reflow Ovens

Reflow ovens are large machines that are used to assemble PCBs. These machines provide soldering capability to both small and large PCB assembly areas. If you are soldering in a small PCB assembly area, then you can buy smaller reflow ovens according to your rework and assembly area. These machines are tailored to suit the needs of small PCB assembly areas.

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