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Reflow soldering – an important part of the PCBA process


Reflow soldering is the technique of applying solder paste to the contact pads of a PCB and melting it to link the electrical components. It uses SMT solder paste to connect one or more electronic components to a contact pad and then melts the solder by controlled heating to achieve a permanent bond. It can be carried out using different heating methods such as reflow ovens, infrared heat lamps or heat guns. Reflow soldering is a very important step in the PCB assembly process in SMT.

What is SMT reflow soldering?

What-is-SMT-reflow-solderingSMT Reflow soldering is the method us/ed to put together PCBs. It is a procedure in which SMD components are firmly connected to their solder pads using a solder paste. Then the complete SMT PCB assembly is exposed to a controlled heat environment within a heating device known as the reflow oven. The solder paste is re-flowed in a molten state to form permanent solder joints.

Why is reflow soldering important?

Reflow soldering is the main process technology in SMT, reflow soldering quality is the key to PCB reliability, it directly affects the performance of electronic equipment reliability and economic benefits
The reflow soldering process has two main purposes.

1) For printed tin boards (the process route is: solder paste printing + placement + reflow), the purpose is to heat and melt the solder paste and solder the pins or solder ends of the device to the pads of the PCB through the melted solder paste for electrical connection.

(2) For printed glue boards/dispensing boards (process route: SMT glue printing/SMT glue dispensing + placement + reflow), the purpose is to heat and cure the SMT glue to fix the bottom of the device body by curing the SMT glue and the PCB to the corresponding position for bonding.

What’s the benefits of reflow soldering?

1.It works best for SMT assembly.
2.It merely causes a less thermal heat shock.
3.It is among the most secure soldering methods.
4.It results in reduced waste.
5.It can be used to solder particular PCB components.

What do you need for reflow soldering?

The following are the essential needed elements for a successful reflow soldering procedure:-
● An appropriate device.
● Appropriate reflow profile circumstances.
● Footprint design, for a PCB element.
● A printed PCB with excellent detail and design.
● Options for surface-mount component placement that is repeatable.
● Make sure your PCB is of excellent caliber.
● Selection of the best solder.

How do you reflow solder on PCB?

How-do-you-reflow-solder-on-PCBFollowing are the multiple steps of the procedure:


Reflow soldering preparation involves several significant stages that must be completed.

Solder paste

Apply solder paste to the board that has to be soldered. Only put solder paste where it is necessary to solder.

Stage of reflow soldering

Due to temperature variations, the reflow soldering stage is divided into multiple distinct phases. The solder junctions will be securely fastened if the reflow tunnel’s temperature is suitably managed. Following are the four phases that are employed:
● Preheat:
Bring the board’s temperature to the correct level. Thermal stress may weaken the board if you apply too much heat.
● Heating Soak:
The board temperature enters the thermal soak area at this step. Here, the temperature is kept constant to provide uniform heating throughout.
● Reflow:
Reflow occurs when you reach the maximum soldering temperature, which for a Pb-Free (Sn/Ag) solder is between 240 and 250 degrees Celsius. After melting, the solder forms solder joints.
● Cooling:
Cooling takes place without damaging the board or the components or overtaxing them. The usual range for these temperatures should be between 30-100°C.

How do you reflow a solder joint?

First, add PCB flux to the bridge and a little portion of the solder wick to reflow a solder junction. Next, apply the soldering iron to both the flux-infused portion of the wick and the undesirable solder. The solder will start to be absorbed by the wick.

How many reflow soldering defects are there in SMT?

The following list includes common SMT reflow soldering flaws and remediation techniques:

● Bridging:
Solder that flows between connectors creates a “bridge,” or short, which is the cause of the problem.
● Solder balls:
The flux on the wave’s surface gases and spits out, creating solder balls, or the solder itself bounces back from the wave, creating solder balls.
● Mid-chip balling:
When some solder paste separates from the bulk deposits on the pads and becomes lodged underneath the component, it results in Mid-Chip Solder Balls.
● Poor wetting:
When the components’, pins’, or PCB pads’ solder ends are not wetted or only partially soldered.
● Voids:
The voids are typically caused by the reflow-induced release of trapped flux from the sandwiched solder.
● Tomb-stoning:
Different wetting speeds that lead to uneven torque are the cause of the tombstone problem.
● De-wetting:
When the flux is not given enough time to clean the surface or when early evaporation of flux occurs and allows the surfaces to re-oxidize before reflow, this is what causes De-wetting.

Considerations for Reflow soldering during PCBA processing

Following are some of the design considerations that should be considered for reflow soldering:
● Thermal properties of the material
● Component and footprint alignment issues
● Board warping

How many times can you reflow a PCB?

With reflow soldering, most assemblies can be soldered in a single pass. If components are applied to both sides and you don’t want to “glue” the pieces just on the bottom side, some require two.

Additionally, this sets aside a few cycles that could be performed locally on the board if it needs to be modified.Every time the assembly is heated, all the parts are under stress which can lead to the failure of the complete assembly.

What temperature does solder reflow?

What temperature does solder reflowPb-Free (Sn/Ag) solder typically reflows at temperatures between 240 and 250°C for 40 to 80 seconds above 220°C. It should be noted that modest variations in the temperature of equipment and components normally do not cause soldering issues and that the recommended Sn/Pb reflow temperature range is less critical.

Reflow soldering warming zone temperature setting: the warming rate should be set at 2 to 4°C/sec. The warming rate in the preheat zone is too fast, which tends to deteriorate the flow and composition of the solder paste and is likely to produce burst beads and tin bead phenomenon.

Reflow soldering preheat zone temperature setting: temperature at 130 to 190 ℃, time to 80 to 120 seconds suitable, if the temperature is too low, there will be solder after reflow soldering is not molten situation occurs.

Reflow soldering back to the soldering area temperature setting: back to the soldering area temperature is the peak temperature of reflow soldering, the peak temperature is set at 240 to 260 ℃, melting time is recommended to 240 ℃ above the time to adjust to 30 to 40 seconds.

Reflow soldering cooling zone temperature setting: Reflow soldering cooling zone rate should be at 4°C/sec.

What are the different types of reflow ovens?

What-are-the-different-types-of-reflow-ovensThere are many different types of reflow soldering ovens based on their functioning technology. Some of them are mentioned below:

Based on the working technology

● Convection reflow oven
● Nitrogen reflow oven
● Infra-red reflow oven

Based on the shape

● Bench type reflow oven
● Vertical flow soldering oven

How many zones are in reflow?

Four temperature zones are typically included in a reflow oven. Varied temperature zones have different reflow effects and temperature curves.
1.Preheating zone of the reflow oven functionality.
2.The reflow oven’s heat absorption zone function.
3.The reflow oven’s soldering area.
4.Reflow oven’s cooling area.


There is a perfect reflow solder profile for every assembly. Reflow soldering can take some time to set up, but it is necessary to make sure that every component is thoroughly soldered without being damaged. Because the allowable temperature range for a lead-free assembly is lower than that of a tin-lead assembly, it is even more crucial when profiling a lead-free assembly.

The extra time and effort required to create a meticulously designed profile will pay off in the form of a repeatable procedure that consistently produces the desired results. More information about PCBA and PCB knowledge, please browse our website-IBE.



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