Everything About the SMT Assembly Reflow Soldering Process

Soldering is a method generally of thermal nature (this means that it uses heat at temperatures much higher than those of the environment), by means of which two or more components can be joined mechanically and in the case of areas related to electronics the electrical connection of electronic components. Then this welding must provide a union of electrical conduction, which is a union of electromechanical characteristics that will allow the components are fixed together to a chassis structure or a printed circuit board and at the same time can be electrically connected to the terminals of this or pads as appropriate, in a reflow oven the union is usually terminals or wires and in the case of printed circuit boards are pads.

There are numerous soldering methods (wave soldering, reflow soldering, manual soldering and more), some used in the industrial field, others in the amateur and hobbyist environment and some techniques are used in both fields. In this article, we will refer to reflow soldering, a type of soldering which is mostly used in the industrial field, although nowadays there are some small reflow soldering devices that can be purchased by the amateur and used in the small workshop of the electronics hobbyist.

Below is the detailed video of PCBA in the reflow oven!

How to Reflow Soldering

Reflow soldering system is a process that involves applying a layer of solder paste to the pads of a printed circuit board (PCB) and then placing surface mount devices (SMD) in their designated positions. The PCB is then subjected to controlled heating in an oven, which causes the solder to melt and solidify, creating both electrical and mechanical connections between the components and the PCB.

Let’s delve into the process in more detail to gain a better understanding of how reflow soldering works.

SMT assembly in PCBA manufacturing requires the presence of exposed pads where the electronic components will be soldered. These pads are left uncovered by the StopMask layer to allow the application of solder.

The solder paste used in reflow soldering is a mixture of solder metal (typically alloys like silver, tin, lead, etc.) in the form of small spheres. It is combined with solvent pastes and flux agents, which aid in component adhesion and keep the solder spheres suspended in the paste. The solder paste typically resembles toothpaste in texture and has the characteristic color of electronic solder.

To apply the solder paste onto the pads, a PCB stencil is used as a template. The stencil, typically made of thin metal, features perforations that align with the PCB’s pads. The pattern of these perforations is derived from the StopMask layer of the Gerber files generated during the PCB design stage.

Each pad on the PCB is then coated with a small amount of solder paste, where each SMD component will be placed.

The next step involves accurately placing the electronic components onto the PCB, ensuring that each terminal of the component aligns with the corresponding pad on the PCB. This task is typically carried out using a pick and place machine—an automated device that precisely positions each component.

For smaller PCBA Prototyping projects or ones with a limited number of SMD components, this process can also be done manually. In such cases, tweezers are used to place each component onto the corresponding pads. The creamy texture of the solder paste facilitates component retention due to surface tension.

Once the components are in place, the PCB is transferred to a reflow machine where the soldering process takes place.

The heating process in the reflow oven consists of multiple stages. The application of heat and precise temperature control over time are critical factors for ensuring proper component adhesion, achieving high-quality solder joints, and preventing damage to the internal circuitry of the components.

This heating process requires meticulous temperature control to reach the necessary temperatures for strong and consistent solder connections while avoiding excessive heat that could potentially harm the semiconductor materials of the components.

As a PCBA company, FS PCBA understand the importance of employing precise reflow soldering techniques to ensure reliable and high-quality electronic assemblies. Our experienced team utilizes state-of-the-art equipment and follows industry best practices to deliver outstanding results for your projects.

Four Processes of Reflow Soldering

Ramp to Soak

The first stage of heating for reflow soldering is called “Ramp to soak” and consists of gradually increasing the temperature of the PCBA board so that the temperature rises safely, it allows all components and the entire assembly to reach a safe, homogeneous and constant temperature, also during this stage the volatile solvents of the solder paste are dissolved avoiding that there are remnants inside this, the mission of these liquids that suspend the solder; that is to say, they make it pasty, is to facilitate the union of this one with the components, nevertheless if residues are left inside the solder with the time these can affect the quality of the electronic adhesion for that reason it is important that they evaporate during the soldering process and it is started during the stage of preheating zone.

Thermal Soak Zone

The next stage is called thermal soak zone and consists of exposing the assembly to a constant temperature and for a duration of 1 to 2 minutes, allowing to complete the elimination of volatiles from the solder paste by evaporation, will activate the fluxes such as PCB flux, the components will begin to adhere to the pads of the printed circuit board and allow the adhesion free of oxide to be performed by being involved in an environment of evaporated fluxes. Too high temperature in this process can cause the solder to spatter or form lumps as well as oxidize the solder paste affecting the bonding and solder termination. The fluxes may not be fully activated if the temperature is too low.

In the soak zone, thermal equilibrium is desired throughout the assembly; the solder temperature must be homogeneous before the assembly can be taken to the next reflow zone. The soak zone profile can decrease any temperature differences between different components or between different areas of the PCB, especially if the PCBA board is very large, also the soak profile is recommended to decrease soldering problems on components with arrays such as components with BGA terminals.

Reflow Zone

The third zone is the reflow zone, also known as the “time above reflow” or TAL temperature above the liquid level. At this temperature the maximum temperature is reached, it is important that the maximum temperature is kept under control for the process to be successful. It is typically between 20 and 40 degrees above the solder melting temperature (which is the temperature at which the solder enters its liquid state).

As semiconductor components (transistors, integrated circuits, mosfets) and discrete electronic components (resistors, capacitors, coils, inductors) have different solder tolerance temperatures; that maximum temperature reached in the reflow zone must be below the component most sensitive to thermal damage. A good rule of thumb is to subtract 5 degrees from the maximum temperature that the most fragile component can withstand; or to the lowest temperature specified in the data sheet as the maximum temperature during the reflow process, it is important to closely monitor the whole procedure and never exceed this limit. High temperatures beyond 260℃ can damage the internal elements of surface mount components and generate the growth of intermetallic oxides that affect semiconductor performance, on the other hand if the temperature is not high enough it can prevent the paste from melting smoothly.

During this zone, the temperature is above the liquid point, the flux reduces the surface tension and causes the solder components to separate allowing the metal powder spheres to join, completely evaporating the rest of the flux material.

The temperature and time should not exceed the solder manufacturer’s specifications in any case, if it is too high it can prematurely consume all the flux generating a dry solder that will give a bad formation, appearing broken solder joints inside the soldering and drying of the solder, bubbles inside and rust.

The miscalculated temperature/time relationship can lead to a reduction in the efficiency of the flux whose job is to clean the solder of impurities and will result in poor wetting, inadequate solvent and flux removal from the solder leads to defective solder joints.

It is recommended that the reflow zone period be as short as possible however most solder pastes specify that this period should be at least 30 seconds although no reason for this particular time seems to be explained, one possibility is that in some places on the PCBA the temperature is not reached with the profile as programmed, then setting a time of 30 seconds reduces the chances that there are areas that fail to melt due to lack of temperature. Considering that obtaining a homogeneous temperature on the PCBA is quite difficult, a minimum reflow time will reduce the temperature changes inside the furnace.

The reflow temperature should not be sustained above 60 seconds as this time is excessive for many electronic components.

On the other hand, a too short reflow time or too low temperature can trap solvents and fluxes inside creating cold or unsound joints due to small voids (bubbles) that may appear inside the solder. Any slight mistake that occurs in the SMT reflow oven can lead to PCB assembly errors, causing common PCB problems.

Cooling Zone

The cooling zone allows to gradually reduce the temperature of the board during the soldering process to allow them to solidify properly, this reduces excessive inter-metallic formation by thermal shock of the components, i.e. by a too rapid change of temperature, This is a damage that is observed inside the components.

Temperatures in the cooling zone range from 30 to 100 degrees, the fast cooling rate generates a more mechanically sound fine-grained structure. An acceleration rate is often ignored or not strictly monitored, maybe that the ramp rate is less critical. The maximum allowable slope for any component should be applied whether the component is heating or cooling, a cooling rate of 4℃ per second is suggested. This parameter should be considered during PCB assembly process, especially on critical parts for aerospace and military applications.

Reflow Soldering FAQ

Both wave soldering and reflow soldering are crucial processes in PCB assembly, serving the purpose of establishing connections between PCB boards and components.

Reflow soldering is commonly employed in scenarios involving SMD due to their densely packed pins and high sensitivity to temperature variations.

On the other hand, wave soldering is typically utilized for DIP components, which possess larger pins. In this process, the circuit board is passed over a wave of molten solder, allowing the liquid solder to cover the entire pad area. Through the principle of surface tension, the molten solder forms solder joints with the component’s pins, ensuring a robust and dependable connection.

One of the key factors in selecting a partner for PCBA manufacturing is to pay attention to the production equipment employed. When it comes to reflow soldering equipment, the temperature control capability of each temperature zone plays a crucial role in ensuring superior soldering quality. Manufacturers, such as FS Technology, prioritize the use of advanced wave soldering equipment in temperature zone 10, which offers enhanced convenience and precision.

By investing in equipment with independent temperature control in each zone, manufacturers can achieve optimal thermal profiles and precisely regulate the temperature throughout the soldering process. This level of control contributes to improved soldering quality, enhancing the reliability and performance of the assembled PCB.