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.

How to Reflow Soldering

Let’s get into the subject of reflow soldering, in a nutshell, this consists of applying a layer of solder paste on the pads of the printed circuit board, then place the SMD components in the correct position, the printed circuit board will be taken to an oven which will heat the entire assembly allowing the solder to melt and then solidify fixing all components electrically and mechanically to the printed circuit board.

Now let’s look at the process in more detail so that you can understand how this soldering method works.

In SMT assembly, surface mount assembly requires that the printed circuit boards have pads on which the electronic components will be soldered. These pads are uncovered from the StopMask layer because solder will be applied to these pads. 

The solder paste used in the reflow soldering method consists of small spheres of solder metal (or the alloys used in it silver, tin, lead etc) combined with solvent pastes and agents that help the adhesion of the components such as flux keeping the small solder spheres in suspension. Solder paste can have a texture similar to toothpaste with the characteristic color of solder used in electronics.

To apply this solder paste on these pads a sheet known as a “stencil” is used which will serve as a template in the application of the solder paste, this stencil is usually constructed in a thin sheet of metal with perforations that match the pads of the printed circuit board. This perforation pattern is obtained from the StopMask layer of the Gerber files generated during the PCB design stage.

The PCB will then have a small amount of solder paste on each of the pads where a surface mount or SMT component will be located.

The next stage is to place the electronic components on top of the PCB in the exact correct location so that each terminal of each component matches the corresponding pad on the PCB. This task is carried out by the pick and place machine which is an automated machine that will accurately place each component in the right place. 

This process can also be carried out manually, when the project is small or has very few SMT surface mount components, using tweezers each component can be placed on top of the corresponding pads, as the solder paste is a creamy texture it will allow to hold the components slightly because of a physical phenomenon known as surface tension.

The printed circuit board is taken to a reflow machine where the soldering process as such will take place.

The heating process within this oven is separated into several stages and the way in which the heat is applied and its control of temperature increase over time is a critical factor in order for the components to adhere properly, to have a quality finish soldering  and to avoid damage to the internal circuitry of the PCB components.

This heating process requires strict control so that the required temperatures can be reached for the solder to achieve a strong and consistent connection to the printed circuit board, but that these temperatures also do not reach dangerous levels for the semiconductor material of the components as they could damage them.

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 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 PCBA assembly especially on critical parts for aerospace and military applications.

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