Reflow soldering

- information, tutorial, article about the basics of infra-red reflow soldering and how it can be used in for PCB assembly in mass production and for prototype PCB assembly.

Reflow soldering is the most widely used method of soldering used within PCB assembly whether it is used for mass production or for prototype PCB assembly with surface mount components. The technology uses two main stages. First a solder paste is applied to the board, and then secondly the board is heated to enable the solder to melt. This stage in itself has several steps that are needed to ensure that the board is heated and cooled correctly.

Using reflow soldering technology it is possible to reliably solder surface mount components, and particularly those with very fine pitch leads. This makes it ideal for use with the components used in mass produced electronics products.

Preparation

The first stage in reflow soldering for PCB assembly is to apply solder paste and components to the board. These stages are covered in more detail in a separate page on this section of the website.

• Solder paste:   In essence solder paste is applied to the board. The paste is only applied to the areas that require soldering. While boards have solder resist layers added to them, it is necessary to only add solder paste to those areas where the solder is actually required. This is achieved by having a solder mask and solder paste "machine". This only allows the solder paste to be added to those areas of the board where it is needed. Once added the solder paste has been added to the board, it can move on to the next stage.

• Pick and place;   With the solder paste on the board the components can then be set in place. Normally an automatic pick and place machine is used because the numbers of components used these days and the accuracy required make manual placement non-viable.

  The pick and place machine places the components onto the board, and they are held in place by the surface tension of the solder paste. This is quite sufficient for normal handling, although some care is obviously needed. It is possible for the components to be glued to the board, but this makes rework very difficult. Then with all the components in place they can be move to the reflow soldering machine.

Reflow soldering

The reflow process itself consists of a number of individual processes. These are required to ensure that the board is brought up to the correct temperature for reflow soldering without applying any unacceptable levels of thermal shock. Correctly profiling the temperature also ensures that the resulting solder joints are of the highest quality. The four stages normally used are as follows:

• Preheat

• Thermal soak

• Reflow

• Cooling

Pre-heat

The boards need to be brought steadily up to the required temperature. If the rate is too high, then the board or the components may be damaged by the thermal stress. Additionally if the board is brought up to temperature too quickly then areas may not reach the required temperature because of the thermal mass. If the board is brought u to temperature too slowly then the board may not reach the required temperature.

The temperature rise rate that is often used for infra-red reflow soldering is between 2 and 3 C per second, although rise rates down to 1C per second may be used on some occasions.

Thermal soak

Having brought the board up to temperature it next enters what is often termed a thermal soak area. Here the card is maintained at temperature for two reasons. One is to ensure that any areas that are not adequately heated because of shadowing effects come up to the required temperature. The other is to remove the solder paste solvents or volatiles and to activate the flux.

Reflow

The reflow area is the area of the soldering process where the highest temperature is reached. It is here that the solder is caused to melt and create the required solder joints. The actual reflow process involves the flux reducing the surface tension at the junction of the metals to accomplish metallurgical bonding, allowing the individual solder powder spheres to combine and melt.

Very careful control of the temperature and time is required to ensure that the process provides optimal quality.

Summary

Reflow soldering is the most widely used for of soldering for PCB assembly both in mass production and for prototype PCB assembly. It enables SMT components to be reliably soldered to the printed circuit boards, and enables the very fine pitch leads that are in widespread use to be handled with comparative ease, where other methods of soldering would not be feasible.