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.
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