Desoldering by hot air

Preliminary trials of hot air de-soldering

In order to carry out desoldering tests a JBC AM 6500 Air Flow Station has been used (Figure 2) . This station serves for de-soldering and soldering manually all types of SMDs and PLCCs of any size. The process for desoldering SMT (surface mount technology) components consists of three stages: (1) fix the vacuum extractor on the component, (2) hot air flow reflows solder and (3) the SMT component is removed automatically. Preliminary, have been carried out laboratory de-soldering tests of SMDs (surface mount devices) from end-of-life Pb solder PCBs and also from Pb free solder PCBs. The objectives of these initial de-soldering tests are to study the influence of solder, component finish and board finish in desoldering time. The results are shown in Table 1.


Figure 2. Air flow station

According to these results some conclusions have been obtained:

• Type of solder: SAC (SnAgCu) takes a bit longer than SACBi2 (SnAgCuBi) for de-soldering.

• Component finish: Sn longer than SnPb

• Finish on board: Not a significant influence.

However, an important condition in de-soldering time is the size of the component. The larger it is, longer is necessary in order to heat all the pins.
In the same way, by means of hot air flow the re-melting of through hole solder points have been studied. These solder points are from lead and lead free PCBs. The results are shown in Table 2. According to this, the conclusions are as follows:

• Lead-free solders longer to melt. Slight differences in melting time among lead free solders.

• Melting time depends on solder point size.

• As air temperature increases melting times decrease and become more similar for all solders.

Advanced hot air de-soldering tests

The objective of the laboratory tests has been to determinate for surface mount components (lead-containing and lead-free soldering):

• Disassembling time considering: solder nature, component size, air temperature, flow and nozzle distance to component surface.

• Component temperature in disassembling moment.

• Definition of a proper procedure to recover electronic components

In these tests, the disassembling time and temperature of several surface mount components from lead-containing and lead-free PCBs have been determined following two different procedures (Figure 4):

• The hot air flow is directly focussed on the component centre. Connection pins and package are heated at the same time.

• The hot air flow is conducted to the connection pins using an experimental nozzle.
  The aim of this second procedure is to protect the package from high temperatures and to make de-soldering time shorter.

In both cases, the temperature on the component surface have been measured by means of a non-contact infrared thermometer. Different distances between air nozzle and components have been considered. The results are summarised in Table 3.


Figure 4. Hot air flow station and the experimental nozzle

Click here to open table 3: Advanced hot air de-soldering tests

According to the results some conclusions have been obtained:

• With same solder and under same de-soldering conditions, disassembling time could vary slightly for the same components.

• Lead-free solders take longer to re-melt than Sn/Pb containing ones.

• Board and component finish do not have an important effect in the de-soldering time.

• Hot air flow must be focussed on connections pins directly in order to reduce disassembling time and not to subject the component to very high temperature

• Hot air flow must be distributed homogeneously around all connection leads. All leads should be de-soldered simultaneously in order to avoid damages when the component is recovered (Figure 5).

Figure 5. Component must be homogeneously disassembled in order to avoid damages in connections


Next:  Desoldering by infrared heating