PCB | July 06, 2009

What's it all with vapor phase soldering?

Technical article: In the first process step, the cold PCB is delivered to a chamber in which there is vapor from a special liquid (medium) for this process – perfluoropolyether (PFPE).
The liquid is chemically inert and with a density almost twice as high as water. Due to the temperature difference between the PCB and the vapor, condensation immediately starts on both sides of the PCB. The condensed vapor forms a thin film covering the entire PCB.

The high surface tension of the liquid used allows to thoroughly cover every critical gap on the PCB - such as spaces underneath BGAs, closely placed or odd shaped components, etc. Due to the high density of the vapor, the liquid film on the PCB pushes away the Oxygen. Therefore the soldering process takes place in an Oxygen-free environment. As the film conducts heat quite well, the thermal energy is transmitted to the PCB homogeneously.

The liquid is heated by electric heaters. Once the boiling point has been reached, the liquid will not continue to heat up further. Any further energy being supplied will be used to produce more vapor (enthalpy of evaporation).

The amount of liquid condensing on the PCB depends on the amount of vapor in the chamber. If you supply more heat, the amount of vapor increases. By controlling the heat that is transferred from the heaters to the liquid in the chamber, the amount of condensed liquid on the PCB can be changed accordingly - thus allowing to change the heating of the PCB after it has entered the chamber.

The condensation of vapor stops when the PCB reaches the temperature of the vapor. Therefore the boiling point of the liquid defines the maximum temperature to which the PCB is exposed. For lead free soldering, the use of a liquid with a 230ºC boiling point is recommended; for Sn/Pb soldering it is 200ºC.

When the solder paste is completely reflowed, the PCB is removed from the chamber. After the PCB leaves the vapor zone, the remaining condensed fluid on the PCB evaporates, due to the residual heat of the board. In the last process step the soldered and dry PCB is cooled in a cooling zone.

Many tests show that the number of voids on the PCB after vapor phase soldering is smaller when compared to convection reflow soldering. To reduce the amount of voids even further, Germany-based Asscon developed a vacuum section which can be added to some vapor phase ovens. The picture below on the right shows the reduction of voids using such section on the same component under the same conditions, compared to the standard vapor phase soldering (the picture on the left).

The key advantages of vapor phase soldering:
- soldering in oxygen free environment (eliminates the need to use nitrogen)

- lower peak temperature making possible to reduce the cost of PCB by 10-15% by using the substrates with lower thermal resistance and to avoid overheating the components. Guaranteed control of the max. temperature in the oven due to the physical properties of the liquid used – typically 230ºC for Lead Free and 200ºC for Sn/Pb boards

- better heat transfer to the PCB due to the use of liquid for that and not air and nitrogen. This reflects in considerable reduction of electrical power consumption. The maximum power consumption of exampled smallest oven is 3,5 kW, while the average consumption during soldering is 1,5 kW. The maximum power consumption of exampled large inline oven is 12 kW, while the average consumption during soldering is 6 kW

- flexible and easy control of the PCB temperature until reaching the reflow values

- faster and easier profiling

- smaller footprint in comparison with convection ovens

- reduction of the voids in the solder joints. For further decreasing the amount of voids, a vacuum section can be added to the vapor phase systems

- most complex and demanding PCB’s can be soldered with ovens that cost less compared to the forced convection ovens

- the number of soldering defects is usually smaller

Author: Marek Petryk, Amtest Poland
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