Vapor Phase Soldering

Vapor phase or condensation heating is a direct contact process that takes place in a zone of heated, saturated vapor. Heat is transferred when the hot, saturated vapor condenses on a surface and gives up its latent heat of vaporization. This heating mode has been adapted to mass reflow soldering, paint and polymer coating and polymer fusing, where it provides precise temperature control in a clean, non-oxidizing and non-flammable environment. High reliability soldering minimizes the high cost and delay of defective board rework.

Vapor phase soldering offers a higher rate of heat transfer than other reflow heating modes, conducting energy uniformly without dependence on circuit assembly mass, geometry, color or composition. Component temperature never exceeds the vapor temperature in a vapor phase system—a significant benefit for complex board architectures and low-mass, heat - sensitive devices, including surface mount technology. And it allows lower temperature processing than infrared or hot air processing.

As mentioned above, in this method, a special inert liquid is heated by a heater and the product to be soldered is immersed in the saturated vapor atmosphere acquired by the boiling of that liquid, and the vapor that contacts the product releases its latent heat of vaporation as it condenses. This results in highly efficient and even soldering of the product.

Figure 1 shows the structure of the equipment used in this method. This equipment consists of the first vapor phase used for the batch reflow soldering, a preheater, cooling, and a second vapor phase to prevent splashing of the liquid from the first vapor phase.

Figure 1: Vapor phase soldering method

Vapor phase soldering has the following characteristics.

1. Advantages

• The efficiency with which heat is transmitted to the work is extremely high and the whole work is heated evenly regardless of the shapes of the components.
• Since the latent heat of vaporation is used, the temperature can be controlled precisely.
• Since soldering is performed in an inert atmosphere, there is minimal oxidation or contamination of the soldered sections.
• As a result of the above features, the heating conditions can be kept low and the processing times can be short. As a result, the thermal stress applied to the packages is minimal.

2. Disadvantages
High running costs.

Types of production soldering equipment

There are three types of commonly used production soldering stations: wave solder, convection and vapor phase reflow (VPR). The convection types are further subdivided into three classifications: radiant dominant (Class 1 IR), a combination of convection and IR dominant (Class II IR), and convection dominant (Class III). These system classifications have been established by the Surface Mount Equipment Manufacturers Association (SMEMA).

For all their differences the three systems basically do the same thing: the board and components to be soldered enter the equipment, they are heated up to a temperature above the liquidous point of the solder, it will wet and flow, then exit the equipment and cool down. Differences between the various types of soldering equipment can be found in the method used to heat the boards, when (and how) the solder is applied to the board, and the rate the temperature is changing during the time the boards are in the soldering equipment.

This application note deals predominantly with convection solder equipment, although much of the information presented applies to wave solder and VPR systems also.