4.1 Vapor-Phase Reflow. IPC IPC-7530A

IPC-7530A Draft Document for Industry Consensus Only August 2016

Figure 4-2 Profile for VPS With Preheat Resembles Convection Profile

Vapor-phase reflow can be operated as a single- or two-fluid system, utilizing a primary and a secondary fluid. The process was developed using the two-fluid approach in batch equipment, but modern in-line systems are normally operated with only one fluid. Whichever system is used, the maximum temperature reached by assemblies in VPS reflow depends on the choice of the primary fluid. Primary fluids are available in a number of temperature ranges, with 218 °C to 222 °C being common with SnPb solder and 235 °C to 245 ºC being common with lead-free solder.

While all the primary fluids can be classified as perfluorocarbons, the basic structure (amine, cyclic or ether) will determine the key properties of in-use stability, solder paste chemical solubility and overall process economics.

The choice of a fluid is normally based on the melting point of the solder alloy to be reflowed.

For the range cited, lower temperatures are suitable for the typical SnPb or SnPbAg alloys used for standard attachment processes. The upper end of the range will permit reflow of high-Pb alloys, which are used to attach pins to PGA packages. Users faced with specialty alloy reflows have been successful in mixing two primary fluids to tailor a VPS for a specific stable boiling point. They have found that higher temperatures will permit shorter times, which may be advantageous with some solder pastes.

VPS heats uniformly, so no part on the board (irrespective of its geometry) exceeds the fluid-boiling temperature.

This is suitable for soldering odd-shaped parts, flexible circuits, pins and connectors, as well as for reflowing SnPb and Pb-free surface-mount package leads.

It is important to note that the soak zone for convection is almost impossible to duplicate in VPS, and convection pastes with soak zone for activation may not work in VPS. Users typically address this issues by adding secondary cooling coils to VPS. This controls the rise in temperature by increasing the water circulation when the PCB enters the VPS chamber. Pastes for ramp-to-peak are also common for both of these systems.

The use of nitrogen in convection systems for soldering in an inert environment has increased due to the widespread use of low-solids and no-clean fluxes and solder pastes.

It is difficult to provide uniform heating in larger ceramic and plastic BGAs–even with convection-dominant systems–VPS is being considered because of its efficient and uniform heating characteristics. In the case of backward compatibility, you cannot go to a higher reflow temperature for just a few Pb-free components, as the majority of SnPb components are likely to be adversely affected. VPS is being considered as a solution to provide a middle-of-the-road compromise reflow profile to produce acceptable solder joints in both SnPb and Pb-free components.

Because VPS provides an inert soldering environment without the use of nitrogen, users are giving it another look.

They have found VPS to be a suitable option for Pb-free soldering, low-volume niche applications or for boards with numerous large ceramic BGAs with practically no leaded or chip components. Although VPS has very efficient and uniform heating characteristics, it is unlikely it will reclaim its status from years ago.

4.1 Vapor-Phase Reflow

Vapor-phase reflow can be operated as a single- or two-fluid system, utilizing a primary and a secondary fluid. The process was developed using the two-fluid approach in batch equipment; but modern in-line systems normally operate with only one fluid. Whichever system is used, the maximum temperature reached by assemblies in vaporphase reflow depends on the choice of the primary fluid. Primary fluids are available in a number of temperature ranges, with 218 °C to 222 °C being common with tin-lead products and 235 °C to 245 °C for lead-free products.

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