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112 SMT Magazine • September 2015 by robert voigt DDM novaSTar smt QuICK tIps selecting a reflow Oven, part 2 Column In this follow-up to Part 1, which focused on selecting and evaluating a basic reflow oven configuration for a circuit board assembly en- vironment, we'll address heating technologies, methods of board transportation, and inerting systems. Heating technologies Regardless of the technology used for heat- ing boards, the primary objective is to control the thermal profile to prevent undesirable con- ditions, such as: • Tombstoning: a condition that causes the component to stand up due to micro-explosions in the solder • Delamination of the substrate (a partial separation of the layers of circuit board material, similar to blisters) • Poor adhesion/contact: caused by over-heating and under-heating Typically, there are three main methods used for reflow oven heating. They are: 1. Vapor phase 2. Infrared 3. Convection vapor phase Vapor phase is a simple and very reliable method of soldering. Complex components and assemblies can be done in a small process window, but this comes at a cost, so understand the process and what is involved before making a purchase decision. Vapor phase heating is performed in a batch process using a special liquid with a very par- ticular vaporization temperature of 240°F, en- suring that materials are never overheated and damaged. Parts are placed in a single chamber above a liquid bath, which is then heated to create a vapor at the perfect melting point of solder. The board is enveloped in the vapor to accomplish the soak and reflow over time, and the chamber temperature is reduced to con- dense the vapor back into the liquid bath. The parts are then removed and replaced with a new board (or set of boards). figure 1: vapor phase reflow oven showing board suspended above heat transfer fluid bed.