Selecting a Reflow Oven, Part 2

Estimated reading time: 2 minutes

In this follow-up to Part 1, which focused on selecting and evaluating a basic reflow oven configuration for a circuit board assembly environment, we’ll address heating technologies, methods of board transportation, and inerting systems.

Heating Technologies

Regardless of the technology used for heating boards, the primary objective is to control the thermal profile to prevent undesirable conditions, 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 particular vaporization temperature of 240°F, ensuring 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 condense the vapor back into the liquid bath. The parts are then removed and replaced with a new board (or set of boards).

Infrared (IR)

IR systems were widely used throughout the 1990s because they were relatively inexpensive to buy and operate. IR heating can be used in either batch or continuous conveyorized units. While still available for specialized applications, IR heating is affected by the color of the target components, so different colored components absorb heat at different rates and can produce different heating profiles, not entirely desirable.

Today, IR heating is often used for special applications where heat ramp rates need to go quickly with very little soak time. The direct heating technique of IR makes modulation and control very difficult. Factors that can be used to vary the heating profile include distance, speed and heat intensity. For these reasons, IR is not often used in low- to medium-volume production environments which need to accommodate a wide range of board configurations and heat profiles.

Convection

Convection heating is the most widely used method for reflowing circuit boards today. In this system, air is circulated throughout one or more chambers (or zones), either vertically or horizontally, to surround the entire board assembly with even, uniform heat. Each zone in a multi-zone oven retains its own heat profile very reliably. Both the vertical and horizontal methods of heat distribution share the same end result—even heat around the boards.

A plenum heat distribution system typically directs air vertically from above and below toward the board surfaces. They often require preheating plenums to condition the air temperature before heating the board, but directing airflow from two different directions (top and bottom) allows the user to adjust heat settings independently and with good control to create a very precise profile. This type of system is usually more complicated to build and thus more expensive than a horizontal convection type.

Read The Full Article Here

Editor's Note: This article originally appeared in the September 2015 issue of SMT Magazine.