Figure 1. A vertical oven is well suited for lean manufacturing situations because a single operator can load and unload the oven.

Many different processes require the use of an oven in the manufacture of electronic components and assemblies. These include curing sealants within the assemblies; potting; stress relieving the housing; curing an adhesive; and burn-in testing. Three key factors -- work space size, loading method and operating temperature -- should be considered when selecting an oven for processing electrical components and assemblies.

Some manufacturers test a small sample of a production lot while others test each individual component. Regardless of whether you choose to sample test or test each device, the size and number of parts to be processed in the batch must be determined to establish the necessary workspace size. A number of loading methods are possible and should be considered in view of production floor layout and available personnel. Parts can be loaded directly onto shelves in a shelf oven or placed on material-handling trucks that are loaded into a truck oven. These trucks can be used to convey the material through other processing areas in the plant.

Vertical conveyor ovens also are being used for electronic component and assembly heat processing for several reasons:

• They can accommodate large production rates.

• They take up little floor space when compared to conventional horizontal belt-conveyor ovens.

• They require only one operator because parts are loaded and unloaded from a single position in front of the oven.

Its layout makes a vertical conveyor oven well suited for use in cells developed for lean manufacturing. The oven can be placed in close proximity to other processes and tended to by a single operator (figure 1).

Enlarge this picture Figure 2. In ovens employing horizontal airflow (left), heated air is blown sideways between shelves and across parts. In ovens employing vertical airflow (right), heated air is pulled up through the load. Vertical airflow should not be used in ovens that use solid trays or shelves to hold the parts as they will impede airflow.

An industrial oven with a micro-processor-based temperature controller and well-planned air circulation will provide the uniform temperatures required for processing electrical components and assemblies. Uniformity throughout the oven workspace should be +/-4 to 6^oF (+/-2 to 3^oC) when measured 6" off the oven walls, ceiling and floor. Choose an oven with control accuracy of at least +/-0.3 percent of the temperature controller's span.

Either horizontal or vertical airflow can be used for processing electronic components and assemblies. Horizontal airflow commonly is used where work is loaded densely on shelves and the least restriction to free air passage is in the horizontal direction (figure 2). Vertical airflow is used where work is hanging or is sitting on the oven floor and the least restriction to a free air passage is in the vertical direction. Blocking the airflow within the workspace will adversely affect the distribution of heated air and, therefore, the temperature uniformity.

Overtemperature protection is extremely important when choosing an oven. The value of the electronic assemblies loaded into one batch can be more than $100,000. A properly equipped oven should include a high limit temperature controller to shut down the heat if the oven air temperature exceeds setpoint by more than 15^oF (8^oC). The overtemperature control system also should include backup heating element control contactors and a recirculating blower airflow safety switch. For applications with extremely critical product loads, two overtemperature devices may be desirable.

Burn-in testing, one process that requires an industrial oven, is used to detect potential defects in electronic components and assemblies. By applying power in a heated environment, components are subjected to thermal stress. This will cause the component to fail at the interface of different materials within the component due to the differences in the rates of thermal expansion. In essence, thermal stressing is used to detect poor connections. Individual electronic components and, in the case of entire assemblies, the entire housing are checked at elevated temperatures (thermally stressed) for possible failure.

Figure 3. Class 100 ovens are well suited for applications where eliminating particle contamination is required.

Most powered burn-in testing is conducted at relatively low temperatures of 120 to 200^oF (49 to 93^oC). An assembly's packaging material sometimes limits the temperature at which the assembly can be tested.

Due to the low operating temperatures, electrically heated ovens used for burn-in are designed with less heat input than conventional industrial ovens of the same size. When power is applied to electronic components, heat normally is generated. The amount of heat generated by the load can be so large that it must be removed by a powered forced exhauster to maintain the oven temperature. When an exhauster is necessary, an exhauster airflow safety switch also should be installed. This switch will shut off power to the heating elements if the exhauster fails.

Many electronic components and assemblies are manufactured and tested in controlled, clean room environments. This is especially true of the semiconductor industry. Clean rooms can range from simply a cleaner-than-average room all the way to a Class 1000, Class 100 or Class 10 clean room.

If used in a clean room environment, the oven itself has to be clean (figure 3). That is, it must provide the desired low particle count atmosphere inside the workspace and have an exterior construction that minimizes particle generation in the clean room. Class 100 clean room ovens are available for this purpose in bench-, cabinet- and truck-style configurations and should include:

 

• A recirculation HEPA filter that is 99.97 percent efficient on 0.3 micron particles.

• Type 304 stainless steel interior.

• Continuously back-welded interior seams.

• Interior ductwork that is easily removable for cleaning.

• Silicone rubber door seal.

Ovens come in many sizes, designs and temperatures. No matter what the application for processing electronics components and assemblies, the right oven will provide many years of quality service.