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Good Thermometry Techniques Understanding the thermal impact of the oven on the board and the board on the oven is critical for a controlled and reproducible reflow process. Thermometry is the only practical method for validating these influences by means of oven profiling. Oven-and-board interactions are explored in this presoldering step and the oven is adjusted for proper time-temperature profile of the PWB with the parts to be soldered. Some of the most important process points determined through board and oven profiling are:

Heating-cooling ramps and ensuring that they are within the recommended bounds prescribed in the surface-mount device (SMD) component and solder paste specifications Peak process temperature and duration Dependency on the heater types, quantities, cooling methods on board, and component materials Conveyor speed Heat distribution along the oven s width Heating characteristics of the PWB and the SMD components upon it Rate at which the oven reacts to the board s presence

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Product Profile Board All of the preceding and many more variables have dramatic impact on the soldering process The only way to assess these is to gauge accurately the thermal energy distribution in the oven in the presence and absence of the board Most process engineers trust their favorite profiling board to determine the health of their reflow oven or assess the performance of a new oven prior to purchase The profiling board is simply a thermocouple-instrumented product board or test vehicle that is run through the oven for the purpose of determining the oven s effect on the board for given heater and conveyor speed set points The thermocouples trace the temperature profile of the oven as seen by the component lead or package as the board is conveyed through the oven for reflow.

However, reliance on a product board to determine oven performance may mask some inherent problems with the reflow oven If the board is not thermally balanced in terms of components or board innerlayers, it may not be able to detect a strong transverse (rail-to-rail) thermal differential It is for this reason that a balanced oven diagnostic profiling device should be made and run to assess oven performance independent of a product board That is not to say that the oven diagnostic board is a direct replacement for the product profiling boardThe latter is to verify that the critical sectors of the board or, better yet, device leads are heating per set profile Thermocouples for this board are deployed at the leading edge, trailing edge, center, and sides, with the thermocouple beads ideally embedded within the solder joints of disparate component types.

Conventional system 15875 grams 15 8 5 cm 35 W 35 g s US $ 20,000 Micromachined system 10 grams 2 2 cm 1 mW 100,000 g s US $ 500

Additional thermocouples can be placed on unusually massive components as well as on adjacent smaller components to determine whether a large component is thermally shadowing its diminutive neighbor The number of thermocouples deployed depends on the package types, known oven characteristics, board thickness, board layout complexity, etc The following sections describe good thermometry practices and the oven diagnostic board, as well as some of the thermocouple measuring innovations that are now a routine for large-scale mass reflow manufacturing operations 473101 Oven Diagnostic Board The oven diagnostic board can be a very simple or a rather complex device Its job is to probe the normal operating conditions of the oven and determine whether all the heaters and forced-convection fans are working properly.

It is useful in assessing not only the oven s time-temperature profile parallel to the tunnel s long axis, but also perpendicular to it at the rail edges The data recorded from this board are not necessarily be translatable to the establishment of a product time-temperature profile, but are more suited.

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FIGURE 47.9 Schematic plan view and detailed side view of an adjustable oven characterization fixture (oven diagnostic tool) composed of pairs of thermocouples, shielded from one another by a rigid, heat-resistant insulator. The thermocouple positions are adjustable in width and length for investigating temperature distribution along the tunnel width.

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