Flow Visualization in PCB Testing: Part 1

In our first post about flow visualization, we discuss the benefits of flow visualization, along with several primary methods for creating successful flow visualization.

Part 1: Best Techniques for Air Flow Visualization

PCBs support a multitude of components with varied geometries, electrical functions, power dissipation and ther­mal performance needs. For a PCB to work properly, a component’s thermal requirements must be met locally or at the system level. Regardless of the type of housing that surrounds a PCB, its cooling system must be designed so that diverse components are electrically functional and run at temperatures that help them reach their expected life spans.

Much effort is needed to meet a component’s thermal re­quirements, whether by enhanced fluid flow (liquid or air) or by adding a cooling solution, e.g., heat sink, onto the component. Except for a conduction cooled PCB, where a cold plate extracts heat from the board, electronics are typically in contact with some sort of cooling fluid. In many cases, the PCB is in direct contact with the coolant. This creates a very complex problem along with a unique opportunity.

The problem stems from the intricate topology of the PCB. Highly complex flows are observed on PCBs due to their three dimensional protrusions, i.e., components. A typi­cal PCB sees every imaginable flow structure. These include laminar, turbulent, separated flow, reversed flow, pulsating, locally transient and others. Flow visualization has the potential to yield more insight into a fluid flow or convection cooling problem than any other single method. Many misconceptions can usually be cleared up by flow visualization. However, it is important to use the technique most suited to a given problem.

Air Flow Visualization

Smoke entrainment is the most common visualization technique for laminar air flows. But it has somewhat limited use in turbulent flows due to its rapid diffusion by turbulent mixing. Smoke can be produced from many sources, but essentially it is made by either smoke-tube or smoke-wire.

In the smoke-tube method, vaporized oil is used to form a visible whitish cloud of small particles as the hot oil vapor condenses. Consideration must be given to the vortices shed by the smoke probe itself, since the most visible small-scale features often arise from the probes own wake. This effect is important for probe diameter-based Reynolds numbers exceeding about 15. Because it is impractical to reduce the probe diameter beyond a point and still get a reasonable amount of smoke flow, the smoke can be injected upstream of a convergence section to eliminate wake effects. A key disadvantage to the smoke-tube method is that the smoke is produced hot and rises due to its own buoyancy, thus it doesn’t follow the local flow faithfully. To reduce buoyance effects, the smoke can be cooled in a long length of tubing from the point of generation before its introduction into the flow.

In the smoke-wire method, smoke is generated as a sheet by coating a thin wire with oil, stretching it across the flow, and heating it with a pulse of current. Almost any wire and power supply can be used in this technique. The oil should be chosen carefully to have a broad boiling plateau, rather than a single temperature, in order to generate good smoke. Model train oil is suitable for this method.

The end result of board level flow visualization is PCBs that are thermally optimized and require no re-spin because of thermal constraints. If the board is thermally laid out, heat sinks and other cooling solutions are often not required.

Click  here for part 2 of this three part series
Click here for part 3 of this three part series

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