A fixture for characterizing underfill: this inexpensive and adaptable non-proprietary hardware aids visibility.

Underfill can improve the reliability of BGAs during exposure to thermal cycle or vibration. The development of the best material and processing is often hampered by visibility: you cannot see what you are doing. Flow fronts, voids, mixing, coverage and uniformity typically cannot be characterized without destroying expensive hardware. This fixture might help: a simple slab of aluminum, a slot to contain a little pane of tempered glass, ringed by De-Sta-Co or other adjustable clamps (Figures 1 and 2).

To use it: Set the BGA, adjust the clamp height and pressure just enough to hold the BGA in place, dispense the underfill material, cure (if necessary or if part of a study), unclamp the glass and view the underside. Clean and try another variant.

Studies could include characterization of flow behaviors: temperature vs. viscosity; perimeter-fill processing; flow-around a center heat slug or adhesive dots; damming, mixing and sequencing, etc; or they could involve processing details: dispense nozzles and pressures; access, interference and traverse patterns; studies of damage to columns; remove and repair-replace processes; displacement of the part or spacers due to side filling, etc.

Other studies could concentrate on materials properties and effects: location and extent of voids before and after cure; CTE deformation; z-axis geometry effect of cure frothing and voids; degassing effects; chemical compatibility; effect of simulated PWB surface roughness on flow, etc. Or they might cover other special objectives: component keep-outs and adjacent-part interference; investigation of special rework/removal issues; simulations of varying collapse geometries due to different levels of temperature/ time/weight, as that affects underfill flow, etc.

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These special objectives go beyond the usual demonstration of viscosity, gap, extent and fill location, and could also benefit from adaptations of this simple glass-slide fixturing approach. A possible side benefit: simple show-and-tell visuals based on glass specimens from this fixture could be used to illustrate development issues and status.

The design is not critical, and the value is in ease-of-use and the visualization capability. The fixture shown is for maximum adaptability. Alternate designs could be tailored for small-size (first-level flip-chip, inside-the-package technologies) or high-temperature clamps for cure studies; designs with adjustable side x, y or z features to study dispense-access or removal constraints; calibrated vertical stops and loading to study z-axis CTE effects. For simple tasks, the clamps need not be articulated: simple bucktooth cleats, with finger-tightened knurled- nuts similar to those shown Figure 2, could offer clearance and size options to suit the task.

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Figure 3 shows the results of a study on several types of BGAs (a large ceramic column grid array, PBGAs, a medium-sized CBGA and [U]BGAs) in an exploratory dispense test of a candidate underfill formulation. The closeup (Figure 4) of underfill on a PBGA shows clearly unacceptable results but illustrate the visualization and characterization capability. These typical bottom-side seen-through-the-glass images demonstrate the ready visibility of the extent and characteristics of the flow. Certainly, the development engineer's tasks, process controls and results would be much more precise, discriminating and relevant, embodying precise dimensional and photo documentation.

Figures 5 to 8 show more dramatic examples, for demonstration purposes only, of the underfill processing characteristics that can readily seen and documented by using the bottom-side see-through capabilities enabled by this fixture.

This fixture concept is not new, and certainly is not revolutionary, but is offered to possibly benefit industry development of underfill materials and processes.

Acknowledgments

Appreciation is extended to the Sunnyvale SMT manufacturing and materials & process engineers for their support and utilization of this project.

Tom Clifford is group lead, advanced electronics packaging, Lockheed Martin (lmco.com); [e-mail].