Electronics Assembly Knowledge, Vision & Wisdom
Underfill Limitations for Future Packages
Underfill Limitations for Future Packages
The paper addresses the issue of underfill as die thickness diminishes, the interconnection bumps get smaller and their number increases.
Materials Tech

Authored By:
Horatio Quinones and Tom Ratledge
Nordson ASYMTEK
Carlsbad, CA, USA
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Summary
The capillary underfill (CUF) although a well established manufacturing assembly process, is being challenged as die thickness diminishes, the interconnection (bumps) get smaller and their number increases. Denser populated packages demand very tight tolerances for keep out zones (KOZ); the total package thickness challenges the process throughput since die contamination from underfill fluid is not allowed and multiple fluid dispense passes may be needed.

All this challenges translate in lower capillary surface energies, increase in fluid flow drag, smaller particle size fluid that often results in increase in viscosity and therefore slow flow-out-times. The present work addresses these issues. A series of mathematical models based on surface energy evolution for CUF accounting for these new geometries and processes is proposed. In particular the problem of component proximity is and the gap topology issues are studied. Experimental data for CUF in the presence of these future assembly demands is shown. Although there are practical physical limitations for the CUF as experienced today if one were to implement it for future packages, new hybrid CUF methods that overcome such shortcoming are recommended
Conclusions
A comprehensive analysis of capillary fluid flow has been presented and validated by actual data. The coupling of molecular forces adhesion and cohesive in nature and the minimization of surfaces, as in the case of elastic membranes that yield Willmore critical surfaces in the differential geometry scheme gives an adequate way to model capillary fluid flow. These solutions were obtain in a way by perturbation theory where Variational of the functional derived from Poisson integral formulation mimic very closely the observation of capillary fluid flow under various boundary conditions that included several different geometries.

For dispensing in presence of tight spacing and high density packages, the Jetting technology lends itself in a very transparent and practical manner. Above results can be used as guidelines in situations where uneven surfaces and arrays are present and capillary flow is used. Design rules for packaging design need to include the capillary physics present during underfilling and fluid dispensing in general.
Initially Published in the SMTA Proceedings
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