Andrew Skipor

Mechanical characterization of plastic ball grid array package flexure using moire interferometry

Most previous studies of PBGA packaging reliability focused on the effect of thermal cycling. However, as portable electronic products such as cellular phones and laptop computers are reduced in size and become more readily available, isothermal flexural fatigue also becomes an important reliability issue. Solder interconnects subjected to mechanically induced deformation may result in failure. In the current work, moire interferometry is used to investigate the influence of PCB flexure on interconnect strains. A versatile testing apparatus is developed to load PBGA packages in four point bending. Moire fringe patterns are recorded and analyzed at various bending loads to examine the variation in displacement and strain between the components. Solder balls across the entire array experience large shear strains, often resulting in plastic deformation, which reduces service life of the package.

Residual Stress Development during Relamination of Woven Composite Circuit Boards

The manufacture of multilayer circuit boards involves relamination of alternating B-stage woven glass/epoxy prepregs, which act as bonding sheets, C-stage cores and copper foils. The high relamination temperature and mismatch of thermo-mechanical properties between the copper and woven glass/epoxy composite substrate lead to development of residual stresses in the boards. In the current work, numerical and experimental studies are performed to study residual deformation and warpage in a model multilayer circuit board construction of a common composite substrate (7628 fabric style). A numerical procedure based on classical lamination theory with non-isothermal viscoelastic constitutive relations is developed to predict the deformation and residual stress state due to relamination. Experimental values of the substrate stress relaxation modulus and coefficients of thermal expansion (CTE) are used as inputs in the numerical procedure to predict warpage of model circuit boards with a non-symmetric lay-up of 7628 style composite substrate. Boards with exactly the same construction as used in the numerical analysis were fabricated according to the prescribed pressing cycle, and the time dependent warpage measured using an ultrasonic contour scan technique. Comparison of the experimental warpage data with numerical predictions provides insight into the effects of processing cycle and substrate properties on residual stress development.

Free-standing quantum dots for electronic applications

Free-Standing Quantum dots (FSQdot), also known as nano-crystals, are semiconductors composed of II-VI, III-V, or IV-VI materials. Ranging in size from 2-10 nanometers in diameter, these FSQdots have size-tunable band gaps. Unlike Qdots produced on semiconductor wafers, FSQdots can be manufactured using scalable colloidal solution chemistry. These FSQdots have high photo-luminescence quantum efficiencies, good thermal and photo-stability, narrow emission line widths (atom-like spectral emission), and are compatible with solution processing. Solution processable FSQdots can be modified using innovative surface functionalization schemes. Once appropriately functionalized, the FSQdots can be incorporated into printable inks and bound to surfaces. These capabilities unlock a realm of possibilities for producing novel commercial electronic products, from unique surface coatings to new display constructions.