In-situ stress and warpage measurements to investigate reliability of flip-chip on board assembly without underfill

Current printed wiring boards (PWBs) are all organic, the most common being epoxy-glass laminate FR-4 due to its cost effectiveness and overall performance. However, for high-density wiring (HDW) and assembly of flip-chips directly to the substrate without the use of underfill, substrate materials with low CTE and high elastic modulus are needed. Novel low CTE-high stiffness organic and inorganic boards have been evaluated for flip-chip on board technology without the use of underfill. Standard liquid-liquid thermal shock tests were carried out on test vehicles with different board materials and failure modes were characterized. In-situ warpage and stress measurements were made to analyze the observed failure modes and to set guidelines for optimal board material selection. The effect of interlayer dielectric thickness on the package reliability has also been studied. The reliability test results are in accordance with the inferences from the in-situ warpage and stress measurements and it can be concluded that along with low CTE, high modulus is an inevitable substrate property requirement for flip-chip reliability without underfill in next-generation packages. This paper also presents Photo-stimulated Luminescence Spectroscopy (PSLS) and Raman Spectroscopy as non-destructive and direct techniques for the in-situ and residual stress measurement in microsystems and thus a powerful means for reliability assessment. Experimental results have also been supported by finite element models and analytical solutions.