Liquid encapsulant stress variations as measured with the ATCO4 assembly test chip

We have examined the use of the ATCO4 piezoresistive Assembly Test Chip to measure the mechanical surface stresses produced by liquid encapsulation of die mounted directly on ceramic substrates. In our experiment, two groups of parts, each with about 70 samples, were encapsulated with two different materials, one a standard and one a low stress formulation. Results are presented for the measured stress components; σ_xx-σ_zz, σ_xy , and σ_xx-σ_yy. We observe an ≈20% reduction in the compressive stress between the normal stress and low stress materials, and a small but measurable difference in the average in-plane shearing stress. This experiment demonstrates the ability to resolve stress differences produced by different encapsulants and provides guidelines for selecting appropriate sample sizes