Surface munt packages are heated up above solder melt ing temperature during reflow soldering process. If t h e p l a s t i c e n c a p s u l a n t h a s absorbed m o i s t u r e , package cracking may occur. In t h i s study, an evaluat i o n method of Lhe package cracking is developed by means of moisture diffusion analysis of p l a s t i c and deformation and s t r e s s analysis of packages.

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The strength of plastic encapsuiaiIts is analyzed from the viewpoint of crack propagation. With a preexisting crack length a and a specific number of applied load cycles N, fatigue crack propagation rates da/dN of the encapsulants were measured with laboratory test specimens. It was found that the da/dN of encapsulants can tie expressed as functions of the stress intensity factor range 11K. The crack propagation behavior in the package was estimated from the data of da/dN and 11K at the lowest temperature of the test cycles. Reasonabie correlation is found between the estimated crack propagation behavior and the observed one; The applicability of fracture mechanics to the package cracking problem is demonstrated.

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