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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Long life corrosion fatigue behaviour is discussed. The importance of (i) quantification of corrosion pit depth (or surface roughness caused by corrosion) as a function of service period and (ii) corrosion pit modelling is emphasized. Extreme value statistics is useful for corrosion pit depth quantification. It is demonstrated that a corrosion pit can be treated as an elliptical crack with the same depth and surface length as a pit. From these considerations, a method for determination of a reasonable allowable stress in corrosion fatigue is proposed. Finally, an example of an allowable stress is given, i.e. the allowable stresses of a real machine component under rotating bending stress are determined from estimation curves of pit depth together with corrosion fatigue crack growth data. NOMENCLATURE a = crack length area = projected area of a hole on a horizontal plane b, C, C i , C2, ho, m, n, a , , a2 = constants d = pit (or hole) diameter f = frequency F = correction factor h = pit (or hole) depth h, = pit depth, at which corrosion fatigue crack growth starts h,,, = maximum pit depth hf= final depth of crack AK = stress intensity factor range K,,, = maximum stress intensity factor AK,, = threshold stress intensity factor range (for R > 0) KmaX.,, = threshold stress intensity factor (for R < 0) AKtaCF = threshold stress intensity factor range in corrosive environments (for R > 0) Kmai,th,CF = threshold stress intensity factor in corrosive environments (for R < 0) AK,,, = allowable value of AK,,.,, Kall = allowable value of Kmar,th,CF N, = number of cycles to failure R = stress ratio ( = urmn/amai) r = correlation coefficient S = safety factor t, = design lifetime rf= fatigue crack growth period T = stress increasing time (or stress rise time) in a stress cycle Aa = stress range Au,,, = allowable stress range (for R z 0) amax = maximum stress AuW = stress range at the fatigue limit in tension or bending ( R > 0) Aa; = Aa,,, for h = 0 u,,,~~,~,, = allowable maximum stress (for R < 0)Authors' proposal uw = maximum stress at the fatigue limit in tension or bending ( R < 0) c&=u, for h = O uWCF = fatigue limit in tension or bending under corrosive environments ( R < 0) uLcF = uWCF for h = 0 T~ = fatigue limit in torsion ( R < 0).

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Thermal stress in silicon chips encapsulated in IC plastic packages is discussed. A stress-sensing chip which can detect the three-dimensional stress components separately is developed. Thermal stress occurs in the silicon chip almost linearly with temperature at the steady state. The stress path with temperature during heating is the same as that during cooling. On the other hand, during temperature cycling, stress hysteresis is observed, and the stress increases gradually with increasing cycles. The stress increase phenomenon occurs due to the viscoelastic phenomenon of the molding compounds. The stress-increase ratio is determined by the relationship between the cycle period and the stress relaxation time of the molding compounds at temperatures of interest.