In many of today’s high speed, high density circuits, there is a need to remove large amounts of heat. To facilitate this removal of heat, it is common to adhere a sheet of a high thermal conductivity material (such as aluminum or copper) to the substrate (which may be alumina ceramic). This can result in large expansion mismatches which cause stresses and bowing, with the possibility of delamination, cracking, stressing solder joints, loss of hermeticity, or shorting of a metal lid to wire bonds inside a cavity. One approach to this problem is to use a compliant adhesive to decouple the materials. The present paper is an experimental and theoretical study of the strains as a function of temperature from −40° C to 140° C in a trilayer structure of 0.030 in. or 0.76 mm thick aluminum, 0.006 in. or 0.15 mm thick adhesive, and 0.021 in. or 0.5 mm thick low-temperature cofired (glassy) ceramic. The strains are analyzed using E. Suhir’s theory, and they are measured using strain gages for three adhesives: an epoxy, a fabric-reinforced epoxy, and a silcone elastopolymer. If the adhesive has an elastic modulus below 10 psi or 70 kPa, theory predicts almost complete de-coupling. Between 100 and 105 psi or 700 kPa and 700 MPa, there is partial decoupling, depending on the in-plane dimensions. Above 10,000 psi or 700 MPa, the decoupling is negligible, and the same bowing results for any elastic modulus between 10,000 and 1,000,000 psi or 70 MPa and 7 GPa. For temperatures below 80° C, only the elastomer has enough compliance to provide any de-coupling. Above 80° C, the elastomer de-couples the most, and the unreinforced epoxy the least. Almost all of the observed effects are understandable in terms of the Suhir theory, along with the fact that the elastic modulus of the epoxy materials decreases with increasing temperature. In particular, when there is some decoupling of the materials, the amount of decoupling depends on the in-plane dimensions of the sample.
The nonlinear optical properties of thin ( -1 mm) lithium niobate integrated optical substrates can be obtained from the interference between the free and bound second harmonic waves (Maker fringes). The harmonic power is affected by the homogeneity of the plate, and this provides us with a method of quality control. We use a Q switched YAG laser to generate the harmonic power and the equipment is computer controlled. Fringe patterns that deviate appreciably from those expected for hon:oceneous plates indicate substrates that may not function properly in devices.In addition, the method can be used to analyze lithium niobate boules to ascertain if they are potentially useful for substrates.
The measurement of the refractive index of lithium niobate integrated optical substrates of various stoichiometries is difficult because of their thinness (til mm). We use a prism coupling method and calculate the refractive index from the angle at which the intensity of the light from the total internal reflection in the prism base drops. The equipment is computer controlled.In addition, the method can be used to analyze lithium niobate boules to ascertain if they are potentially useful for substrates.
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