Mamoru Kurashina

Low warpage coreless substrate for large-size LSI packages

Due to inadequate rigidity, warpage of coreless substrates is generally large compared to other types of LSI package substrates. Therefore, the most important problem in the application of coreless substrates is warpage reduction during the reflow process. So far, there have been only a limited number of reports on coreless substrates for large-size LSI packages. Moreover, there have been very few examples that discussed substrate layer structure designs for warpage reduction and reliability improvement in the LSI assembly process. In the present study, we focus on developing coreless packages for large-size LSIs. To achieve our goal, we adopted the following development processes. First, we designed analytical models with different layer structures comprising two kinds of materials, and investigated the effect of layer structure on warpage reduction using warpage simulations. Next, we made four kinds of real coreless substrates with layer structures identical to the simulation models, and verified the actual thermal warpage behavior. Finally, we investigated the thermal stress reliabilities of these substrates after LSI mounting. From the results, we found that warpage reduction and reliability enhancement of coreless substrates were realized by arranging the high rigidity materials on the external layers of the substrates.

Precision improvement study of thermal warpage prediction technology for LSI packages

A finer interconnection pitch of LSI packages has enhanced the importance of precise prediction technology of temperature-dependent warpage. In our research, we prepared a model package with minute wiring and vias, and examined a method of improving the agreement accuracy between numerical analysis and measurement of temperature-dependent warpage. To improve the precision of warpage prediction technology, we paid attention to warpage measurement technology, especially the temperature distribution in a sample, in addition to improving the accuracy of the numerical analysis model and material properties. We succeeded in heating a substrate with a temperature difference of 20°C or 3°C between the top side and bottom side, by controlling the heating conditions. Furthermore, the numerical analysis with a fine wiring model was performed under conditions where the temperature varied in consideration of the thermal conductivity of the substrate. The material properties for the numerical analysis, such as Coefficient of Thermal Expansion and Relaxation Modulus were measured very carefully with original setups, because they are essential for improving the accuracy of our numerical analysis. As a result, we found that substrate warpage with an uneven temperature distribution is quite different from such warpage with uniform temperature. To predict the temperature-dependent warpage with a high accuracy, we found that the temperature distribution in a substrate should be considered in the numerical analysis, besides applying the precise model and material properties.