Electronic Packaging Enhancement Engineered by Reducing the Bonding Temperature via Modified Cure Cycles

Abstract

Semiconductor packaging continues to reduce in thickness following the overall thinning of electronic devices such as smartphones and tablets. As the package becomes thinner, the warpage of the semiconductor package becomes more important due to the reduced bending stiffness and driven by thermal residual stresses and thermal expansion mismatch during the epoxy molding compound (EMC) curing to create the package. To address this packaging reliability issue, in this study, we developed a modified cure cycle that adds a rapid cooling step to the conventional cure cycle (CCC) to enhance the reliability of the EMC molded to a copper substrate (EMC/Cu bi-layer package) by lowering the bonding temperature of the EMC/Cu bi-layer package. Modeling of the package via Timoshenko theory including effective cure shrinkage allowed the rapid cooling step to be quantified and confirmed via experiments. The modified cure cycle resulted in a 26% reduction in residual strain, a 27% reduction in curvature, and a 40% increase in peel strength compared to the CCC, suggesting that this is an effective new method for managing warping effects in such packaged structures.

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“…The cure start temperature at which the degree of cure begins to increase is 103 °C, and the lowest viscosity, gelation, and solidification temperature of EMC were 128, 147, and 166 °C, respectively. Meanwhile, in the ramp rate of 7.5 °C/min (hot press), the cure start temperature, lowest viscosity, gelation, and solidification temperatures were 97, 123, 143, and 161 °C, respectively . The results indicate that the ramp rate affects the rheological characteristics of EMC, and thus, the bonding temperature between EMC and copper substrate was also affected.…”

“…The results show that the residual internal strain and peel strength of the FE-CH-cured EMC/Cu bi-layer package were slightly higher and lower than those of the hot press-cured specimen, respectively. From our previous paper, the rapid cooling of the modified cure cycle affected the lowering of the bonding temperature between EMC and Cu . Accordingly, to analyze the effect of rapid cooling on the bonding temperature, the rapid cooling temperature was set at 100 °C (near the cure start), 120 °C (near the lowest viscosity temperature), 140 °C (shortly before the analytical bonding temperature w/ ECS), and 160 °C (near the solidification temperature) using the FE-CH process, respectively.…”

“…To inhibit the curing reaction of EMC, the FE-CH process utilized a simple method of rapidly cooling the specimen to room temperature by turning off the power supply. Meanwhile, in the hot press process, the specimen was subjected to cooling by immersion in liquid nitrogen . After the temperature decreases to room temperature, the specimen was reheated again to 175 °C and kept at that temperature under the same conditions as the CCC.…”

“…T zero‑curvature w/ECS is lower than the bonding temperature ( T bonding ) because EMC’s ECS has an additional effect on the curvature besides the CTE mismatch. Therefore, to rapidly cool shortly before the bonding temperature, the bonding temperature using Timoshenko theory, which took into account the ECS of the EMC, was calculated by measuring the curvature of the EMC/Cu bi-layer package in our previous work as follows T normalz normale normalr normalo .25em normalc normalu normalr normalv normala normalt normalu normalr normale .25em normalw / .25em normalE normalC normalS − T = 1 α 2 .25em − .25em α 1 [ k true{ h [ 3 ( 1 + m ) 2 + false( 1 + m n false) true( m 2 + 1 m n true) ] true} 6 ( 1 + m ) 2 − ε E C S ] T normals normalh normali normalf normalt = ε normalE normalC normalS α 2 − .25em α …”

“…ε ECS is the effective cure shrinkage of EMC. The CTEs and ε ECS used in the equation refer to the values used in our previous paper …”