SMTA International Conference Proceedings


Author: Jesse Phou
Company: Motorola Semiconductor
Date Published: 9/21/2003   Conference: SMTA International

Abstract: The use of a 4-layer HDI-1 laminate substrate was necessary to meet highly constrained routing requirements and BGA assignments for a particular device. The substrate was subsequently redesigned with a rotated die into a 2-layer conventional laminate substrate using etchback technology. This change presented a cost reduction opportunity but had several trade-offs to consider. Substrate etchback technology and die rotational placement were technical challenges that must be better understood since these could be manufacturability concerns, as well as potential package reliability and quality issues.

The use of etchback in the design caused openings to be made in the soldermask layer on the bottom side of the substrate. It was not certain whether these openings would make the package more susceptible to moisture ingression during moisture testing. But if that were the case, this would add great concern because this package had to pass preconditioning at MSL3 and be capable of 260°C reflow temperatures due to a product Pb-free requirement. Secondly, the 15-degree die rotation may cause potential die placement and die attach issues, or even bring the possibility of substrate strip warpage.

This paper presents the background of the design dilemma and how it evolved from a 4-layer to a 2-layer substrate. The packaging challenges and risks for this MAPBGA are described and discussed, and the package reliability results are reviewed. Electrical simulation showed that the 2-layer redesign was overall equal to or better than the 4-layer design. Results from assembly showed that there were no major issues with the risk items considered, but there were minor issues at wirebond and die placement that had to be worked out. The package successfully passed qualification at MSL 3, with the capability of 260°C reflow temperatures. The qualification units also successfully passed both 200 and 500 temperature cycle readpoints. In conclusion, the 2-layer design provided a robust package solution, with a cost advantage by ~40%.

Key words: substrate, rotated die, etchback.

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