SMTA International Conference Proceedings

CCGA - Solder Column Attachment for Absorbing Large CTE Mismatch

Author: Martin Hart
Company: Topline Corporation
Date Published: 9/27/2015   Conference: SMTA International

Abstract: Replacing solder balls with solder columns makes a device known as a Column Grid Array (CCGA). CCGA packages provide a trusted solution for overcoming 10ppm/oC mismatch of coefficient of thermal expansion (CTE) between large ceramic IC packages and glass-epoxy printed circuit boards (PCB). Solder columns are compliant and absorb strain in applications typically found in the fields of military, aerospace and defense where significantly varying temperatures, high vibration and shock are present.

Plastic ball grid array (PBGA) packages using solder ball interconnections perform reasonably well in commercial applications. This is not the case with large size ceramic ball grid arrays (CBGA). Solder balls on large CBGA ceramic packages encounter dangerously high stress due to the inherent mismatch between the ceramic's CTE and the PCB.

The array of solder balls are stretched and squeezed, especially at the corners of the ceramic substrate and directly under the corners of the silicon die or flip chip. As the temperature swings hot to cold in repeated cycles, the strain induces physical movement in solder balls measured from the neutral point, typically the center of the IC package. While the amount of movement may seem small - about the thickness of a sheet of paper - such stretching may be equivalent to 10% of the overall diameter of the solder ball. Under conditions of repeated thermal cycles, solder balls will delaminate from the IC package and/or the PCB, resulting in catastrophic electrical failure.

Replacing solder balls with solder columns provides stress relief on large ceramic area array IC packages. There is also interest to study improvements in reliability by attaching solder columns to large plastic BGA packages as a way to reduce stress for certain applications.

This paper reviews the types of commercially off-the-shelf (COTS) solder columns commonly available, including plain Pb90/Sn10 columns, copper wrapped Pb80/Sn20 columns and the NASA invented Micro-coil Spring. This paper also reviews practical ways to attach solder columns and Micro-Coil Springs to IC packages in a high-mix, low volume benchtop environment.

Key Words: 

CCGA, column grid array, solder columns, Micro-Coil Spring, Pin-Pack

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