Device Packaging 2019

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CCGA Braided Copper Compliant Solder Columns - High Thermal Conductivity Interconnect Transfers Heat and Absorbs Stress Caused by CTE Mismatch Between Large Area Array IC Package and PC Board.
Keywords: Column Grid Array, Copper Braided Column, High Thermal Conductivity Interconnect
Ceramic Column Grid Array (CCGA) packages are an important component in surface mount technology used in harsh environments found in Military, Defense and Aerospace applications. Solder columns for CCGA was originally invented by IBM in the 1970s, which lead to the creation of the ubiquitous Ball Grid Array (BGA) invented by Western Electric. Solder columns provide a practical way to reduce stress caused by CTE mismatch due to differences in material properties of the IC package, such as ceramic, silicon, and the FR4 (organic) PC board. IBM's original plain Pb90/Sn10 solder column designed to reduce stress between large ceramic packages, but does not provide sufficient thermal path to conduct heat away from today's heat generating IC packages. While a structural improvement has been achieved by wrapping copper ribbon around high-temperature melting Pb80/Sn20 solder columns, the increase in thermal path to conduct heat away from the bottom of CCGA IC packages is still not adequate. This paper compares a new type of braided copper solder column to traditional solder columns and copper ribbon wrapped solder columns. Braided columns are constructed from 32 strands of copper wire versus just a single copper ribbon found in copper wrapped solder columns. During prolonged stress events, eventually a column constructed from a single copper ribbon will fracture causing a full failure condition. Whereas if a few strands of copper in the braided column fracture, then the remaining strands of copper will continue to conduct signals from the IC package to the PC board. Another supposition is that the electrical path along a spiral copper wrapped column is longer than the linear length of the cylindrical column, due to the necessary pitch angle to have the wrap placed on the column. Whereas a full diamond patterned braided column provides a shorter electrical path because of a higher allowed pitch angle. This is due to the multiplicity of the wires, allowing the mesh to be more present with a shorter electrical path. Through cross-sectional and geometric analysis techniques, as well as known material constants, we can find the thermal conductivity and thermal transfer coefficient for these solder columns. Thus, we can design solder columns to have a higher thermal conductivity in order to transfer more heat away from the underside of CCGA packages without the need for top mounted heat spreaders. However, developing a column for higher thermal transfer capabilities, sacrifices the elastic capabilities of the column. In this way, the modeling of the thermal conductivity in these columns is directly analogous to the Young’s modulus. This means there is a limit to the thermal conductivity of the column, based on the maximum allowed Young’s modulus for a particular set-up. Knowing this relationship, however, gives the opportunity to optimize the possibilities, allowing for an advancement in Solder Column development. An improvement in structural compliancy of the braided copper solder columns also reduces stress by absorbing differences in CTE materials mismatch between the IC package and the PC board. Reduction in stress contributes to longer life with reduced solder joint cracking.
Matthew Wyatt Pearson, Research and Development Associate
Milledgeville, Georgia

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