Authored By:
rian Roggeman, David Vicari
Universal Instruments Corp.
Binghamton, NY, USA
Roggeman@uic.com Martin Anselm, Ph.D.
Lee Smith, Ahmer Syed
Amkor Technology, Inc.
Chandler, AZ, USA
Summary
The successful integration of package-on-package (PoP) stacking utilizing through mold via (TMV) technology hinges on a robust assembly process. In this study, seven dip materials were investigated for high quality TMV PoP assembly by optimizing machine settings to achieve proper material transfer. Film thickness was varied for each material to transfer enough material (target of 50% ball coverage) while preventing parts from sticking within the film. Assemblies were reflowed in both air and N atmospheres and yields were quantified.
It was determined that flux dipping provides for better TMV assemblies in air reflow due to the flux's ability to wet to and subsequently protect the TMV solder ball during reflow. All paste dipped materials experienced significant fallout in air reflow due to a non-coalescing of the TMV solder joint. All materials provided 100% assembly yields in N2 reflow.
Conclusions
The successful integration of PoP using TMV technology relies on careful attention to each aspect of the assembly process. Most of the attention is focused on soldering the top package to the bottom package because this relies on either a flux or paste dip process. Consideration over the material selection, film thickness and other machine variables is necessary to achieve best results.
Seven dipping materials, including two fluxes and five solder pastes, were investigated. Machine parameters were optimized for each material by setting a target of 50% ball coverage of the dip material, while preventing any parts from being stuck within the dip film. This exercise is necessary when evaluating new materials.
In this investigation, assembly yields were highly influenced by the reflow atmosphere. Nitrogen atmosphere produced 100% yields of both top and bottom packages for every material used in the study. Air reflow atmosphere resulted in noncoalescing TMV solder joints when the SCSP 200 package was dipped into solder paste. This is presumably due to oxide formation on the unprotected TMV solder ball which is not sufficiently removed with the minimal flux available in dipping solder paste.
Five different dip pastes were used with varying degrees of success, while the flux dipped samples achieved excellent yields in air reflow. Additional efforts are taking place to enhance the assembly yields in air reflow, and ultimately quantify the reliability of these devices as a function of the assembly process and material selection.
Initially Published in the IPC Proceedings
|
