Is Cleaning Critical to POP Assemblies?

Harald Wack, Ph.D., Umut Tosun, M.S., and Jigar Patel, M.S.
ZESTRON America, Manassas, VA, USA

Package on package (PoP) assemblies are widely growing in use for applications that require small footprint technology. Typically, this integrated circuit design stacks and integrates logic and memory packages, thereby increasing board density and substantially expanding functionality within the same footprint of a single BGA. As a result, PoP's have become an ideal component selection for products such as advanced mobile platforms and digital cameras.

Cleaning is a critical process within the electronics manufacturing industry. Effective cleaning improves product reliability by ensuring optimal surface resistance and preventing current leakage that can lead to PCB failure. This paper will address the cleanliness level of PoP assemblies including underneath PoP components and in between packages.

The test procedure will utilize 14mm test boards with 3 x 5 arrays for 15 Package on Package components each incorporating 0.65mm ball pitch top and 0.5mm ball pitch bottom with daisy chain patterns in both packages. The boards will be populated with a no-clean paste and flux and reflowed with and without nitrogen for comparison. Subsequently, they will be cleaned utilizing an inline cleaner and several types of engineered aqueous cleaning agents.

Cleanliness evaluations will be made utilizing visual inspection, ion chromatography and SIR. A comparative cleaning analysis will include populated reflowed boards without cleaning and multiple cleaning scenarios with variable combinations of wash, rinse, dry and bake.

Expected results will empirically confirm the effectiveness of cleaning technologies for this rapidly evolving technology.

The results of all three phases of the study show that with new generation cleaning agents and an improved mechanical design in the wash section of an inline cleaning machine, PoP assemblies can be effectively cleaned.

As originally published in the SMTA International Conference Proceedings. As for this paper only two package components were used on a test vehicle, ZESTRON is planning to collaborate with industry partners to develop and validate cleaning process parameters for multiple stack PoP assemblies in the future.

This research paper is part of a series written by ZESTRON on optimizing precision cleaning processes for electronics manufacturing industry. These studies have been presented at the industry's known conferences SMTAI and IPC/APEX. Based on our findings, key market developments have been initiated, thereby addressing the current shortcomings observed in the industry.