Determining Critical Cleaning Process Parameters for QFNs

A two phase study assess how to completely clean underneath leadless devices following soldering with Pb-free water soluble and Pb-free No Clean pastes.

QFN or Quad Flat No Leads or MLF (Micro Lead Frame) packages are one of the fastest growing package types within the electronics industry. These components physically and electrically connect integrated circuits to printed circuit boards. The QFN package is similar to the Quad Flat Package, but the leads do not extend out from the package sides. This is a near CSP plastic encapsulated package made with a planar copper lead frame substrate. Perimeter lands on the package bottom provide electrical connections to the PCB. The exposed thermal pad included in the package increases heat transfer from the integrated circuit into the PCB. Heat transfer can be further facilitated by metal via the thermal pad. The body size of the component ranges from 2 mm x 2 mm to 12 mm x 12 mm and the lead pitch could be as low as 0.4 mm. 

Since the leads of the QFN package do not extend out from the component body, but are located underneath the component body, cleaning the flux residues after the soldering process becomes a challenge. The low standoff associated with such components, typically 1 mil, and the large thermal pad at the center of the component body pose barriers to the complete removal of flux residues. Partially removed or untouched residues can lead to component failure resulting from electrochemical migration via dendritic growth as well as electrical leakage currents. 

This study is designed in two phases and will determine the critical cleaning process parameters for complete cleanliness underneath leadless devices or QFNs. Two groups of paste types will be used; lead-free water soluble and lead-free no-clean. The cleaning process will employ a dynamic surfactant-based cleaning agent and spray-in-air cleaning equipment.

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Determining Critical Cleaning Process Parameters for QFNs