Cleaning for Reliability After QFN Rework

Mike Bixenman, D. B. A.
Kyzen Corporation, Nashville, TN, USA

Michael Konrad
Aqueous Technologies Corporation
Rancho Cucamonga, CA, USA

The need for cleanliness under individual components increases as the spacing between connector leads decreases and power increases. The low standoff height of QFNs traps flux between the ground pad and component leads. Entrapped flux under the QFN is a reliability concern. The purpose of this paper is to make an argument for removing flux residue under the QFN post rework and present cleaning process options for meeting this cleaning challenge.

The assurance that the product works for its designed life expectancy is an important issue when reworking electronic assemblies. Flux residue under Quad Flat Pack No Lead (QFN) components can impact performance when conductive ions migrate within the electrical field.1 The common cleaning equipment used for rework operations is the dishwasher style aqueous spray-in-air machine. Engineered cleaning materials are needed to dissolve flux residues commonly used in the rework operation. To successfully clean all flux residues under the QFN, process factors must be understood.

Cleaning under low feature components requires an optimized process. Neither the cleaning agent nor the cleaning equipment accomplishes the cleaning need in unison. Integrating the right cleaning agent with the right cleaning machine and other process factors are the key to cleaning leading edge circuit assemblies. The data presented in this paper builds from this premise.

When designing optimized QFN rework processes, the authors recommend that users view cleaning as an integrated process. Cleaning agent and cleaning machine science has dramatically improved with time. Integrating best in class technologies provide proven performance that accomplishes this demanding cleaning need.