Electronics Assembly Knowledge, Vision & Wisdom
Cleaning for Reliability After QFN Rework
Cleaning for Reliability After QFN Rework
Paper makes an argument for removing flux residue under QFNs post rework, and present cleaning process options for this cleaning challenge.
Production Floor

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

Michael Konrad
Aqueous Technologies Corporation
Rancho Cucamonga, CA, USA
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Transcript
The need for cleanliness under individual components increases as the spacing between connector leads decreases, and power increases.

The low standoff height of QFNs trap 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 QFNs after rework, and present cleaning process options for meeting this cleaning challenge. The test vehicle used for this study was fabricated from FR4 laminate.

Stainless steel pins were placed at specific standoff heights, and positioned to hold 1"by 1' by 2 mil think glass slides.

The glass slides could be locked at ten separate standoff heights from 1 mil up to 10 mils above the laminate surface. Eight paste fluxes used in commercial solder paste products were tested.

Three of the solder pastes represented water soluble technologies, two rosin technologies, and three no-clean technologies. Two milliliters of the flux was applied to each of the test sites.

The glass slides were positioned and locked in place. Then the test vehicles were reflowed using a standard tin-lead reflow profile.

Following the reflow process, the test vehicles were placed in a batch cleaning system.

So what were the conclusions?

Flux residue under Quad Flat Pack No-Lead, or QFN components can impact performance when conductive ions migrate within the electrical field.

Engineered cleaning materials are needed to dissolve flux residues commonly used in the rework operation. To successfully clean all flux residues under QFN components, 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 by itself.

Integrating the right cleaning agent with the right cleaning machine and other process factors, are the keys to cleaning leading edge circuit assemblies.

The data presented in this paper builds from this premise. When designing an optimized QFN rework processes, the authors recommend that users view cleaning as an integrated process.

Integrating best in class technologies with proven performance will accomplish this demanding cleaning need.
Summary
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.
Conclusions
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.
Initially Published in the SMTA Proceedings
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