Techspray - High Reliability and Low Variability Results with Benchtop PCB Cleaning

Our thanks to Techspray for allowing us to reprint the following.

By Lindsey Shehan, Chemist, Techspray

Introduction


Historically, dip-and-brush, often with isopropyl alcohol (IPA), was the most common method used in benchtop (manual) cleaning of printed circuit boards (PCBs). More stringent cleaning standards, necessitated by miniaturization, have caused many to question tried-andtrue methods. Even no-clean residues have raised questions; should they be cleaned? Or should they be left alone as the name implies? The dip-and-brush method was innately flawed and even more problematic when used on more tenacious no-clean flux residues. Boards were cleaned with contaminated solvent and brushes held onto contaminants, leaving the question as to how much residue was removed from the board by this process and how much cross-contamination was introduced through this process. Did you end up cleaner, dirtier, or no better, no worse than when you started?

By its nature, aerosol cleaning eliminates both of these problems. Aerosol cleaning introduces a continuous supply of clean, or virgin, solvent throughout the entire cleaning process. Aerosol cleaning prevents the introduction of new contaminants. While there are some obvious pros of aerosol cleaning, questions remain as to whether this method of benchtop cleaning can consistently and reliably clean a board. This paper examines the role that different variables play in aerosol cleaning and offers guidelines to improve an aerosol cleaning process.

Testing Method

Boards were built using Kester’s FL250D (Sn63Pb37), no-clean paste to attach two QFNs and one QFP. Two components (QFN B and QFP) were fluxed, with Amerway #100 Type "R" Non-Activated Rosin Flux, to simulate rework. QFN A was untouched to act as a control and also so that it could be checked for crosscontamination. Two drops (from glass pipette) of flux were added, one to the top and one to the bottom of QFN B. Four drops were added to the QFP, one to each side. The boards were then reflowed using...

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