Circuits Assembly - April 2008 - (Page 28)

Benchtop Cleaning The ‘Big Brush Off’ Revisited By Mike Jones In rework and repair, if you can’t rinse, you can’t clean. ast December in this journal, Terry Munson offered up a unique analysis of benchtop cleaning. 1 In one brief piece, he managed to be both exactly right and precisely wrong – a phenomenon typically observed only in members of Congress. This is a brief attempt to set the record straight. First, let’s review the parts Munson got right. He asserted localized brush cleaning is widely used to remove flux buildups after assembly, rework and repair. He noted few, if any, SMT manufacturers have ever properly “qualified” their benchtop cleaning process to determine it was delivering the desired results. He scored a perfect “10” when he noted the result of conventional brush cleaning was merely “the redistribution of flux residue to nearby areas of the assembly.” Munson also observed erratic cleaning is a particularly severe problem with no-clean fluxes, which is true. Last, he diagnosed the problem correctly when he observed that localized cleaning without rinsing posed a significant risk of contamination, voltage leakage, corrosion and ultimately premature failure of the PCB. When you’re right, you’re right; and he’s absolutely right on every count. L So the Process Doctor has correctly identified the problem, but neglected to write a prescription to cure it. Luckily, I know some experts in benchtop cleaning. And any of those experts would report there are a handful of acceptable ways to clean a circuit board after hand-soldering, but “dip-andbrush” cleaning isn’t one of them. Why clean? Mr. Munson never challenged the requirement to clean. Yet unless you’re making the cheapest possible product, any circuit upon which hand-soldering has been performed must be cleaned. Even “no-clean” fluxes need to be cleaned, unless the soldering process is totally automated. Fluxes work because they contain halites (which are salts, also called “activators”), which lower the surface tension of the melted solder and thereby improve the solder surface wetting. Salts corrode metals, so it always has been a requirement with traditional fluxes (R, RA, RMA, SA and so on) that those halites be removed from the board by cleaning. In contrast, most no-clean chemistries use the heat of the soldering process to encapsulate the activators in a microscopic, protective bubble. This physically separates the salts from the metals, which keeps the halites from corroding the components and board. The encapsulation process is performed chemically when the flux reaches a critical temperature during the soldering process. And that is the heart of this story: Because cleaning “breaks down insulative 28 Circuits Assembly APRIL 2008 circuitsassembly.com http://circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - April 2008

Circuits Assembly - April 2008 Contents Caveat Lector Industry News Market Watch Talking Heads Screen Printing Better Manufacturing Design and Modeling of High-Speed, High-Density 3-D CSPs and Memory Modules The ‘Big Brush Off’ Revisited Impact of Soldering Atmosphere on Solder Joint Formation Beyond Moore’s Law ESD Control For Class 0 ESDS Devices Growing Your Brand This Year’s Model Tech Tips Reflow Soldering Process Doctor Pb-Free Lessons Learned Getting Lean Equipment Advances Apex Product Preview Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - April 2008

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