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Cleaning for High-reliability Applications
December 31, 1969 |Estimated reading time: 5 minutes
Military and other high-reliability PCBs are cleaned after reflow because of the crucial nature of electronics performance in the field. Why then, with reliability an expected trait of most electronics devices in consumer and commercial applications, does clean not automatically correlate to reliable outside of the military sector? The bigger question is what happens if your product fails. Residues — flux, oils, trapped moisture —are more harmful than ever.
In days gone by, post-solder cleaning was as much a normal process step as component insertion and soldering. We stuffed boards, cut and clinched the leads, ran them through the wave soldering machine, and dunked them into a bath of boiling 1,1,1-trichloroethane or Freon TMS. It was that routine and it was that easy.
The cleaning pendulum swung in the other direction in the days following the Montreal Protocol (the essential end of popular cleaning solvents), at least for most of us. Cleaning was relegated to manufacturers of military electronic assemblies through various military specifications and grandfathered contractor-based regulations. In the world of military electronics assembly, cleaning never left the norms of electronics production.
There are two primary reasons why the practice of cleaning electronic assemblies after reflow remains in practice in military and other high-reliability circles. Old habits die hard. This is particularly true in military-related endeavors. Also, products must work or people die. In the military world, missiles must reach their destination, guidance systems must be accurate, early warning radar must accurately detect threats, and the list goes on.
For reasons familiar but not understood by this author, much of the civilian world chose not to embrace the relationship between clean and reliable. While some civilian electronic manufacturers choose to clean their assemblies (medical manufacturers are a pertinent example), others chose to allow residues to remain on their assemblies. Are those residues harmful? Unfortunately, the answer is maybe no or maybe yes.
The determination of harmful can be based on two individual criteria. Will the residue cause product failure? What is the cost of product failure?
Allow me to first identify what I mean by residue. Most readers will assume that I am referring to flux residues. While flux residue is at the top of most assembler’s lists of concerning residues, there are many other potential residues that may be present on a post-reflow assembly (Table 1).
Table 1.
Board fabrication
Component fabricationAssembly processEtch residuesWater quality rinsesSolder paste flux Developer chemicalsPlating bath residuesFlux from wave solderingWater quality rinses for inner layersDeflashing chemicalsCored solder fluxWater quality rinses for outer layersMold release agentsRework/repair fluxesHASL fluids (HO)Preplating oxide cleaningCleaning (defluxing) chemicalsAlkaline cleanersTinning flux residuesWater rinse qualityWater quality for final rinses Rework cleaner
Will these residues lead to performance or reliability issues now or in the future? Will flux residue (even nearly invisible residues from no-clean fluxes) react with electrical current and moisture to create electrical migration (dendritic growth)? Will (as a major cell phone manufacturer reported) mold-release from component reels transfer onto an assembly and cause solderability problems? Will board fabrication process residues or operator skin oils degrade the adhesion quality of conformal coating? In fact, all of these residues are capable of lowering a product’s accuracy, reliability, and lifespan.
The bigger question is what happens if your product fails. In some applications, failure leads to inconvenience. In others, it leads to injury, death, or property destruction. Industry experts acknowledge that board-level residues (from board fabrication, component fabrication, and assembly) are capable of producing multiple causes of failures including electro-migration, electrical creepage, conformal coat adhesion failure, RF waveform abnormalities, etc. While this has always been true, it has never been truer. Personally, I love living in a miniaturized world. I remember the days of brick-sized mobile phones, laptop computers that took up your entire lap, and the Walkman “portable music device” that was too large for any pocket. It has been said that we have left the age of revolution and entered the age of evolution. The revolution of the 1980s (birth of mobile phone, video games, personal computers, etc.) has been replaced with the miniaturization and faster speeds of the current evolution. When it comes to technology, society demands smaller, faster, and cheaper. And, unlike the 1970s when gadgets were mostly cheap, low-quality electro-junk, today’s buyers demand performance and reliability. Today’s speed, miniaturization, and low cost mandate clean assemblies more than any time in our electronic history.
The fact is, the smaller an electronic assembly is, the closer components are to each other, and as a result, the closer a cathode is to an anode. Evolution pushes cathodes and anodes so close together that it now takes only a very small amount of residue to carry electrical current across the shrinking divide. Removing assembly residues after reflow prevents any chance of electro-migration as it removes one of the three required base ingredients for this growing phenomenon (conductive residues, moisture, and electrical current).
Today’s consumer expectations are extremely high. If my mobile phone fails, I turn it in for a new one. I have no expectation that I will have to purchase a replacement. I expect it to last even under the most demanding environments. Modern consumers utilize modern communication platforms (ie social media) to express their likes and dislikes for various consumer products. Lighting-fast communication and twenty-four-hour news cycles and news accessibility all contribute to a product’s success or failure. Products that fail do so in a big way. Recent recalls by Toyota involving unintended acceleration of their cars seem to be pointing to an electronic throttle control system. Will this turn out to be a simple case of floor mats sticking against an accelerator pedal or will it be due to a failure in the electrical control systems? Only time will tell. It is safe to say that if the assemblies were free from residue, electrical migration would not be a factor. Given the fact that it only costs about $0.05 to clean an assembly, that would be money very well spent.
Conclusion
The worlds of high-reliability products (military, space, medical) and commercial products (personal electronics, transportation, communication) are rapidly moving closer together. Consumers of military hardware and iPods alike demand quality, reliability, and performance. In the low-margin world of consumer products and the punitive liability-fueled society we live in, few can afford well-publicized quality problems. Cleaning electronic assemblies adds a measurable level of reliability at a relatively low cost.
Michael Konrad is an SMT Advisory Board member and president of Aqueous Technologies. Konrad also is an IPC SMEMA Council APEX Committee Member. He was a "High Performance Electronics Assembly Cleaning Symposium" panelist. Contact him at konrad@aqueoustech.com.
Other Articles by Konrad:So, You Need to Clean? A Defluxing Process Roadmap Part ISo, You Need to Clean? A Defluxing Process Roadmap Part IICleaning for Reliability Post QFN Rework
SMT, February 2010
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