March, 2014
www.us-tech.com
Page 59 For Electronic Assemblies, Reliability Rules By Michael Konrad, President, Aqueous Technologies T
he electronic assembly industry has many rules, each with its own definition. For exam- ple, there is the “rule of thumb.” This refers
to “a rough and practical approach, based on expe- rience, rather than a scientific or precise approach based on theory.” In the electronics assembly industry, the rule
of thumb has long dictated that if an assembly looks clean, it is clean. In the late 1980s, assem- blers en masse defected from using rosin and water-soluble fluxes and transitioned to the use of “no-clean” fluxes. The promise, mostly delivered, was that there
would be no visible residues after soldering. The formerly popular flux types all left visible residues prior to cleaning. No-clean fluxes left behind little or no visible residues. With the transition, assem- blers have enjoyed the use of no-clean fluxes for years of blissful tranquility until one day, when the rules changed. In English, the word “rule” re -
fers to “a statement of what is allowed or what will happen within a particu- lar system (such as a language or sci- ence).” In terms of the cleanliness of electronic assemblies, many types of residues are commonly found on those assemblies. Residues from cir- cuit-board fabrication are common. Because a circuit board is bare does not mean it is clean. On the contrary,
The assembly process is
rife with potential residue sources: hand oils, equip- ment transfer residues, residues from food prod- ucts (such as potato chips), hand lotion residues, and, of course, flux residues.
bare boards may be surprisingly dirty, covered with invisible forms of contamination, some benign and oth- ers harmful. Residues can also form during component fabrication. In one case, a silicon mold release was trans- ferred from a component reel, to the component, and finally to the circuit board, causing tremendous problems with the soldering process. Perhaps the most prolific of all
the circuit-board residues are the assembly residues. The assembly process is rife with potential residue sources. Human-derived residues include hand oils, equipment transfer residues, residues from food products (such as potato chips), hand lotion residues, and, of course, flux residues. One rule that many circuit assem- blers forget is that if one doesn’t remove flux residues, one doesn’t remove any residues. Even with the belief that no-clean flux leaves no residues on a circuit board, there are many other residues to be found there.
For those who may have won-
dered why, after 20 years of not clean- ing their electronic assemblies, that they are experiencing problems only now, the answer is simply that the “rules” have changed. At one time, a circuit-board’s geography resembled “fly-over” country, with an ample sup- ply of available real estate. Twenty years ago, components were also con- siderably larger than many of today’s components. Circuit boards were larger and components were more widely spaced. There was also a con- siderably higher level of tolerable residues 20 years ago. Today, with the drive toward
Dendritic growth caused by a combination of residues, moisture, and electric current. (Photo courtesy of Foresite)
miniaturization, both component-to-component and component-to-board spacings continue to grow tighter. Miniaturization, combined with increased product life spans and greater reliability expecta- tions, have resulted in the reduction of tolerable residue limits. In other words, a specific volume of residues
on an assembly a few years ago would go unno- ticed; the identical volume today would be cata- strophic. The closer components are spaced, the more likely residues will have a detrimental effect. For those who may question how to justify
cleaning an assembly that has been produced with a “no-clean” flux, the answer is to forget the term “no clean” and to relabel their solder paste jars with the description “low residue” which is more
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