‘Contamination is Seldom Even’ Print E-mail
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Written by Mike Buetow   
Friday, 12 August 2011 11:25

As lead-free residues wreak havoc on the potential reliability – not to mention the aesthetics – of printed circuit boards, interest in post-assembly cleaning has reemerged. In response, cleaning equipment OEM Aqueous Technologies has joined with chemistry provider Kyzen to offer a series of workshops aimed at addressing some of the misconceptions and hurdles to cleaning PCBs. Aqueous Technologies CEO Mike Konrad spoke with CIRCUITS ASSEMBLY editor in chief Mike Buetow about the workshops.


CA: What is the basic driver behind your workshop?

MK: The cleaning industry, specifically the cleaning equipment industry, is unlike most other electronic assembly equipment segments. If one were to contemplate purchasing a new pick-and-place machine, for example, one would compare the prospective machine's flexibility, placement speed, flexibility, etc., against one's current machine. In other words, there is a point of comparison; there is a current benchmark.

When it comes to cleaning equipment, there is rarely a relevant point of comparison. There are few benchmarks. Most assemblies either have no experience with cleaning systems or have outdated experience. The fact is almost all assemblers cleaned before 1989. Also true is the fact that most modern assemblers have not cleaned assemblies since 1989. 1989 was a long time ago. Many production engineers who had experience with cleaning have long retired. Those who have not yet retired have not necessarily kept up with the evolution within the cleaning industry.

The absence of modern, relevant information necessitates cleaning workshops. The primary purpose for the workshops is to educate the participants – to bring them into the 21st Century with relevant, timely, and accurate knowledge of current best practices, environmental regulations, chemical and equipment advances, etc. We have either produced or co-produced more than eleven workshops in the past year. Audiences are growing, as cleaning becomes increasing relevant.

CA: Does there tend to be much variability in cleanliness across a board surface?

MK: There are vast variables in cleanliness levels across an assembly's surface area. An assembly's surface area is not normally uniform. Densities and standoff heights vary. A good cleaning process will produce consistent cleanliness results, regardless of geographic variances across the assembly's surface. A less-than-good cleaning process may produce vast differences in cleanliness across the board.

CA: Should the IPC standard of 10 micrograms/in2 be changed? If so, what would be an appropriate cleanliness level?

MK: IPC's cleanliness standard of 10 micrograms NaCl/in2, originally a military standard, was first published when Fantasy Island was the number one show on television. Just consider how circuit assemblies have changed since the 1970s. The fact is there is no single cleanliness number appropriate for all assembly types. Not only does the volume of contamination required to cause a failure vary based on assembly design, but an assembly's end-use environment also plays a defining role. A cleanliness result of 2 micrograms NaCl/in2 may produce a highly reliable assembly for one company, while it could be a death sentence for another. Some assemblies tolerate a specific level of contamination, while others may suffer. The only cleanliness result that is acceptable across the board is 0.0.

CA: Are cleaning tests ultimately a means to an end, or should they be looked at primarily as process indicators?

MK: Cleanliness testers, all cleanliness testers, should be used as process indicators. R.O.S.E. (Resistivity of Solvent Extract) testers, for example, the most popular method of determining post-reflow cleanliness levels, average the volume of detected contamination across an assembly's entire surface area. Contamination is seldom spread evenly across an assembly. Because acceptable levels of contamination levels vary from design to design and application to application, detected results must be collated with other data. Most important, changes in detected contamination must be investigated. A move from 1.5 micrograms to 3.5 micrograms, while both numbers technically represent “clean” boards, signals a change in process, which may lead to problems down the line.

CA: Does R.O.S.E. still have a place in cleanliness testing today? If not, what would you recommend to use?

MK: Yes. R.O.S.E. testers remain the only rapid, low-cost method of cleanliness determination. While Ion Chromatography provides a more detailed analysis of an assembly's contamination, it is normally an outsourced test and frequently costs in excess of $1,000. Most assemblers utilize R.O.S.E. testing and only use Ion Chromatography if the R.O.S.E. tester indicates an unacceptable raise in contamination. In this case, Ion Chromatography may be used to identify the contamination species and assist in the identification of its source.

CA: Discuss the variability of different types of fluxes (e.g., low solids) and no-clean pastes insofar as meeting the IPC cleanliness standard. Do they give users a false sense of security?

MK: The primary flux used today is no-clean. No-clean fluxes have very low solids contents. The lower the solids content, the lower the amount of residue after reflow. IPC cleanliness standards allow up to 10 micrograms NaCl/in2. Most non-cleaned no-clean fluxes will produce results lower than 10 micrograms NaCl/in2. Two issues are associated with allowing no-clean residues, albeit fewer residues to remain on an assembly:

  1. As I stated, the threshold for tolerable contamination varies based on component density, alloy, and in-use environment. While no-clean flux yields fewer residues, the volume of contamination may still result in electro-migration or electrical leakage problems.
  2. When we no longer remove flux residues, we no longer remove all other residue types. Back in the day, when all assemblies were cleaned, the cleaning process, while primarily targeting flux, also removed other residue types. Residues from bare board fabrication, component fabrication, and operator assembly (human contamination; e.g., finger oils) are not removed when the assembly is not cleaned. The machines used to clean assemblies are referred to as “defluxers.” More appropriately, they should be referred to as cleaners. While the intended target is flux residues, dozens of potential "stowaways" on an assembly can contribute to contamination-related assembly failures.

CA: So, how clean is clean?

MK: Zero point zero (0.0) is clean. Any detectible contamination above 0.0 needs to be evaluated. For some assemblers, the answer is < 10 micrograms NaCl/in2. For others, < 2 micrograms NaCl/in2 is more appropriate. One must consider the cost of failure. For space, medical, and military products, the cost of failure is high. For many commercial products, the cost is less. What we know as a fact is contamination (flux, fabrication, and assembly residues) increases the opportunity for electrical migration and leakage, increased corrosion, conformal coat failures (under-coat corrosion and delamination). The cost to effectively clean is literally pennies per assembly. A few cents of prevention can save costly repairs, reputations and lives.

Last Updated on Monday, 15 August 2011 20:10


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