One Clean Source reader asked a question about the need to run cleanliness testing.
“Have you ever written an article that explains why/not ionic contamination testing is required if using no-clean fluxes?”
In a global sense, the question is related to: How clean is clean enough? When do you need to do cleanliness testing? What tests do you need to pass? Manufacturers often groan at the thought of testing, because it takes time and money. The answer applies to metal fabrication and finishing, optics, medical devices, aerospace – all sorts of applications, not only electronics. We haven’t yet written a full article, but we can provide a few ideas about testing. The answer is complex and application-dependent; and the principles of why ionic contamination testing might or might not be required can be applied to the need for testing of all manner of residual soils.
“No-clean” flux
First, you need to understand why there might be contamination problems. This involves understanding the nature of the soil. For example, the term “no-clean flux” is misleading. Solder fluxes allow more effective soldering; they remove surface oxides and improve wettability. Fluxes contain an assortment of chemicals, liquid and solid. Fluxes act as cleaning agents.
Way back when, flux was cleaned off of electronics assemblies, often with solvents. These cleaning operations were termed defluxing. The primary concern was with ionic contamination from the flux residue; ionics can impair short and long-term function of electronics assemblies. Starting in the mid-1980’s, the effort to replace ozone-depleting chemicals (ODCs) lead to increased development and testing of so-called no-clean fluxes. No-clean flux is not “chemical-free” and it does not magically disappear. It’s just that lower levels of conductive soils remain.
What has changed in the last quarter century or so? Electronics assemblies have become more complex; that means more solder flux can be entrapped in small spaces. Also, there are more and more varied ways in which electronics assemblies are used. In some instances, the risks and consequences associated with residual soil, including fluxes, may not be an issue. Many consumer electronics are replaced every year or so; and the low cost of replacement means that electronics repair is a thing of the past.
Why clean “no-clean”?
In other applications, such as in aerospace, automotive, military, and healthcare applications, reliable performance over a number of years may be a must. Gradually, manufacturers began to clean the no-clean fluxes. Sometimes, water alone is enough. Increasingly, a full cleaning process, including wash, rinse, and dry steps are used. In such instances, cleaning, testing, and process documentation are essential.
Ionic testing requirements are not an absolute. The only two requirements in life are death and taxes; and we hear that with skillful lawyers and accountants, that latter can be minimized. In contrast, ionic contamination testing may be required to demonstrate that the residue level is low enough for the application at hand. This involves determining what “low enough” means.
What is “clean enough”?
One common complaint from electronics assemblers is that an assembly passes in-house ionics testing but fails customer acceptance tests. There are a number of reasons. Are the assembler and the customer on the same page? The assembler pass/fail criteria for ionics may be less stringent than that of the customer. The customer may want the specificity of ion chromatography rather than more general resistivity testing.
What does the customer actually want? It is important to understand what standards or test methods are being invoked. There are industry-standard test methods; many of them are generated by IPC. We tend to associate IPC standards with aerospace applications. However, ASTM standards or ISO standards are often used in other areas such as in medical devices. The customer may not even be concerned with ionic contamination. They may require determination of particulates or non-volatile residue.
What does the customer actually need? Of course, as an electronics assembler, you need to meet customer requirements. However, on occasion, the testing that customers demand is not what they actually need to assure reliability. Even if the contract with the customer does not specify ionics testing, as an assembler, you may want to add it in, depending on the ultimate use requirements. Risk/benefit analysis that can include factors such as long-term reliability and operating environment have to be considered.
Testing and Quality
Testing is part of good manufacturing practice, of a good quality program. Notice that we discussed the cleaning process, ionics testing, and other possible tests. We suggest practices that combine the aspects of “how clean is clean enough” with “what processes do we use to achieve cleanliness.” This means that even if you don’t clean the no-clean flux, documenting the specifics of the flux and the assembly process is essential. Even if you don’t do ionics testing, justifying why ionics testing is not done- and doing so in writing – is also essential. Even if your manufacturing application is not electronic assembly, these principles of understanding why cleaning is needed, how cleaning processes perform, and how you know that you have reached, or not reached, a desired performance level are important to embrace.
