Why failure-inducing OA fluxes are passing testing.

In recent years, several factors have converged to create a perfect storm in the cleaning world, significantly affecting the long-term reliability of improperly
cleaned assemblies.

First, exciting design innovations continue to support the increasing demand for miniaturization. The resulting complex geometries, densely packed assemblies and low-standoff spacing increase the difficulty of removing flux residue, thereby elevating the risk of partically cleaned residues under low-standoff components.

Next, this challenge is increasing at a time when processes have been rapidly converted from eutectic to lead-free pastes. The associated higher reflow temperatures of lead-free processes result in increased amounts of “burned in” residues that are polymerized and hardened more than their eutectic counterparts.

Finally, the demand for high-reliability products is at an all-time high, and field failures can be devastating not only for manufacturers, but for the end-users of these products as well. All too often, electronic assembly failures are traced back to residues remaining under low-standoff components, even after boards have passed visual inspection and ionic testing. Therefore, the ability to fully remove post-soldered flux residues is an essential requirement practiced by high-reliability assembly manufacturers in military, aerospace, automotive, medical, and other critical segments of the electronics industry.

To fully remove residues and ensure reliability, many customers strive to overcome these challenges by turning toward the use of improved cleaning agents. Alternatively, others employ the use of water-soluble (OA) flux, which avoids the use of cleaning chemistry and offers the promise of lower cost and ease of removal. For many, the implementation of OA flux followed by a water cleaning process successfully results in boards that pass inspection methods and are problem-free. Correct? In reality, this is far from the truth.

We are increasingly contacted by customers who experience reliability problems and field failures directly resulting from aggressive OA flux residues remaining under components. Without exception, these customers are completely surprised to learn that their cleaning processes and inspection methods failed to prevent this from occurring. One can certainly understand their surprise, as OA flux is specifically formulated to facilitate ease of removal with water only. And when cleanliness assessments are performed properly, the customer would be correct to assume that they have successfully eliminated the possibility of cleaning-related reliability issues. However, this is not a problem related to flux formulation or the ability of water to remove these OA fluxes. The problem is really twofold. First, there are the limitations of water to penetrate tight spaces to access and fully remove the OA flux. Next, the problem is related to the limitations of nondestructive test methods such as visual inspection and ionic contamination to detect these residues.

There is no disputing the fact that heated spray-in-air water processes are fully capable of removing OA flux residues from the surface. However, complex geometries and tight spacing negatively affect the ability of DI water to reach and remove OA flux from underneath critical components due to its relatively high surface tension. This results in boards that easily pass visual inspection requirements.

Furthermore, assemblies cleaned in this manner very often pass ionic contamination testing, since the test result depends on the ability of IPA and water to fully extract residues from not only the board surface, but from underneath components. Herein lies the problem. How can one reasonably expect IPA and water under the static conditions of ionic contamination testing to succeed in extracting residues that remained trapped through a heated, pressurized spray-in-air machine such as an inline?

Based on the widespread use of OA flux and water-only cleaning processes in a market, we have seen a dramatic increase in cleaning-related issues with companies that have successfully cleaned with water for many years. We believe this trend will continue.

Does this mean that we are advocating a migration from processes utilizing OA fluxes? Absolutely not. However, customers concerned about long-term reliability should certainly be aware of the limitations of DI water cleaning processes to penetrate under low-standoff devices to fully remove residues. This ability varies greatly depending on board design differences and wash process capabilities. Therefore, when implementing a process, it is important to look beyond visual inspection of the surface, which is easily cleanable and includes destructive testing of representative test vehicles to look underneath components to see if a water cleaning process is suitable. This is especially important, since ionic contamination has been shown to also be unreliable as well, given the inherent problems associated with IPA/water to fully extract residues from under difficult components.

Rest assured, residues will be found under low-standoff components. There are design considerations related to cleaning processes, cleaning equipment modifications or chemistry that can aid in removing those residues. We’ll discuss more on that in future columns.

Michael McCutchen is vice president Americas and South Asia at Zestron (zestron.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it .