Process engineers are fully aware a number of contaminations, created during the soldering processes, will significantly impair the cross linkage of conformal coatings but are safely removed by an adequate cleaning process. There has been a clear and consistent increase in the demand for no-clean (eutectic as well as lead-free) products over the last few years. This trend coincides – and has been correlated – to the growth of observed in-field service failures, particularly with conformal coated assemblies.

Despite potential risks and shortcomings, the no-clean concept has established itself as a dominant production process for a majority of product segments. Unfortunately, many companies have learned a difficult lesson by adopting no-clean processes that include additional production steps, such as component underfill, or applied conformal coatings. At the same time, lessons learned within the cleaning community, have promoted new innovations, such as broader process windows, improved economics, higher industrial safety, and full material compatibility.

These improvements have assisted the return of cleaning as a value added production step. It has been said that cleaning behaves similarly to coating in that when introduced after the fact, it severely impacts the entire assembly process and becomes unnecessarily expensive! It is commonly accepted that the cleanliness of assemblies plays a vital part in the quality of subsequent production steps. Industry standards such as J-STD 001D, item 8, are the most commonly applied and are the generally accepted procedures for assessing cleanliness. Light-optical magnification, for example, is used within the electronic manufacturing industry to identify visible contamination. Unfortunately, some of the critical residues are not easily visualized. Ionic contamination measurement is currently the most widely used method. It quantifies the sodium chloride equivalent (VNaCl) in accordance to TM650. Geometries and surface areas of populated assemblies are unfortunately not taken into consideration. In other words, the measured values are only benchmark values and not scientific cleanliness assessments. For proper statistical control, three to five measurements of identical PCBs is the minimal requirement.

Current users of no-clean processes can attest to the very thin layered, transparent and often unrecognizable (i.e. by light optical inspection) flux-based films that remain. The latter are neither reliably authenticated via ionic contamination measurement nor can such residual contamination be cost-effectively substantiated using charge contrast measurements. Innovative and simple-to-use procedures (i.e. Flux Test kits) can complement the above-mentioned shortcomings. By means of selective discoloration of residual organic acids, this test not only provides proof of their existence but also visualizes their local distribution. In addition, new innovative tests have recently attracted considerable attention.

As the transition to lead-free materials continues, more users will refer to new innovative methods such as a contact angle measurement (CAM) or Flux test kits. Through the emergence of new levels and types of contamination, their analytical detection becomes significantly more important than ever. With gained control over the contamination and the adequate cleaning process, users will then have to address the types of conformal coatings and their respective material properties and behaviors. The susceptibility of assemblies to failure can range from delamination of conformal coatings to electrochemical migration, and even to the actual design of the circuit.

It has been demonstrated that the removal of contamination prior to conformal coating provides significant improvements in the adhesion of conformal coatings and at the same time reduces the possibilities of contamination induced failures. Furthermore, it has been established the onset of lead-free products, the level of contamination, and its respective impact add to the complexity of adequate process control. In light of this ongoing transition, research engineers are extending their investigation to provide even greater insight into lead-free initiated phenomena. This in turn will lead to more adequate analytical tools for detection and complementary know how for coating materials and their respective performance.

For additional information on the above article or to schedule a demonstration at the EMPF, contact Robert N. Berta, 610-362-1200 ext 253 or via e-mail at rberta@aciusa.org.