Clean Sweep Print E-mail
User Rating: / 0
PoorBest 
Written by Mike Buetow   
Monday, 06 December 2010 16:21

Bix’s tricks: Kyzen CTO Mike Bixenman

Relegated to the highest-reliability products by the advent of no-cleans, cleaning has made a major comeback, even in certain handheld devices. As chief technical officer of a leading supplier of cleaning chemistries, Mike Bixenman of Kyzen has watched the pendulum swing both ways. He reflected on the cycles with CIRCUITS ASSEMBLY'S Mike Buetow in October. 

CA: Was there a tipping point that turned the cleaners back on?
MB:
Over the past decade, we have witnessed the rapid rate of disruptive product innovations. Cleaning gained importance when reliability and yields depended on device functionality. With increased performance standards, residues that can cause leakage currents are once again cleaned.

CA: In the past year, much has been made of pH neutral products. What are the advantages, and what are the limits?
MB: The rapid conversion to Pb-free requires improved flux compositions driven by higher soldering temperatures, poorer alloy wetting, and miniaturization. The bar has moved, making cleaning more difficult. Components with standoff gaps less than 0.002" allow flux residues to converge between conductions.

So where do pH neutral cleaning agents fit? One approach is to design a pH neutral cleaning agent. This has worked for stencil cleaning, wet solder paste and uncured adhesives. A second is to design a pH neutral cleaning agent that improves water-soluble flux cleaning. Pb-free and miniaturization push DI water as an effective cleaning agent for water-soluble fluxes. One viable alternative is a pH neutral cleaning agent that improves hydrogen bonding, permanent dipole interactions and solvation. A third approach is to design pH neutral cleaning agents that work within the neutral range. Expanding the cleaning agent design within the neutral range improves cleaning effectiveness on rosin and no-clean flux residues.

The limiting factor for pH neutral cleaning agents is the ability to clean no-clean and rosin-based flux residues. When the properties are a poor match for the residue, cleaning will suffer.

CA: How tied is chemistry effectiveness to the equipment itself?
MB: To achieve a repeatable and reproducible process, the cleaning agent must be matched to the soil and cleaning equipment. By rule, the static cleaning rate (rate at which the cleaning agent dissolves soil in the absence of impingement energy) plus the applied dynamic energy from the cleaning equipment equals the process cleaning rate. A boundary cleaning agent (poorly matched to the soil) processed in an excellent cleaning machine will produce subpar cleaning results. Conversely, a highly effective cleaning agent processed in a machine that limits exposure time, wash temperature effects, and impingement energy will produce subpar cleaning results.

The best cleaning processes first take into account both upstream and downstream processing conditions. The ultimate goal is a process that provides repeatability and reproducibility over time.

CA: J-STD-001D departs from earlier soldering standards in that it establishes cleanliness requirements only for R0 and R1 rosins. What does this mean for RE and OR flux users? Is Kyzen planning to make recommendations to customers on cleanliness levels?
MB: Assemblies are far too complex to provide a one-size-fits-all cleaning requirement. The standards are recommended as a reference to be used in the establishment and evaluation of design and process requirements. Standards rely on process control methodology to ensure consistent quality levels during the assembly process. J-STD-001 notes that flux types and cleaning agents shall produce products acceptable to the standard and product classification. When major elements of the proven processes are changed (e.g., flux, solder paste, cleaning media or system, solder alloy or soldering system), the changes are validated. The common test approach uses verification testing to ensure conformance to the end-use environment. It is up to the OEM to determine the limits that render an acceptable product for its intended use.

CA: What impact would the presence of resin residues have if one were to conformally coat? Would a coated board need lower residue levels than called out by standards for uncoated PCBs?
MB:
Gases and vapors permeate conformal coating material [and] research suggests some permeability to acids in flux. Ionic contaminants sealed within the coating can be solubilized by moist air moving into the coating, and then recrystalize when moist air moves out. This form of osmosis leads to conformal coating blistering, which compromises the integrity and adhesion of the coating. Under an electrical field, the ionic contaminants propagate, leading to electrochemical migration.

Typically for Class 2 and 3, conformal coatings are necessary for protection from harsh environments. With the advancement of more environmentally friendly conformal coatings and considerably more advanced application methods, conformal coatings are finding their way into mainstream assemblies as well. This has been compounded by increased complexity and miniaturization: Consumer manufacturer’s reputations are at risk.

Conformal coatings provide adequate protection to a certain environment only when applied correctly and film integrity is achieved. Film integrity can be characterized by even film thickness, minimal defects in the film, especially over or around conductors and coating adhesion. Most issues relating to coating thickness and defects are application- and material compatibility-related. Defects like dewetting, capillary flow and delamination are typically related to surface contaminants remaining on the assembly; specifically, nonionic contaminants, and thus the integrity of the film, are compromised, which will have implications on reliability.
Even when coating integrity is achieved, sufficient adhesion is not guaranteed. Adhesion depends on a coating material surface tension lower than the surface energy of the substrate; otherwise the material cannot wet the surface. Major compatibility concerns are related to solder resists that tend to have a low surface energy, mold release agents remaining on components and flux residues after soldering.

One common misconception is that conformal coatings are impermeable barriers; this is certainly not the case. Gases and vapors permeate conformal coating material. Although they are typically a good barrier to ionic and nonionic contaminants, research suggests some permeability to acids in flux. Within the flux residue, contaminants on the assembly are sealed prior to conformal coating. Ionic contaminants sealed within the coating can be solubilized by moist air moving into the coating, and then recrystalize when moist air moves out. This form of osmosis leads to conformal coating blistering, which compromises the integrity and adhesion of the coating. Under an electrical field, the ionic contaminants propagate, leading to electrochemical migration.

Last Updated on Thursday, 09 December 2010 12:56
 

Columns

Eastern-US: China’s New Competitor?

Parity emerges among EMS Factories from Asia, Mexico and the US.

For the first time in years we see parity in the Eastern US among EMS factories from Asia, Mexico and the US. This EMS market condition will permit American OEMs (the EMS industry refers to OEMs as customers) to have more EMS pathways to choose from. Now more than ever, such EMS assignments will require deeper investigation relating to the OEMs’ evaluation of manufacturing strategies.

Read more...
 
The Human Touch

For those who count on the electronics industry for big feats, it’s been a remarkable couple of years.

Read more...
 

Features

Advances in Concentration Monitoring and Closed-Loop Control

Contaminated bath water skews refractive index results. New technology can accurately measure aqueous cleaning agent concentration.

Read more...
 
Circuits Disassembly: Materials Characterization and Failure Analysis

A systematic approach to nonconventional methods of encapsulant removal.

Read more...
 

Search

Search

Login

CB Login

Language

Language

English French German Italian Portuguese Russian Spanish
 

Products

Panasonic Debuts PanaCIM Maintenance with Augmented Reality
PanaCIM Maintenance with Augmented Reality software provides instant communication and information to factory technicians -- when and where it is needed -- so they can respond to factory needs more...