At the EMPF Demonstration Factory, most printed circuit boards (PCB) assemblies (those with no clean fluxes and pastes) go through an inline or batch cleaning process before inspection. PCB assembly cleaning is a process. It requires equipment and a basic knowledge of chemistry.

In recent years, the cleaning of PCBs and stencils has become an important issue for an increasing number of SMT manufacturers. High reliability for PCB assemblies by an effective cleaning process is especially necessary in areas such as military, aviation, telecommunication, medical and the automotive industry where densely populated PCB and devices with small lead spacing are used. The need for increased reliability is growing and complex circuit boards (e.g. chip on board and Micro BGA applications) and small lead spacing are requiring strict cleanliness guidelines for the use of conformal coatings in automotive, radio frequency and telecommunication applications. Dramatic growth is also expected to occur in the US, Europe and Asia caused by the move towards lead-free solders. This is due in part to the higher temperatures "baking" lead-free pastes and fluxes. Cleaning applications can be divided into three main areas:

Removal of flux residues from PCB assemblies after soldering
Removal of solder paste or adhesive from stencils or misprints
The cleaning of tools (dispensing nozzles, solder pallets, cooling zones of ovens, and squeegees)

In a typical SMT assembly line, solder paste is printed through screens or stencils. Additionally, specially formulated SMT adhesives can be printed through stencils as an alternative to using dispensing needles. The solder paste and adhesive must be cleaned periodically from the stencil in order to maintain quality demands. Decreasing pitches down to less than 0.3 mm give rise to stringent requirements regarding stencil cleanliness. Under these conditions a flawless printed image is very important in order to achieve acceptable soldering results. Scrapping misprinted PCBs can be very expensive, particularly if components are already mounted onto one side of the board. Studies have revealed that inadequate paste printing causes more than 60 percent of the defects arising during the SMT process. This can be the result of insufficient cleaning of solder paste mis-prints that is oftentimes done manually. Due to its nature, manual cleaning does not guarantee repeatable results. Therefore a specified and well working stencil cleaning process is imperative.

Satisfactory results in stencil cleaning with alkaline, surfactant-based systems can only be achieved with certain solder pastes, however new requirements for the removal of adhesives and flux residues have prompted the development of more advanced cleaning technologies.

These problems set the stage for development of technology referred to as (Micro-Phase Cleaning, registered by Zestron® Corporation). Under pre-specified conditions the micro phase cleaner changes from a clear, colorless solution to form an MPC emulsion. "Water-like" properties allow it to effectively remove and dissolve ionic contaminants from a surface. Non-polar and organic residues are removed by contact with the hydrophobic or "solvent-like" phase. Rinsing with water becomes unnecessary since the cleaner evaporates residue free. MPC technology is therefore able to remove contaminants such as epoxy adhesives, solder pastes, and most flux residues. The emulsion works as a transfer medium for removing the contamination from the surface to the water phase where it is not soluble and therefore can be filtered. Due to this unique property, the contaminant can be easily filtered from the MPC bath, thereby allowing the cleaning agent to be continuously regenerated.

Although stencil cleaning drastically improves the printing results, misprints cannot be completely avoided.

• The type of contamination
• The design complexity of the substrate
• Equipment requirements

MPC technology has resulted in a halogen-free, non-flammable cleaner with a low odor level that is suitable for cleaning all types of flux residues, solder pastes, and adhesives. The non-flammable and non-foaming properties of these cleaners allow it to be used in most types of equipment (e.g. in-line and batch applications). In many cases, this agent can even be used as a drop-in replacement in equipment such as ultrasonic equipment, which are capable of applying the cleaning agent as a liquid.A MPC based cleaning agent, VIGON® A 200, removes contaminants from the board's surface and transfers them to the aqueous medium where they are filtered. The cleaning agent does not become progressively depleted of active components and is therefore self-regenerating. Its surfactant-free formulation eliminates time consuming bath monitoring and frequent foaming issues, a problem associated with many surfactant-based cleaners. The prolonged service life of the cleaner results in a much lower consumption rate, which significantly reduces labor-intensive bath changes as well as transportation and disposal costs. Additionally, the low VOC level is in compliance with the latest health standards and environmental regulations. Over the last four years, more than 200 fluxes and solder pastes have been successfully tested in high and low-pressure spray equipment such as in-line, batch and centrifugal equipment.

The following methods provide accurate information on the level of contamination:

• Measuring refractive indexes of the cleaning agent
• Determining surface insulation resistance
• Conductivity measurements
• Ionic contamination measurements
• Flux tests (Figure 3)

In the near future, a combination of more densely populated boards, decreases in lead spacing, and a constant need for increasing reliability will make cleaning more of a challenge. As the number of choices increase, it becomes more difficult for the user to find the best possible cleaning process for their specific needs. It is recommended to discuss your requirements with a specialist who is knowledgeable with all of the cleaning processes (solvent, semi-aqueous or aqueous) to develop an effective solution.

Upon request, the EMPF is able to perform cleaning demonstrations of in-line and batch cleaning equipment. The EMPF uses these products for cleaning boards in its Demonstration Factory. Specific customer testing and research projects for lead-free solder applications to remove RMA, OA and no-clean residue from the PCB assemblies are also performed using these products.

If you would like to have a demonstration of this cleaning process, please call Jeff Stong at the EMPF at (610) 362-1200 x224 or e-mail jstong@aciusa.org