When technological requirement for PCBs influence
board designs and assembly, manufacturers modify cleanliness specifications
to maintain cleanliness requirements. When technological requirement for PCBs influence
board designs and assembly, manufacturers modify cleanliness specifications
to maintain cleanliness requirements.
Manufacturers must consider the materials used, the board design, and the operating environment in order to establish custom cleanliness specifications to meet industry standards and specific customer needs.
Why is cleanliness a hot topic? Take a look at how ionic contamination influences the relationship between high-performance design requirements and the assembly environment and sources of field failure.
Higher impedance circuitry increases the impact of electrical leakage between leads, particularly under humid conditions. A blood serum analyzer is more sensitive to ion residues, and is more susceptible to ionic contamination than a board designed for lower impedance levels.
Higher trace potentials increase the risk of electromigration. This is particularly true as designers push the recommended limits for spacing between traces while fitting circuitry into smaller packages. Applications such as hybrid electric vehicles are seeing increased potentials coupled with almost infinite available power.
Harsher assembly environments increase the potential for corrosive residue. Assemblers are using a wider variety of materials than ever before. Because some of these materials perform masking or structural functions, the initial surface cleanliness is more critical since cleaning after these applications is difficult.
Due to variations in materials, assembly environments, and board designs, there are no all-encompassing residue levels to define clean or unclean status. Pauls and Munson with CSL (Contamination Studies Laboratory) generalize, "the higher the ionic residues, the greater the contribution to the overall assembly residues detected and the risk of electrochemical failures" (1).
These factors demonstrate that when design trends change, manufacturers must update their cleanliness requirements. Otherwise, customers may experience significant performance variances due to slight differences in cleanliness levels.
Cleanliness-conscious PCB manufacturers use only qualified and accepted fluxes in the manufacturing process. HASL fluxes contain amine hydro-bromide flux activators in a polyglycol carrier. If the solder mask does not cure completely, bromide residues penetrate the porous surface, causing corrosive damage. HASL water soluble flux (WSF) also increases levels of weak organic acid (WOA) and chloride residues. To prevent electrochemical failure, chemical content is closely monitored.
Mr. Tech Dweeb Tech Tip dsi recommendation for bareboard WSF levels:
- Chloride levels should be below 2.5 micrograms per square inch.
- Bromide levels should be under 2 micrograms per square inch.
Deionized water cleans more effectively than tap water, and typically has a higher resistive value and lower contaminant levels than reverse osmosis (R/O) treated water. Using highly resistive deionized water in the final rinse stage lowers the amount of ionic residue that can dry on the panel's surface. Use of a saponifier can help ensure complete residue removal.
Temperature levels throughout the HASL process affect cleaning. To increase the dissolution rate of flux residues, rinse water must be kept above 140°F. Depending on the thermal threshold of mask and laminate materials, manufacturers must maintain temperatures that allow cleaning materials to penetrate the surface completely. Rinse water temperatures can be raised above 140°F to the highest tolerable level for deeper cleaning and surface pore expansion.
Brush systems also improve board cleanliness. Oscillating approximately 200 times per minute, the bristles furrow into HASL flux layers and increase cleaning potential. This furrowing action disrupts the surface tension of the flux residue, increasing the dissolution rate and improving the effectiveness of the cleaning process.
Incoming components can also contribute to overall residue levels. PCB assemblers devoted to cleanliness cannot assume incoming components and boards are clean. In a recent study, CSL analyzed incoming bare boards and components, and found high methane sulfonic acid, chloride, and bromide concentrations, and high sulfate levels (2). These contaminants significantly increase electrical corrosion and electromigration risk.
Assemblers can reduce ionic residue with a saponified wash before assembly. However, ionic residues under the solder mask cannot be removed by the assembler and may continue to leach out during subsequent processing.
Cleanliness is also maintained through proper handling, a closely supervised assembly process, careful monitoring of rinse water quality, and initial cleanliness specifications. Equipment used to test PCB cleanliness and surface insulation resistance (SIR) will be discussed in future issues of the dsi Quarterly Tech Review.
Sources:
1. Munson, Terry, and Pauls, Doug. "Component Residues." Circuits Assembly. Nov 1999.
2. Munson, Terry. "Incoming Components as a Source of Contamination." Circuits Assembly. May 2001.
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