PCB Cleaning Equipment Configurations, Techniques, and Why Cleaning Is Important

Posted on: 01/01/2001 PCB Cleaning Equipment Configurations, Techniques, and Why Cleaning Is Important By Mark J. Palmer Cleaning equipment configur...
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Posted on: 01/01/2001

PCB Cleaning Equipment Configurations, Techniques, and Why Cleaning Is Important By Mark J. Palmer

Cleaning equipment configurations and techniques reduce bare-board contamination levels. The article diagrams cleaning unit layouts to give the fabricator the ability to meet assembler or OEM incoming material acceptance specifications.

Traditionally the PCB has been cleaned with tap water after HASL and at final clean. Whether water is tap, deionized or supplied via reverse osmosis it has an inherent high surface tension of 73 dyne1 per centimete Tap water also has the drawback of possibly containing ionic material such as nitrates, sulfates and chlorid Overcoming these physical limitations is addressed by showing stepped improvements.

Basic HASL cleaning systems that are now employed are of an "inline" configuration with three cascade forward water chambers and a drying section. The first chamber, wash: second chamber, primary rinse: third chamber, final rinse: and the fourth a heated forced air dryer as in Figure 1. Post HASL conveyor speeds are generally in excess of six feet per minute and traveling through cleaning spaces of less than two feet in length. The use of 140°F tap water will increase the dissolution rate of the HASL flux residues versus the use of ambient temperature water. Air or "oil quench" cool down is necessary after HASL to prevent warping and thermal stress to the laminate as well as the masking materials. Depending on the mask and laminate's thermal threshold; elevating or keeping the pane core Post HASL temperature between 210-250°F is recommended for a deeper cleaning. Masking polymer epoxy resins laminates (like FR-4 laminate) vary in the amount, size and depth of the micro-pores/pittings th are inherent to these types of material surfaces. Keeping the panel hotter, before it enters a 140°F + cleanin medium, will keep the micro-pore/pitting areas expanded. This expansion from the elevated core temperatu will allow the cleaning media to penetrate deeper. Consulting with the manufacturer of the material and sett a design of experiment for thermal stress testing is highly advised. Internal heating elements can be used in conjunction with the external heating source(s). The use of heated deionized water will require heating elements and tanks that do not contain copper, brass or other reactive metals. The installation of stainless steel jacketed heating elements along with using high temperature plastics, stainless steel or glass lined tanks is recommended. Otherwise reactive metals will eventually

corrode.

The 140°F temperature that is used in Figure 2 and following diagrams can be increased for application or throughput considerations. Higher cleaning and/or rinsing temperatures will generally equal lower contamination levels.

Figure 3 shows the addition of an oscillating nylon brush system to the wash section. These brush systems will oscillate approximately 200 times per minute. The addition of the brush system allows for furrowing by the brush tines into the HASL flux layer. This furrowing effect breaks the surface tension of the HASL flux residue and decreases the time required for the cleaning media to penetrate and dissolve the HASL flux layer.

Figure 4 shows the addition of a pre-wash module that precedes the wash module. By utilizing this module, the gross residue of the HASL flux layer can be substantially reduced, allowing the brush system in the wash section to work more effectively. If the HASL layer is reduced as the panel enters the wash, after the pre-wash, the brush will be contacting the PCB's actual surface sooner; therefore the embedded residues in the pores of mask and exposed laminate have a better opportunity of being deep cleaned.

In Figures 1-4, the equipment has been supplied with tap water. The cleaning equipment diagrams in Figur 5-9 show the incorporation of deionized water. Deionized water is preferred due to its normally higher resist value and negligible contaminant content when compared with reverse osmosis (R/O) treated water. If the l rinsing of the panel is being done with a pure and highly resistive water (before it enters the drying section), there will be less ionic contaminants that can be potentially dried to the panel's surface.

Figure 5 will also use a tap water wash, with the addition of a heated tap water prewash. A hard water pre-wash is preferred as it tends to be ion laden and will actually give a more aggressive washing/rinsing action versus softened or deionized water (see Figure 5).

Figure 6 has been designed with a brush system in the pre-wash module. This additional brush system increases the prewash's effectiveness and allows for elevated conveyor speeds needed for higher production throughputs.

Figure 7 will incorporate four features that enable a cleaning unit to hold and then utilize chemical additives. These additives can be rinse aids for reducing the surface tension of the water(s) or saponifiying agents that help dissolve and neutralize the acidic HASL flux residues. Solvents are rarely used in HASL flux residue removal due to today's disposal and environmental concerns. The first feature (A) is the isolated recirculation of the wash module. The second (B) is the utilization of High Volume Low Pressure (HVLP) sprays in the wash module(s). The third (C) is the use of ambient air knives at the end of the wash module that decreases the down stream "drag-out" of chemistry from the chemical wash section(s). The fourth (D) is the addition of a "wet" isolation" section. This "wet" isolation section is installed to remove wash chemistry that has not been removed by the ambient air knives. These "wet" isolation sections are

generally set with low volume spray nozzles. Depending on the spray bar diameter, line pressure and nozzl apertures, the residual chemistry "drag-out" that has passed the ambient air knives, can be reduced by dilu the wash solution by a factor of 103. This reduction will minimize the amount of chemical entering the rinse water modules. If the rinse water is being regenerated through R/O membranes or deionizing resin beds; th "wet" isolation protects the membranes from excessive chemical fouling and will prolong the life of the resin the D.I. exchange tanks. In Figure 7, this Post HASL cleaning configuration can address almost any PCB cleaning challenge. Any module can be elongated for conveyor speed requirements. Retrofitting can be costly, so analyzing the cost of the purchase of a new properly configured unit is prudent. If an older machine is to be retrofitted for with a chemical, it needs to be thoroughly cleaned, inspected for leaks and the unit's construction materials checked for compatibility.

Case Study Table 1 is a comparative study of three groups of PCBs (A, B, C-three boards per group). All groups were cleaned and rinsed directly after the HASL process, with ambient tap water only, at the original fabricator. Subsequent remedial cleaning was performed on Groups B and C. All the groups were the from the same lot. Group B was remedial cleaned with a heated saponifier wash and heated deionized rinses in a post assembly "inline" cleaning unit. Group C was remedial cleaned with a lower temperature saponifier wash and brush system using lower temperature deionized rinses in a standard HASL post-cleaner. Table 1 lists the cleaning histor used for the three groups.

The #9619s were very dense mixed technology PCBs. The HASL Post cleaner used to clean Group C was very similar to the unit diagrammed in Figure 8., with the exception of wash and rinse temperatures, 125°F versus 140°F.

Table Data Conclusions Group A: water alone, and especially tap water alone, is insufficient to reduce harmful residues to levels which do not produce electrical failures. Group B: the remedial cleaning of this group in the "inline" assembly cleaning unit with 3% saponifier effectively reduced the residues to acceptable levels. This was due to the saponifier, HVLP nozzles and deionized water. Group C: the shorter PCB post HASL cleaning unit reduced chloride levels significantly below the larger PCA "inline" cleaner. These results are believed to be due to the high volume of wash solution turn over and the brush system.

Figure 8 shows the standard HASL Post cleaner (Figure 1) re-configured with several of the improvements that have been aforementioned. See Figure 8.

If a production floor's footage accommodates the footprint, an enhancement to the system in Figure 8 would be the addition of a heated tap water pre-wash module with a brush system.

This pre-wash module, as in Figure 9, would decrease the "loading of the chemistry in the wash module, by removing the gross HASL flux residue upstream.

Conclusion Today's use of halide free "no clean" flux by the assembler has made it imperative that the fabricator provid a cleaner starting platform. With the rapid size reductions in PCB SMT pad spacing, via diameters and plate through holes combined with the inherent limitation of water, there must be changes to existing processes. Destructive electrochemical migration/dendritic growth is becoming more frequent as circuitry shrinks. A corrosion cell's formation is usually the result of improper cleaning. A cleaning protocol that was designed fo 20-mil pitch and 18-mil tented vias may be ineffective for 5-mil pitch and 13-mil tented vias. Water alone can penetrate very tight geometries, however it can not always be rinsed back out effectively. A example of this is the rinsing of tented vias or under the interface of a solder mask overcut where it meets t laminate. Water alone can not remove fingerprint oils or solvent residues that have out-gassed from an LPI mask during "bake-out." Unless water is sprayed at exceptionally high psi, it has difficulty removing routing particulate. New machine designs using deionized water, in conjunction with the use of chemical additives (that reduce the surface tension of water to 30-40 dyne per centimeter range) and final cleaning before shipping are all viable options for improvement.

Acknowledgments I would like to thank Mr. Al Rowe and Mr. David Belovsky of Marseco, Inc. for supplying machinery diagram and technical information; and Mr. Terry Munson of Contamination Studies Laboratories for contributing ion chromatography data. To contact Mark Palmer, phone 408-894-9901.

This article was presented at IPC Printed Circuits Expo, April 2000, and is reprinted here with permission.

References 1 Richard J. Lewis, Sr. Hawley's Condensed Chemical Dictionary Twelfth Edition, Van Nostrand Reinhold, New York 1993 pp. 1109.

2 Ibid. pp. 582-583.

Mark J. Palmer is with Envirosense Inc. located in San Jose, CA.

Related Websites z

Envirosense, Inc. www.envirosense-inc.com