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Cleaning
December 31, 1969 |Estimated reading time: 13 minutes
By Mike Bixenman
In the era of "no-clean" solder pastes, precision cleaning of assemblies has assumed a new dimension. The question assemblers must ask is determined by whether cleaning is indicated by a PCB's reliability demands; i.e., is the cleaning step required so that the end product will perform in the designated environment for a minimum period or number of cycles?
During the '90s, printed circuit board (PCB) assembly has taken on new dimensions. Vapor-degreasing solvents are phased out because of environmental regulation. "No-clean" processes constitute a desirable technology shift; and when cleaning is contemplated, water-soluble fluxes are generally the choice; finally, cleanliness specifications are not required for bare boards and components, because the completed assemblies are normally washed.
The 'End' of Cleaning?Rosin flux is an excellent "encapsulant," which means that it leaves a nonionic residue when formulated free of ionic activators. Ignoring residue on the board is an attractive option for those looking to eliminate ozone-depleting chemicals. For that reason, considerable time, effort and investment are devoted to no-clean technology. As a result, many technological advances in low-solids flux and machine technology were developed to support this technology shift. The question now becomes, why clean?
Actually, there is a common misconception that the cleaning of assemblies is no longer required. But freedom from harmful contaminants that adversely affect functionality is as important to reliability as the robustness of the solder connections. Cleaning still plays an important role even in the successful implementation of a no-clean process. Incoming parts as well as control of the assembly environment still must meet cleanliness standards if post-solder cleaning is to be eliminated.
High-reliability applications require the benefits of controlled, established post-solder cleaning processes, that use chemistries optimized for the material to be cleaned. Those applications cannot adhere to the requirements of a no-clean process. Moreover, continual developments in SMT have necessitated additional cleaning. For example, the tighter lines and spaces of area-array, chip scale and flip chip packages on assemblies require cleaning under their many I/O interconnects. Cleaning is an important operation for optimum performance of assemblies requiring conformal coatings.
Design the PCB with Cleaning in MindWhen assemblies require post-solder cleaning, there may be tradeoffs to consider when designing for parts layout, soldering and cleaning. With high-density surface mount assemblies, the designer should provide the ability to clean. The following are design options worth considering:
- Optimum orientation of components
- Minimal obstructions for spraying under components
- Use of high-energy jets for wash chemistry
- Use of fine sprays for rinsing (liquid droplets are
- smaller than spaces between components and substrate)
- Elimination of "shadowing," i.e., taller components are placed farthest from spray nozzles; smallest parts are closest
- Parts placement (see sidebar A): Passive chip components (or cylindrical through-hole types) are oriented so that their long axes are perpendicular to the direction of spray. Dual in-line and small-outline integrated circuit (SOIC) components are placed lengthwise along the line of spray. Orientation of components with connections along all sides of a square (e.g., quad flat packs (QFP) or leadless ceramic chip carriers) is such that leading edges are 90° to the direction of spray.
Overall, component placement should be planned so that the "best" evacuation of liquid is accommodated, e.g., in distinct rows with spacing for liquids to drain off the sides of the boards, and with the assurance that cleaning is never impeded when ball grid arrays (BGA) or chip scale packages (CSP) are present. Designers should bear in mind the product's appearance, what it does and how it is to be manufactured. Specifically, how does each step in the assembly process affect all the others? Assemblers who recognize those requirements at the outset of design and integrate them for total optimization hold the key to success.
Pre-assembly DevicesContaminant residues can be introduced to final assemblies via their initial presence on pre-assembly devices and materials. If not removed from parts and boards before assembly, there is no assurance they will be removed later especially if no-clean techniques are used. This is even more urgent in the presence of high-density-interconnect structures where any contamination would likely reside because of the small spacings. Any success with low-residue, no-clean soldering processes depends directly on the cleanliness of incoming bare boards together with overall component cleanliness.
Nonpolar residues in the form of particulates, oils or films are often found on components while polar residues, from incomplete cleaning during parts assembly, may result from plating or flux residues. The source of pre-assembly parts must be held to a cleanliness specification.
Contaminants Encountered During AssemblyContaminants are not foreign to the assembly process. A growing concern with both plated through-hole and surface mount components is the generally tighter spacings encountered. Standoff heights and lead pitches have steadily shrunk, as have intertrace distances. In addition, even with modern manufacturing processes, more tenacious residues are resulting.
Residues consist of ionic (polar), nonionic (nonpolar) and particulate soils. Ionic residues consist of a flux activator, residual plating salts and handling soils; the nonionics contain reacted, nonvolatile residues in flux, oils, greases, fusing fluids and poorly dissociated materials. The particulate residues consist of solder balls or dross, handling soils, drilling or routing dust, and airborne matter.
Polar residues form ions when dissolved in water. For example, when salt in a fingerprint deposit dissolves in water, the sodium-chloride molecule dissociates into positive sodium ions and negative chloride ions. In its ionized form, NaCl increases the electrical conductivity of water, which can cause signal changes in circuits and initiate electromigration and corrosion. Typical polar residues come from plating and etching materials, chemicals from the substrate or component fabrication process, water-soluble solder-flux constituents, activators from rosin or synthetic-activated solder flux, flux reaction products, water-soluble soldermask constituents and deposits from manual handling.
Nonpolar residues consist of contaminants that do not form ions when dissolved in water. They may be hydrophilic (water loving) or hydrophobic (water hating, generally oil loving). Hygroscopic materials may enhance the formation of surface water films, which can result in a decrease in surface resistivity. Also, under favorable conditions, electromigration may occur. Nonionic residues, which are insoluble in water, consist of rosin, synthetic resin, organic compounds from low-residue/no-clean flux formulations, plasticizers from core flux, reaction products, greases and oils, finger print oils, release agents on components, insoluble inorganic compounds and rheological additives to solder pastes.
Particulate residues typically require mechanical energy for their removal. Common particulate residues are siliceous material from dust and dirt, hydrolyzed or oxidized rosin, some flux reaction products (some white residues), silicone greases/oils, glass fiber from laminates, silica and clay-type fillers of soldermasks, and solder balls and dross.
Finally, if the assembler chooses a no-clean process, it is imperative that incoming parts meet required cleanliness standards, board handling be done with gloves or finger cots, and soldering processes be controlled to ensure a minimum level of contamination. If nitrogen is specified for inerting the soldering process, those stricter process controls must be implemented. The cost of a true no-clean process can be similar to post-solder cleaning because of new capital equipment investment plus nitrogen consumption. Subsequent operations, such as wire bonding, die attach, inspection, touch-up, repair/rework, testing, intermediate handling, storage and final assembly, are performed inclusive with soldering operations. If assembly is not committed to proper handling techniques, the level of cleanliness required for a reliable product may be compromised.
Post-wavesolder/Reflow CleaningFlux technology largely determines the required cleaning media. Water-soluble flux removal typically requires water with an additive, high-pressure mechanical energy and temperature. Removal of rosin flux calls for either a solvent, semiaqueous or aqueous cleaning media. Because of the wide range of marketed formulation technologies, low-residue/no-clean flux removal depends on the product to be cleaned. Synthetic flux removal typically requires a solvent or semiaqueous process; a number of cleaning technologies are suitable for removing its residues.
Temperature profiles in the soldering process must be specific, controlled and documented, especially if the product matrix is varied. For example, a densely populated multilayer board will require a specific profile vs. that for a sparsely populated PCB. For those reasons, paste manufacturers provide process guidelines to be followed when selecting the reflow profile. Multiple soldering processes before removal of the flux residue can cause its polymerization. These residues are more difficult to remove because polymerization produces a substance of higher molecular weight, making the contaminant harder to dissolve in the cleaning agent.
Low-solids or low-rosin/resin fluxes typically leave a lower concentration of post-solder residue. A variety of these fluxes are available. However, because the ability to remove a residue can vary with different end products, the assembler must choose a cleaning medium that is compatible with the residue. Overheating or extended heating during the soldering operation can produce residues that are difficult to remove.
Post-solder Cleaning Media and EquipmentSemiaqueous agents comprise a group of materials that "solvate" the soil, followed by removal of contaminants via water rinse. The solvents typically have low vapor pressures, flash points in excess of 150°F, low volatility, affinity for a range of flux residues, long bath life and are non-ozone depleting. Products offered under that category include both water-soluble and insoluble organic solvents for which water is the rinsing agent.
Water-soluble semiaqueous solvents consist of heavy alcohols, glycol ethers and cyclic amines. One of their advantages is ease of removal: Entrapment is not typically a problem. Disadvantages include the complexity of treating the rinse water when closed-loop recycling is desired.
Water-insoluble semiaqueous solvents consist of terpene, hydrocarbon blends, dibasic esters and glycol ether. They require mechanical impingement in the first-stage rinse to ensure adequate removal. Here, entrapment could be an issue. However, one distinct advantage is the ability to separate the cleaning fluid from the rinse stream when closed-loop zero discharge is required. Equipment for semiaqueous cleaning processes typically consists of spray under immersion, centrifugal and ultrasonic agitation.
Overall, the solvents do an excellent job of removing rosin, low-solids and synthetic-activated flux residues. Also, economics are typically favorable because of long bath life, minimal loss and high soil-loading capacity.
Aqueous cleaning offers a series of options for the assembler. The evolution of aqueous cleaning media and equipment has yielded a technology that offers proven processes for the fine cleaning of assemblies. As with any technology, aqueous cleaning has its limitations and peculiarities, which must be understood before process selection is made.
Water is a better solvent for polar-type ionic materials than the organics. Conversely, water is less effective for the removal of nonpolar materials such as greases, fats, oils and rosin. Water, solely or with minor additives, is an excellent solvent for removal of water-soluble organic acid (polar solution) solder flux after soldering. When removing polar and nonpolar soils such as rosin/resin, light oil and synthetic residues, additives are required.
Aqueous cleaning chemistries also offer a range of usage options. However, as stated, only under appropriate process conditions is water suitable for removing many organic-acid (OA) flux residues. Some OA residues require additives, with water, at low concentration, to assist in wetting, defoaming and displacing salts. Aqueous-based saponifiers remove both polar and non-polar soils such as rosin flux. Inorganic-based saponifers have evolved and offer an option for rosin-flux removal in areas where air emissions are regulated. Lastly, organic-solvent mixtures with water offer highly effective cleaning agents for some of the tougher low-residue and synthetic-based fluxes.
The equipment market offers an array of applications for different aqueous cleaning techniques, including use of ultrasonics, centrifugal forces, brush, immersion and spray in air. A broad selection of equipment design features accommodate the many different requirements for performance, cost, floor space, waste management, and safety and health issues (see sidebar B).
Solvent cleaning uses a solvent medium instead of water for washing and rinsing parts and assemblies. It is understood that drying is accomplished via evaporation of any residual liquid generated in the vapor zone by boiling the solvent.
Assembly soils consist of ionic (flux activators, salts) and nonionic (rosin/resin, oils, particulate) soils. Most solvents are nonpolar and excel at removing nonpolar soils, but are poor in removing polar-type ionic soils. In response, bipolar-solvent compositions have been developed that remove both polar and non-polar soils. The nonpolar component is often a halogenated solvent, either chlorinated, brominated, chlorofluorinated or hydrochlorinated. The polar-component solvent is typically an alcohol such as methanol, ethanol, normal propanol or isopropanol. Many of those mixtures take on a common property (referred to as an azeotrope), which permits their behavior as a single substance the vapor produced by partial evaporation of liquid has the same composition as the liquid.
Each type of solvent has advantages and disadvantages, and selection will depend on the type of contaminants to be removed and the material compositions of the assemblies to be cleaned. Equipment to process the cleaning agents is designed to address conservation, workplace exposure, emissions and reclamation. Table 1 displays a flow diagram of the choices for assemblers: aqueous, semiaqueous and solvent cleaning agents.
SummaryElectronic assemblies now provide faster and better operations than ever before; circuit traces are narrower and assemblies more densely populated. Yet, to ensure that those products perform their specified functions in the intended environments for a minimum period or number of cycles, cleaning must still play a large role.
WORKS CONSULTED
- IPC-SA-61, Post-solder Semiaqueous Cleaning Handbook, July 1995.
- IPC-SC-60, Post-solder Solvent Cleaning Handbook, April 1997.
- IPC-AC-62A, Aqueous Post-solder Cleaning Handbook, January 1996.
- E. Surrette, Cleaning in the Era of No-clean, February 1999.
- Requirements for Soldering Fluxes (IPC ANSI/J-STD-004), January 1995.
- Requirements for Soldering Pastes (IPC ANSI/JSTD-005), January 1995.
- L. Hymes, Cleaning Printed Circuit Boards in Today's Environment, 1990.
MIKE BIXENMAN is chief technology officer at Kyzen Corp., 430 Harding Industrial Dr., Nashville, TN 37211; (615) 831-0888; E-mail: mike_bixenman@kyzen.com.
Component Placement Challenges for CleaningSidebar AIn an example of a PCB intended to present difficulties for any cleaning process (i.e., to fail), sides a and b represent the primary and secondary board layouts intended for evaluation (utilizing a fixed front-end reflow-soldering process). The PCB is 3 x 4 x 0.060" of FR-4 material with a liquid photoimageable soldermask; testing follows IPC-specified pad geometries and sizes. Components include PLCC-64s and -32s (0.050"), SOT-23s, 6301 chip resistors, SOIC-16s (0.050") and LCC-32s. Wavesoldered parts include 18-pin DIPs and DL resistor packages. Cleaning challenges include those with components situated on axes X and Y (A); those displaying a tight "bundling" (B); and the small parts placed in mutual proximity (C), theoretically to induce a higher ΔT during reflow than the larger PLCCs. (D, E, F and G present similar difficulties for spray cleaning.) A higher reflow temperature slightly out of spec from a solder paste manufacturer's recommendation will create yet another cleaning challenge, as it tends to render those areas as difficult to clean as large or shadowing components.
ABCs of SMTAzeotrope: A mixture of two or more polar and nonpolar solvents that acts as a single solvent (boiling point is lower than that of either of its components) to remove both types of contaminants.
Contamination: Any foreign materials, such as dirt, oil, epoxy, etc., on a component's leads or pads that impedes solderability.
Dispersant: A chemical additive to water to improve removability of particulates.
Halides: Compounds containing fluorine, chlorine bromine, iodine or astatine that may be part of the flux-activator system, the ionic residues of which must be removed.
No-clean paste: A very-low-residue soldering paste having a solids content between 2.1 and 2.8 percent by weight.
Omegameter: An instrument that measures ionic residues on PCBs by: immersing an assembly into a water/alcohol mixture having a known high resistivity, and measuring and recording the drop in resistivity because of ionic residue taken over a specified period.
Saponifier: An aqueous, soapy solution of organic or inorganic bases and additives for removing rosins and water-soluble fluxes.
Solids content: The percentage by weight of rosin in a flux formulation.