Effective Cleaning after Assembly

Electronics manufacturers have always treated cleaning as an essential process. They clean their products mainly to remove contaminants that are potentially harmful, including mainly solder, flux, and adhesive residue. Cleaning also removes other contaminants of a more general nature such as debris and dust left over from other manufacturing processes.

In the rapidly growing electronics industry, cleaning essentially improves product reliability and lifetime by ensuring high surface resistance and thereby preventing current leakages leading to PCB failure. As electronic equipment become smaller and smaller, the requirement for higher performance and better reliability grows stronger. Achieving high insulation resistance, therefore, requires electronic assemblies are essentially clean. This means electronic engineers must work closely with manufacturers of adhesives, fluxes, cleaning chemicals, and cleaning equipment to ensure reaching an optimal cleaning performance.

Stages Requiring Cleaning

Removing contaminants from the board surface requires cleaning even before the soldering process is completed. In fact, production stages prior to stenciling may also introduce contaminants that must be removed. After stenciling, adhesives may have to be removed, while after soldering it may be necessary to clean the board of corrosive flux residues and excess solder paste.

No Clean Process

The above requirements for cleaning at multiple steps has led manufacturers to adopt the ‘no clean’ process. Although the no clean process has flux with lower solid content as compared to that in traditional types, they still contain activators and rosin, which operators do not remove until prior to the coating or encapsulation process.

Residues left over from the no clean process, together with the additional unwanted chemicals that the missing cleaning stages collect, can adhere to the PCB surface and affect the performance of the protection media applied. Therefore, even advanced technologies involving ‘no clean’ fluxes do not guarantee a clean board after assembly, especially for the high-speed and high-frequency requirements of the electronic industry.

Additionally, cleaning stages are also required for removing the adhesives and coatings when re-work is necessary, for cleaning components, and for keeping the production line operational.

Methods of Cleaning

The electronic industry presently uses two main categories of cleaners. One of them is solvent-based, and the other, water-based. Although solvent-based cleaners dominated the market initially, they had the harmful property of depleting the ozone layer in the atmosphere. Therefore, manufacturers replaced solvent-based cleaners with a diverse range of solvent cleaners.

The new range of cleaners is further subdivided into three sections, non-flammable solvent cleaners, non-flammable halogenated solvent cleaners such as HFEs and HFCs, and flammable solvent cleaners.

Along with their individual advantages and disadvantages, features common to all the solvent cleaners include fast evaporation and single stage cleaning. However, even these new cleaners require specialist equipment including extraction as protection against toxicity and other possible hazards.

To limit ozone-depleting emission from solvents, manufacturers have also developed water-based cleaners, which have several advantages such as low odor, non-flammable properties, low/non VOC, and low toxicity over their solvent-based counterparts.

Applications for cleaning mainly depend on equipment such as spray under immersion, ultrasonic, or dishwasher type application. This makes it essential to identify the proper water-based cleaner for the specific job. In actual practice, water-based cleaners are much more complex compared to their solvent-based counterparts.

Effective Cleaning

Water-based cleaners work in different ways. Some make use of surfactant technology for removing contaminants from the surface of a PCB. They reduce the interfacial tensions and suspend or emulsify the contaminants in solution. Other types of water-based cleaners, mainly flux removers, use saponification to neutralize the flux acids. However, water-based cleaners require multiple stages in the cleaning process to complete the action. These include a two-stage process of rinsing, along with a final drying stage.

Glycols-based cleaners are a new type of surfactant-free water-based cleaners. They combine the advantages offered by solvent-based and water-based cleaners while requiring only minimal rinsing. Essentially, there are two main types of residues—non-ionic and ionic.

Ionic residues are flux residues and harmful material soldering leaves behind after the reflow process. Water-soluble organic or inorganic ionic residue often disassociate in a solution of charged ions, thereby increasing the overall conductivity of the solution. This degrades the reliability of electronic components and assemblies on the PCB as it contributes to current leakages between the circuitry. This also causes increased corrosion and promotes dendrite growth.

Non-ionic residues are mainly rosin, grease, and oils, and usually organic and non-conducting. Board fabrication or assembly processes usually leave behind this residue. The insulating properties of non-ionic residues are a problem for plug-in contacts or connectors on the assemblies. Presence of such residues can cause solder mask, potting compounds, conformal coatings to adhere poorly. Moreover, these can encapsulate foreign debris and ionic contamination.

Both non-ionic and ionic contaminants affect the operation and reliability of the PCB on which they are present. However, a larger proportion of circuit failures are due to ionic contamination.

Assessing Level of Cleanliness

As the electronic industry expands, and the cleaning market develops with it, it is important to define the metrics for assessing the cleanliness a specific application requires. As most flux residues and contaminants are invisible to the naked eye and even under magnification, it is important to use a proper method for determining the level of cleanliness achieved, and assessing compliance to the specified standard. A number of methods are available to assess the level of cleaning for both ionic and non-ionic contaminations.

Optical Process of Examination

Visual examination is an important process usually employed alongside other methods, although it does not provide quantitative information. It is the simplest method for monitoring both types of residue—the visual part, often accompanied by a magnification of around 10-15 times. This process is adequate for most production processes including handling and packaging.

Another method of optical examination is the FTIR or Fourier Transform Infrared Spectroscopy, specifically for measuring non-ionic residue. An analytical method, it efficiently determines the precise quantity of contamination present.

Hi-Vis Spectroscopy and High-Performance Liquid Chromatography or HPLC are methods to identify residual rosin. Other methods of identifying residues and contamination of a PCB are Scanning Electron Microscopy or SEM and Energy Dispersive X-ray or EDX.

Each of the above methods has their own advantages and disadvantages. In general, the equipment necessary to implement these methods are expensive and rarely used in production environments.

Resistivity of Solvent

Measuring the resistivity of the solvent extract (ROSE) is a common method usually employed for determining the degree of ionic contamination. Known also as the solvent extract conductivity or SEC, it involves the resistivity of the solvent decreasing with the rise of concentration of ions in the solution.

A number of electronic assembly houses use a simple automated version of ROSE testing. They utilize the ZeroIon, Ionograph, or the Omega Meter for measurement and quality control testing. IPC-TM-650 defines a standard for the industry, where a solution of deionized water and isopropanol extracts the contaminants whilst the meter measures the change in conductivity. Although offering rapid results and being widely accepted, the above method of testing has its restrictions, being suitable for traditional rosin-based fluxes and ionic contamination.

Measuring Surface Resistance

IC or Ion Chromatography and SIR or surface insulation resistance are two further methods for measuring the level of contamination on the PCB surface. The SIR process measures changes in electrical current over time at higher than normal temperature and humidity levels. As the insulation resistance decreases with the increased presence of contamination, the current flow changes between an interleaving patterns of combs on the PCB.

A more modern method, Ion Chromatography or IC, evaluates the PCB cleanliness by identifying and quantifying specific ionic species present on the PCB. The test method offers a list of ionic residues that specific media can remove.


The required level of cleanliness is entirely dependent on the selection of the most suitable cleaning process, and this is the key to achieving maximum cleanliness and reliability at minimum cost.


Importance of Stencil for PCB Assembly

The surface mount assembly process uses a stencil as a gateway to an accurate and repeatable solder paste deposition. A stencil is a thin sheet or foil of brass or stainless steel with a circuit pattern cut into it, matching the positional pattern of surface mount devices (SMD) on the printed circuit board (PCB) for which the stencil is to be used. After accurately positioning and matching the stencil over the PCB, a metal squeegee forces solder paste through the apertures of the stencil to form deposits on the PCB for holding SMDs in place. The solder paste deposits, when passed through the reflow oven, melt and secure the SMDs to the PCB.

 Importance of Stencil for PCB Assembly

Fig.1: Metal Stencil

The design of the stencil, especially its composition and thickness and the shape and size of its apertures, determines the size, shape, and positioning of the solder paste deposits and this is crucial to ensuring a high-yield assembly process. For instance, the foil thickness and the aperture opening size define the volume of paste deposited on the board. An excess of solder paste causes balling, bridging, and tomb-stoning. Low amounts of solder paste cause dry solder joints. Both compromise the electrical functionality of the board.

Optimum Foil Thickness

The types of SMDs on the board define the optimum foil thickness. For instance, component packages such as 0603 or SOICs with a 0.020” pitch require rather thin solder paste stencils, whereas a thicker stencil is more suitable for components such as 1206 or SOICs of 0.050” pitch. Although stencil thickness for solder paste deposition ranges from 0.001” to 0.030”, the typical foil thickness that a majority of boards use ranges from 0.004” to 0.007”.

Technologies for Making Stencils

At present, the industry uses five technologies for making stencils—laser-cut, electroformed, chemically etched, and hybrid. While the hybrid technology is a combination of chemical etching and laser-cutting, chemical etching is very useful for making step stencils and hybrid stencils.

Chemical Etching for Stencils

Chemical milling etches metal masks and flexible metal mask stencils from both sides. As this etches not only in the vertical direction but also laterally, it causes undercutting, and makes the openings larger than desired. As the etching proceeds from two sides, the tapering on the straight wall causes the formation of an hourglass shape, leading to extra deposits of solder.

As etching the stencil openings does not produce a smooth result, the industry uses two methods for smoothening the walls. One of them is electropolishing, a microetchng process, and the other is nickel plating.

Although a smooth or polished surface helps paste release, it may also cause the paste to skip the surface of the stencil rather than roll with the squeegee. Stencil manufacturers address this problem by polishing the aperture walls selectively, but not the stencil surface. While nickel plating improves stencil smoothness and the printing performance, it reduces the aperture opening, which requires artwork adjustment.

Laser Cutting for Stencils

Laser cutting is a subtractive process, where Gerber data is fed to a CNC machine that controls the laser beam. The laser beam starts from inside the boundary of the aperture and traverses to its perimeter, while completely removing the metal to form the apertures, one aperture at a time.

Several parameters define the smoothness of the laser cut. This includes cutting speed, beam spot size, laser power, and beam focus. Typically, the industry uses a beam spot of about 1.25 mils, which cuts very accurate aperture sizes over a wide range of shape and size requirements. However, laser cut apertures also require post-processing treatments just as chemically etched apertures do. Laser cut stencils require electropolishing and nickel plating to smoothen the inside walls of the aperture. As the aperture size reduces during the latter process, the laser cut aperture size must be suitably compensated.

Aspects of Printing with a Stencil

 Importance of Stencil for PCB Assembly

Fig.2: Printing with a Stencil

Printing with a stencil involves three distinct processes. The first is the aperture-fill process where the solder paste fills the aperture. The second is the paste transfer process, where the paste accumulated in the aperture transfers to the PCB surface, and the third is the positional location of the deposited paste. The three processes are vital to achieving the desired result—depositing a precise volume of solder paste (also called a brick) to the correct location on the PCB.

Filling the stencil aperture with solder paste requires a metal squeegee blade forcing the solder paste into the aperture. The orientation of the aperture with respect to the squeegee blade affects the fill process. For instance, apertures oriented with their short axis in the direction of the blade stroke fill better compared to those with their long axis oriented to the blade stroke. Additionally, as squeegee speed influences aperture fill, lower squeegee speeds achieve better fills for apertures with their long axes oriented parallel to the stroke of the squeegee.

The edge of the squeegee blade also influences the way the paste fills the aperture of a stencil. The usual practice is to print applying minimum squeegee pressure while maintaining a clean wipe of the solder paste on the stencil surface. Increasing the squeegee pressure may damage both the squeegee blade and the stencil, while also causing paste smearing below the stencil surface.

On the other hand, lower squeegee pressure may not allow release of paste through a small aperture, resulting in insufficient solder on the PCB pad. Additionally, paste left on the side of the squeegee near large apertures will likely be pulled down by gravity, resulting in deposition of excess solder. Therefore, a minimum amount of pressure is necessary, which will achieve a clean wipe of the paste.

The amount of pressure to be applied also depends on the type of solder paste being used. For instance, Teflon/nickel-coated squeegee blades require about 25-40% more pressure when using lead-free solder paste than that required for tin/lead paste.

Performance Issues with Solder Paste and Stencils

Certain performance issues related to solder paste and stencils are:

  • Thickness of the stencil foil and the aperture size determine the potential volume of solder paste deposited on the PCB pads
  • The ability of the solder paste to release from the aperture walls of the stencil
  • Positional accuracy of the solder brick printed on the PCB pad

During the print cycle, as the squeegee blade travels across the stencil, solder paste fills the stencil aperture. During the board/stencil separation cycle, the paste releases to the pads on the board. Ideally, all the paste that filled the aperture during the print process should have released from the aperture walls and transferred to the pad on the board, forming a complete solder brick. However, this transfer amount depends on the aspect ratio and area ratio of the aperture.

For instance, with an area of the pad greater than two-thirds the area of the inside aperture wall, the paste can achieve a release of better than 80%. This means reducing the stencil thickness or increasing the aperture size can give a better paste release with the same area ratio.

The ability of the solder paste to release from the aperture walls of the stencil also depends on the finish of the aperture walls. With electropolished and/or electroplated laser-cut apertures, the paste transfer efficiency improves. However, transfer of solder paste from the stencil to the PCB also depends on the adhesion of the paste to the aperture walls of the stencil and on the adhesion of the paste to the pad on the PCB. For a good transfer the latter should be greater, which means the printability depends on the ratio of the stencil wall area to the open face area, while ignoring minor influences such as the draft angle of the wall and its roughness.

The positional and dimensional accuracy of the solder brick printed on the PCB pad depends on the quality of the transferred CAD data, the technology and methods used to manufacture the stencil, and the temperature of the stencil during its use. Moreover, the positional accuracy also depends on the alignment methods used.

Framed Stencils or Glue-In Stencils

Framed stencils are the strongest form of laser-cut stencils available and are designed for high-volume screen-printing during production runs. These are permanently mounted in a stencil frame with a mesh border tightly stretching the stencil foil taut in the frame. Framed stencils with smooth aperture walls are recommended for Micro BGAs and for components with 16 Mil pitch and below. Framed stencils offer the best positional and dimensional accuracy when used under controlled temperature conditions.

Importance of Stencil for PCB Assembly

Fig.3: Framed Stencil

Frameless Stencils

Importance of Stencil for PCB Assembly

Fig.4: Frameless Stencil

For short runs or prototype PCB assembly, frameless stencils offer optimum solder paste volume control. They are designed to work with a stencil tensioning system, which are reusable stencil frames such as universal frames. As the stencils are not permanently glued into a frame, they are significantly cheaper than framed stencils and take up considerably lower storage space.


For achieving good printing results with a stencil, a variety of factors must form a right combination. These include:

The right paste material—correct metal content, viscosity, the largest powder size, and lowest possible flux activity

The right tools—proper stencil, printer, and squeegee blade

The right process—good registration, and clean sweep.

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