Welcome to Summer Issue July 2001
Advancements In Stencil Cleaning Technology
Your resource for information and innovations in SMT stencil cleaning.
In this issue:
"The Impact of Stencil Cleaning on SMT Print Yields" Reprinted from Circuits Assembly March 2001
Stencil Cleaner Reduces Chemistry Use and Wastewater by 40%
StencilScan™ Automatic Screen and Stencil Inspection System
The First Independent Stencil Dryer
"The Impact of Stencil Cleaning on SMT Print Yields"
Reprinted from: Circuits Assembly March 2001 pp 49-50
According to one study, 51-72% of all solder defects are the result of screen-printing problems.1 Particularly when smaller components are required, a less than clean stencil, or a stencil that has been altered by the cleaning process, will increase misprints while decreasing throughput and profitability.
A primary way to assure consistent solder paste deposition is to guarantee stencil cleanliness. Stencils need to be cleaned to facilitate optimal prints. Solder paste residue on a stencil can interfere with good gasketing of the board to the stencil or, result in transferring paste from the bottom of the stencil onto subsequent board prints.
Solder paste left in stencil apertures can impede paste transfer resulting in open solder connections or insufficient solder joints.
However, selecting the proper stencil cleaning process is not as easy as when use of a CFC solvent and a vapor degreaser were permissible. Todays assembler not only needs to be concerned with stencil cleanliness; he also needs to be aware of user safety, environmental concerns and profitability.
Stencil cleaning has been identified as the most hazardous process with the greatest potential environmental impact associated with printed circuit board assembly. The lead contained in the solder paste is poisonous; the chemistries used for cleaning pose concerns for fire, explosion, high alkalinity, user health, fetal disorders, scalding water, CFC / VOC emissions and heavy metal wastewater discharge.
Because few stencil cleaner manufacturers will guarantee the performance or environmental soundness of their systems, PCB assemblers are placed in the difficult position of evaluating stencil cleaners for efficacy, safety and health concerns, while balancing environmental restriction with cost and overall processing speed. In addition, as there are no industry standards for stencil cleanliness, it has been easy for suppliers to claim that they have a safe and effective method of stencil cleaning without necessarily having the hard data to back these claims.
However, the U.S. Environmental Protection Agency (EPA) and some state EPAs such as California now offer programs to help set standards and proactively certify stencil-cleaning processes for environmental safety, user safety and verify the manufacturers claims of efficacy. Processes that have not achieved EPA Certification should be scrutinized closely to determine the reasons why.
In some cases, cleaning the surface of the stencil can be accomplished without the need of any type of chemistry a roll of lint free paper simply wipes the stencil. However, thorough cleaning of the surface and apertures requires the use of a chemistry, and herein lies the problem.
When selecting a stencil cleaning process, the chemistry should be evaluated first. The cleaning machine is secondary and will only determine the initial capital cost, how the chemistry is applied, footprint, user interface and maintenance access. The chemistry will dictate the wetting or solubility of the contaminant, user exposure to health and safety hazards, operational cost, environmental impact, odors or hot water vapors, cycle times, ability to clean different solder pastes, energy use (for those chemistries requiring elevated temperatures), maintenance schedules, storage and transportation requirements, waste management procedures, and exhaust or other special installation requirements especially if the chemistry is a low flash point solvent such as alcohol to isolate potential fire and/or explosion hazards.
Once the chemistry is identified, the method of applying the chemistry can be determined. Manual application is by far the most common methodology worldwide. However, manual cleaning usually yields the least desired results blocked apertures, stencil damage and inconsistent cleaning.
Mechanical application of the chemistry can be achieved by one of three available technologies: spray in air, spray under immersion or ultrasonic cavitation. High-pressure sprays are known to bend delicate land mass areas between fine-pitch apertures. However, without high-pressure, it becomes difficult to penetrate into apertures smaller than 20 mil pitch which results in residual contamination.
Assuming the chemistry is compatible, ultrasonic agitation has become the technology of choice to apply the chemistry. Ultrasonic cavitation is able to deliver the chemistry safely into ultra fine-pitch apertures achieving optimal cleaning results. However, without the proper chemistry, ultrasonic agitation will have little effect.
Whether using sprays or ultrasonics, without the proper chemistry to wet or dissolve the contaminant, results will be similar to washing soiled hands without the proper soap less than 100% effective.
1. Surface Mount International Technical Program. (1993) Vol. 1, pp. 157-166.
For more information or other articles on stencil cleaning go to: http://www.smartsonic.com.cn/article.html
Stencil Cleaner Reduces Chemistry Use and Wastewater by 40%
Smart Sonic advances stencil-cleaning technology with the introduction of the Model 6000 Automatic Ultrasonic Stencil Cleaner. The Model 6000 reduces chemical consumption and wastewater by 40% yet increases ultrasonic energy efficiency over Smart Sonics previous automatic stencil cleaners. The newly designed cleaning tanks are modular and can clean 29-inch stencils yet use 40% less wash water per two-week maintenance cycle compared to the previous automatic systems. In addition, the Model 6000 uses only 3.4 gallons of 440-R® SMT Detergent every 2-weeks compared to 5.7 gallons for previous models.
The new modular tank design enables using the Model 6000 to perform multiple cleaning applications. An ambient wash tank is provided as standard equipment for the safe cleaning of solder paste from stencils and misprinted PCBs. However, an additional heated wash tank may be incorporated for cleaning flux residue from pallets and oven radiators or SMD adhesives from stencils and misprinted substrates - all within the same footprint of the previous single application machines. A touch-pad controller stores multiple programmable cleaning profiles to facilitate easy one-button operation for all users.
Ergonomics has also been improved by reducing the loading / unloading height from 52-inches down to a 36-inch level. Located directly below the loading/unloading station is a built-in drying rack to allow up to three 29-inch stencils to air dry naturally. If automatic drying is required, this area can be converted into a unique low-temperature dryer to safely dry stencils automatically without exposing the stencil to high temperatures that could weaken the bond of the heat-cured adhesives used to attach the screen to the frame and etched foil. Low temperature drying also prevents distortion of fine-pitch apertures and screen tension problems due to irregular expansion and contraction.
The Smart Sonic Stencil Cleaning Process is guaranteed to clean any type of solder paste from any fine-pitch stencil and the Environmental Protection Agency (EPA) certifies the Smart Sonic process environmentally safe, user safe and effective, which makes it ISO 14000 and MP&M Guideline compliant.
For more information on the Model 6000 Automatic Stencil Cleaner, go to: http://www.smartsonic.com.cn/model6000.html
Stencil Inspection System
StencilScan is a fully integrated inspection workstation for screens and stencils after fabrication or after cleaning. StencilScan uses a PC Windows based software package integrated with a high-resolution, calibrated, A3 size flatbed scanner. This combination allows inspection of both screens and stencils for accuracy, damage or contamination and the ability to inspect 100% for absence/presence and correct size of apertures.
StencilScan inputs stencils, screens, silver or diazo film, mylar images, technical drawings, actual PCBs or original hand-taped designs. With these inputs, StencilScan is able to verify cut stencils/screens for accuracy, size, blockage, damage and stretch as well as producing Gerber data for stencil/screen production.
StencilScan permits fast and accurate comparison of screen/stencils to bare PCBs bringing automation and increased productivity to QC departments.
Inspect for cleanliness or absence / presence of apertures
4000 dpi allows inspection of apertures as small as 0.002" / 0.050mm using Local Micro Alignment (LMA)
Guarantees stencil & screen accuracy and cleanliness
Supply production with required Gerber Data
Create stencil files from boards, film and stencils
For more information on the StencilScan inspection system, go to: http://www.smartsonic.com.cn/stencilscan.html
Smart Sonic Introduces the First Independent Stencil Dryer.
The Model LTD-3 (Low Temperature Dryer 3 Stencil Capacity) safely dries up to three 29-inch stencils at one time. The most common bottleneck of a stencil cleaning operation is the drying cycle. While washing and rinsing may only take 5 to 10 minutes, the drying cycle often requires an additional 10 to 15 minutes.
The LTD-3 improves stencil-cleaning throughput by eliminating the drying cycle of the stencil cleaner, allowing the stencil cleaner to continue cleaning stencils while drying is accomplished independently. By incorporating an independent stencil dryer, throughput can improve by 200 to 300% and at a fraction of what an additional stencil cleaner would cost. The low temperature design conserves energy and protects stencils from heat that may weaken adhesive bonds, alter screen tension or distort the etched image due to expansion / contraction. The overall height is only 36-inches allowing for easy loading and unloading of large stencils. Controls are adjustable to suit other applications such as drying pallets, oven radiators and PCBs.
For more information on the Model LTD-3 Stencil Dryer, go to: http://www.smartsonic.com.cn/modelltd3.html
Advancements In Stencil Cleaning Technology is a newsletter and service provided by Smart Sonic Corporation.
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