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Stencil Cleaning
December 31, 1969 |Estimated reading time: 9 minutes
A CLEANING AGENT CAPABLE OF REMOVING EXCESS SOLDER PASTE, ADHESIVE AND FLUX RESIDUES IN AN AUTOMATIC STENCIL CLEANING PROCESS WAS DEVELOPED THROUGH THE DESCRIBED PROCESS. USE OF THIS METHOD CAN GREATLY ENHANCE PRODUCTION AND MINIMIZE LOSS STEMMING FROM MISPRINTED PCBS.
By Ping Seto and Norbert Loew
In recent years, stencil and misprinted printed circuit board (PCB) cleaning has become a significant issue for an increasing number of SMT manufacturers. In a typical PCB assembly line, solder paste is printed through screens or stencils. Additionally, as an alternative to adhesive dispensing, specially formulated SMT adhesives now can be printed through a stencil. Consequently, the solder paste and adhesive must be cleaned periodically from the stencils to maintain the production quality demands.
Figure 1. Solder paste residue stencil (a); adhesive residues in aperture on stainless steel (b); misprinted PCB (c); and solder balls in via (d).
Ever increasing component density and the result required of the printed image give rise to stringent requirements regarding stencil cleanliness. A flawless printed image is very important because misprinted boards no longer can be used without cleaning. Scrapping misprinted PCBs can be very expensive, particularly if components already have been mounted onto one side of a board. Stainless steel stencils with apertures that are either etched or cut with a laser are used for this purpose. Studies have revealed that inadequate paste printing causes more than 60 percent of the defects arising in the SMT process. This, in turn, can be the result of insufficient cleaning that often is done manually. Consequently, an optimally working automatic stencil cleaning process is imperative.
Although stencil cleaning improves the printing results drastically, the production of misprints during the printing process cannot be avoided completely. Yet, cleaning misprinted assemblies is an application that still frequently is ignored because it involves removing the misprinted or smeared solder paste or SMT adhesive from the PCB. Additionally, flux residues from the populated reflowed side should be removed. For example, incomplete removal can leave unacceptable white residues.
Table 1. Experiments performed with micro phase cleaner (MPC) cleaning agent.
The DaimlerChrysler Electronic Plant in Huntsville (Alabama), as part of the process of introducing an adhesive printing step into its assembly lines, evaluated new cleaning media and a new cleaning process for the associated stencil and misprint cleaning application. The plant manufactures approximately 10,000,000 units of automotive electronic components such as engine, body, transmission or transaxle (4WD) controllers per year, and employs almost 3,000 people over two shifts per day. The company was looking for a process in which solder paste and SMT adhesive could be removed from stencils and misprinted boards, in addition to flux residues from the populated reflowed assemblies.
Process RequirementsMany process parameters are important in specifying a cleaning process. When selecting a cleaning process, the cleaning agent should be chosen first, as it has to be adapted to the chemical ingredients of the contamination to be removed. Only after selecting the proper cleaning agent can the application method be evaluated.
Table 2. Process parameters used for evaluating MPC's performance.
The process engineers at DaimlerChrysler conducted an extensive evaluation to find a single cleaning agent capable of removing both solder paste and adhesive for process simplification. An added benefit would be the ability to clean misprinted PCBs (unpopulated and one-side populated), using the same cleaning agent in the same cleaning process.
The basic project parameters for the cleaning process to fulfill included:
- Total removal of the specified misprinted solder paste and three different kinds of SMT adhesives from fine-pitch stainless steel stencils, misprinted nonpopulated and populated PCBs without leaving any kind of residues (Figure 1a-c). An added requirement was that PCB solderability would not be adversely affected and the PCB did not have ionic residue after the cleaning process.
- No flux residues on the populated reflowed side of the misprinted PCBs after cleaning, resulting in minimal ionic contamination values and bright, shiny solder joints.
- No solder balls or adhesive residues in the apertures of the specified stainless steel stencil and the vias of the PCBs (Figure 1d).
- Maximum cycle time of 30 minutes for the complete cleaning cycle including wash, rinse and dry.
- A cleaning agent that assured several essential requirements: Water-based cleaning, eliminating the risk of flammability; excellent bath loading capability, resulting in a long bath life and, consequently, an economical cleaning process; material compatibility with cleaning equipment (pipework, pumps, filters, sealing) and cleaning substrates (stencil, solder mask, components, labels); minimal toxicity and low odor, assuring optimal operator safety and acceptance; and compatibility with spray-in-air equipment.
- No damage to the cleaning substrates through the cleaning process, especially stencils having to withstand 60 wash cycles and more.
The application specialists of Zestron Corp. conducted comprehensive cleaning trials with different cleaning media in their Ashburn, Va.-based Technical Centre to meet these requirements.
Process EvaluationEvaluation of the above factors revealed that a low-pressure spray in air cleaning processes would be the most cost-effective solution for optimal cleaning results.
Table 1 summarizes the cleaning experiments performed with a new technologically advanced micro phase cleaner (MPC) cleaning agent, and Table 2 outlines the process parameters used for this evaluation.
In summary, the cleaning results were excellent for adhesives A and B. Adhesive A proved somewhat easier to clean. Adhesive C proved the most difficult to clean in both stencil and PCB cleaning applications. A slight reddish residue from the adhesive pigment was visible. Cleaning solder paste was not a problem with the newly developed MPC cleaning agent.
Process Reliability TestingNumerous tests were conducted by the two companies to determine that the developed media, VIGON SC 202, exceeded all process requirements.
Reflowed PCB Cleaning Evaluation. To simulate cleaning assemblies populated with reflowed components, 10 board assemblies with reflowed solder paste flux residues and paper barcode labels were cleaned using the adhesive cleaning parameters in a spray in air cleaner. All assemblies were inspected under 60X magnification afterward to determine flux residue changes and label damage. Visually, all samples appeared to be pristine and free of flux residue. The material compatibility with barcodes has been tested and proved.
Secondly, all PCB assemblies were submitted into the ionograph to quantify the amount of ionic contamination. Total amounts of ionics were extracted and divided by board area. All samples were measured well within the ANSI/J-STD-001 limit of less than 1.56 µg/cm2 equivalent NaCl.
Figure 2. Wetting balance results on a cleaned PCB.
Stencil Compatibility Testing. Compatibility of the MPC cleaning agent with stencil material was tested to ensure the cleaning process would not degrade stencil mesh tension and cause printing problems. Two different tests were conducted. A quick first assessment was done on both steel stencil (used by adhesives A and C, and regular paste printing) and plastic stencil (used by adhesive B) to determine general material compatibility. Additionally, a long-term compatibility test was conducted to simulate the cleaning exposure during the usable life of a stencil, which consists of running 60 cleaning cycles (15 minutes per cycle at 77°F) in spray cleaner.
Short-term testing demonstrated no incompatibility between the MPC cleaning agent and either stainless steel or plastic stencils after about five cycles. The mesh on stainless steel stencils ranges from 18 to 20 N/cm, while the plastic stencil mesh measures 17 to 21 N/cm after cleaning. No measurable differences from original sizes or surface tensions were found after cleaning.
Long-term compatibility data was taken with a tension gauge at four different locations on the mesh. There was no significant degradation upon the completion of this test.
Wetting Balance Testing. Wetting balance testing was performed to assess board cleaning effectiveness concerning solderability degradation. The wetting balance test conducted used standard HASL finished test coupons. Such testing provides a quantitative measurement on how quickly solder will wet to the soldering surface. If the cleaning process is ineffective, adhesive residue can cause the wetting time to increase (Figure 2).
Three groups of coupons were used. Each group was exposed to the adhesive misprint with the corresponding adhesive candidate. Adhesive cleaning on the coupons was carried out using the VIGON SC 202.
Wetting balance testing was conducted at an independent testing laboratory. A control group of virgin coupons was tested along with the cleaned samples.
As depicted in Figure 2, the wetting balance results of the coupons after cleaning were very similar to the control samples. There was no observable solderability degradation after the cleaning process with the VIGON SC 202 based on the wetting time and force.
Misprinted Board Cleaning Evaluation. A simple experiment was conducted at Zestron to evaluate the feasibility of manually wiping off uncured adhesive to simulate a misprinted condition. DaimlerChrysler test boards were used with adhesive deposited on the surface. VIGON SC 202 was used to wipe off the glue using a cloth. The results were the same for all three candidates; however, surface cleanliness varied due to the inherent variability of the manual cleaning process. Also, adhesive was entrapped into vias and plated through-holes with high potential for solder failures and insufficient hole fills.
Based on the above experimental results, PCB manual adhesive cleaning is not recommended because of the high process variability; however, misprinted boards can be cleaned effectively using the appropriate board holder with VIGON SC 202.
Evaluation of the Cleaning Equipment. Cooperation with EMC Global Technologies Inc., a U.S. stencil cleaning equipment manufacturer, established the following necessary technical preconditions:
- Cleaning equipment designed for cleaning agents that require a water rinse.
- An effective heating system capable of heating the cleaning agent to the desired operating temperature.
- Effective spray pressure sufficient to remove solder balls from vias.
- Effective hot air drying to assure minimal cycle times.
- Assured environmental and worker safety throughout the equipment design.
The Cleaning ProcessCleaning Media. Further extensive cleaning tests were conducted at Zestron's Technical Centre during which the new MPC cleaner was selected as the optimal cleaning medium for DaimlerChrysler applications. This product is a water-based cleaner without a flash point that removes specified solder paste, flux residues and SMT adhesives without leaving residues on the substrates. Additionally, the cleaner contains no inorganic salts or surfactants, exhibits minimal odor, is biodegradable, and is supplied as a ready-to-use mixture.
Cleaning Equipment. The equipment selected for the application at Daimler Chrysler was designed to run water-based cleaning agents that can be operated at room temperature or elevated temperatures. The cleaning chemistry is kept in a sealed reservoir during product loading and unloading, thereby exposing minimal media to the outside environment, increasing worker safety and decreasing media consumption. The system contained an open loop rinsing unit run with deionized water. The cleaning agent was filtered through a cartridge filter of 20 µm pore size.
Cleaning Parameters. The cleaning substrates were cleaned using VIGON SC 202 at 77°F for 7 minutes. Subsequent rinsing with deionized water lasted for 7 minutes at 122°F, and final drying with circulated hot air for 10 minutes.
SummaryDaimlerChrysler required a new cleaning agent to remove adhesives, solder pastes and flux residues from various substrates associated with an adhesive stencil printing process. The newly developed MPC cleaning agent VIGON SC 202 more than met the requirements laid out by DaimlerChrysler. It was compatible with cleaning all three adhesive candidates, the solder paste on both stencils and PCBs, and the flux residues on one-side populated PCBs. The evaluated media exhibits excellent bath loading capability, resulting in a long bath life and an economical cleaning process. Furthermore, material compatibility with cleaning equipment and cleaning substrates has been proven during the production process. Finally, the cleaning agent's minimal toxicity and low odor assures optimal operator safety and acceptance.
For further information, contact ANDREAS MUEHLBAUER at Zestron Corp., 21641 Beaumeade Circle, Ste. 315, Ashburn, VA 20147; (703) 589-1198 ext. 105; Fax: (703) 821-9248; E-mail: A.Muehlbauer@zestron.com.