In recent years stencil and misprinted PCB cleaning has become a significant issue for an increasing number of SMT manufacturers ultrasonic cleaning system. In a typical PCB assembly line, solder paste is printed through screens or stencils. In addition, as an alternative to dispensing of adhesive, specially formulated SMT adhesives can now be printed through ultrasonic cleaning solution a stencil. Consequently, the solder paste and adhesive must be Prozone cleaned periodically from the stencils to maintain the defluxing chemistry production quality demands.
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 since misprinted boards can no longer be used without cleaning. Scrapping misprinted PCBs can be very expensive, particularly if components have already been mounted onto one side ultrasonic cleaning system of a board. Stainless steel ultrasonic cleaning system stencils with apertures that are either etched Prozone or cut with a laser are used for this purpose. Studies have revealed that more than 60% of the defects arising in the SMT process are caused by inadequate paste printing. This, in return, can be the result of insufficient cleaning that is often done manually. Consequently, an optimally working automatically stencil cleaning process is imperative.
Although cleaning of stencils improves the printing results drastically, the production of misprints during the printing process cannot be avoided completely. However, the cleaning of misprinted assemblies is an application that is still frequently ignored. This ultrasonic cleaning system involves the removal of the misprinted or smeared solder paste or SMT adhesive from the PCB. In addition defluxing chemistry the removal of flux residues from the populated reflowed side Prozone is an important issue that has to be taken into consideration. For example, incomplete removal of the flux residues can leave ultrasonic cleaning solution behind white residues, an unacceptable situation.
The DaimlerChrysler Electronic Plant in Huntsville (Alabama) as part of the process of introducing an adhesive printing step into its ultrasonic cleaning solution assembly lines evaluated new cleaning media and; if necessary, a new cleaning process for the associated stencil and misprint cleaning Prozone application. The Huntsville Electronic Plant manufactures approx. 10.000.000 units of automotive electronic components like defluxing chemistry engine, body, transmission or transaxle (4WD) controllers per year, employing almost 3.000 people running two shifts per day. DaimlerChrysler was ultrasonic cleaning system looking for a process where solder paste and SMT adhesive could be removed from stencils and misprinted boards and in addition flux residues from the populated reflowed assemblies.
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