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Part 3 – The Basics of Cleaning – Stencil Cleaning
by Fernando Rueda, Sales Manager, Kyzen BVBA
Stencil/Screen Cleaning I would like to remind the reader that this article deals with the potential effects (negative and positive) that cleaning, or lack thereof, can have in the manufacturing process, especially, in the printing process where stencils are used. It is not meant to be a troubleshooting guide for your printing process as there are other potential causes, besides a defective cleaning process. In part one of this series, we established that stencil cleaning is a required step in electronics manufacturing. Since more than 70% of defects tend to be due to printing problems, proper maintenance and cleaning of the stencils becomes very critical. Miniaturization of assemblies and high component density increases the importance of stencil cleaning even more. In the printing process, the most common failures
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are solder print (volume, ratio, aperture size, solder paste quality, etc.) and placement (alignment and pressure). A high percentage of these problems come from inadequate amounts of solder paste being deposited on the soldering pads. Clogged stencil apertures (not cleaned properly) can certainly be a common cause for lack of solder paste deposition. Stencil/Screen Cleaning Processes The following are the most common stencil cleaning processes found in a typical electronics manufacturing environment. 1. Underside wipe in printers
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his article is the third part of the series titled “The Basics of Cleaning”. This series of articles will cover the basic aspects that any process engineer needs to cover when looking at a new or existing cleaning process.
2. Cleaning in automatic stencil cleaners 3. Cleaning in semi-automatic stencil cleaners 4. Manual cleaning
We could simplify these four categories by grouping the bottom three into a single category, Offl ine Cleaning.
1. Underside Wipe Cleaning As a general rule, it is very important to keep your printer, and any piece of equipment, well maintained and cleaned. In printers, solder paste buildup in the working or moving areas can be detrimental for the board support and the consistency of the process. It also presents the risk of contaminating boards processed later and potentially causing solder balls. So it is necessary to periodically clean the underside of
the stencil. This very important for fi ne-pitch applications, because any residues of solder paste could potentially cause solder shorts and solder balling. Wiping the stencil with a solvent (cleaning agent) can prevent these defects and avoid inadequate deposition of paste as well as eliminate fl ux build-up that can affect the release of the paste.
Wiping Frequency Wiping frequency is always a question. The frequency of the cleaning will depend on how the main elements (board, stencil and paste) are working together. To determine the wipe frequency, you should follow the procedures recommended by the printer manufacturer. After the number of prints per wipe is determined, it is typically recommended (as a safety cushion) that you subtract one or two prints from that number of prints per wipe.
Solvents/Cleaning Agents Solvent-based chemistry has been, and continues to be, the preference for automatic wipers in printers. In the past, IPA was one of the most common solvents used for underside wipe of stencils in printers. IPA is relatively cheap and quick to dry but it is highly hazardous (for health and safety) and a very low quality cleaner. There have been reports from
www.electronicscomponentsworld.com /
www.electronicsproductionworld.com January 14th 2012
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