| Posted by Bob Willis on 01 May 2006 at 00:00
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(This column, which originally appeared in Global SMT & Packaging magazine 6.4 (Apr 2006), is also available as a free PDF.)
Top left: Damage to the fine pitch stencil bars, stencil should be scraped
Top right: No wetting of solder to gold pads due to poor paste wash-off procedures
Bottom left: Kinked stencil should be replaced with new stencil
Bottom right: Poor reflow of paste, due to solvent in the solder mask after wash-off of board.
Inevitably all stencils need to be cleaned to remove solder paste residues from the surface of the foil and from the apertures to prevent it drying and giving missed/incomplete prints. Most of the paste can be removed from the surface of the stencil and re-used if it has not dried out and continues to print successfully. The stencil should be virtually clean before any cleaning material is used or before the stencil placed in an automatic cleaning system. There are two options to cleaning stencils, manual and automatic, and it will depend on the company budget, production volume and number of product codes which technique is used.
Ideally, separate stencils should be used for tin/lead and lead-free pastes to avoid contamination. Engineers may also choose to introduce aperture changes to improve wetting and coverage of the pads during reflow. In some situations, the same stencil may be used with a particular design for both pastes. In this case, it is crucial that stencil cleaning is performed correctly.
Washing off bare printed boards can and should be avoided. Running test prints on test boards or printing onto a mylar sheet or low tack film applied to a board avoids unnecessary cleaning. Many of the printed board surface finishes are affected by cleaning; simple solderability spot tests or wetting indicators easily demonstrate this fact. The cleaning process and procedure used must not impact on solderability and must remove solder paste from through holes, tooling holes, vias and resist apertures. If not, solder balls will be present on the surface of the boards; after reflow they will be particularly visible on gold boards.
The latest version of IPC610D infers that solder balls located in via holes are a defect, but this depends on your interpretation. In the real world of manufacture, this situation can occur and should not be considered a reject.
Manual Cleaning of Stencils
Remove as much solder paste from the stencil surface as possible using a plastic spatula into a used paste container. A soft blade should be used when removing paste to prevent damage to the surface of the stencil. If the solder paste is still performing well, retain the paste for the next printing job. The paste can be placed directly on the new stencil.
Always place the paste either in a container marked ‘used’ or in the waste container for disposal. A lid should always be placed on the container to maintain the paste consistency for further printing. A set of colour wall charts are available from the SMART Group (www.smartgroup.org) showing the correct use, storage and handling of solder paste.
Place the stencil on a clean, flat surface, print-face down, with a sheet of absorbent cloth under the stencil. Spray the appropriate cleaning agent onto the surface of the stencil and carefully wipe the remaining residues from the stencil surface. Wipes loaded with different cleaning solvents are available for this purpose. Often the paste manufacturer will recommend the best solvent system for cleaning.
Most of the remaining paste will be removed either through the stencil apertures onto the paper or on the wipe. Turn the stencil upside-down so the stencil print surface is off the table and repeat the cleaning operation. Finally, blow out the apertures with a low pressure air line or use a pressurised spray bottle. Make sure that any paste deposits that will be blown out of the apertures will be captured on a disposable cloth on the table. Suitable eye protection must be used.
When the stencil is dry and inspected for satisfactory cleaning, it should be either placed in the original packaging or stored in a properly designed rack. Each of the stencils is expensive and can be easily damaged.
Automatic Cleaning Of Stencils and Boards
Manual cleaning is not ideal and relies totally on the consensus of the operators, their initial training, the space available, and local health and safety requirements. For fine pitch applications, it is nearly impossible to eliminate all residues from a stencil manually. Although an automatic stencil cleaning facility is expensive, it is becoming a necessity based on environmental issues and technology demands.
Evaluation of automatic/semi-automatic systems for stencils and board wash-offs should consider the following points:
- Cleaning material compatibility
- Ability of the systems to remove paste particles
- Suitable clams for stencils, foils and boards so they do not touch
- Automatic cycle or timer
- Ease of machine maintenance
- Running costs
Stencil Storage
Framed or frameless stencil foils are expensive, precision tools that must be correctly handled and stored if they are to provide a long working life. Storage should support the stencils to avoid damage but should also allow easy removal and replacement. The identification of the stencil should be achievable without pulling the stencil out of the rack.
Washing Printed Boards
Although every effort is made to eliminate printing problems, they do occur. If the required standard of paste printing is not achieved, these boards may need to be cleaned. If this is the case, a method needs to be defined and monitored for its effectiveness.
The most common process is to wipe the paste off the board with a cloth or rubber blade that removes the paste from the surface of the pads but deposits it everywhere else. Wiping paste manually places paste in the resist windows, via holes, tooling holes and plated through holes that may not be removed with subsequent cleaning operations. Process problems may be experienced during through-hole component insertion as the paste will reflow, partially filling the holes. Since the fluxing agent in the paste is removed during cleaning, the solder particles normally just bind together in the holes like frogspawn. If the paste is left on the surface of the board, the cleaning system will have to handle more paste removal. Both situations can be evaluated during testing.
Although the cleanliness of the printed board after cleaning is very important, testing for ionic residues is only one issue to consider. The impact on solderability of the surface finish and of the solder particles on the surface of the board, and in the through hole, is also very important to consider. The time taken to clean the boards, either manually or semi automatically, is very important to the solderability of the boards. How easy is it to put boards into a cleaning tank and totally forget they are there?
Final inspection will undoubtedly detect solder balls on the surface of the board after final assembly. On many occasions, the solder paste is blamed for poor reflow and a debate ensues. A simple trick for any boards that are washed off by whatever means is to mark the edge of the card with a felt tip pen. This makes it quite clear that the board has gone through a different process, which may be considered during defect investigation.
The practice of washing off boards should be defined, and a process made available to conduct the operation. The author has been in some factories where the pile of boards for wash off was nearly as high as the printer. The number of washed-off boards should also be monitored as part of a process control strategy to indicate when the process is drifting. Cleaning and drying boards prior to re-printing is important to consider. If the cleaning material is not fully evaporated from the surface of the board or solder mask, it will affect satisfactory reflow of the solder paste. Often this looks like incompletely reflowed paste.
Stencil Inspection
The following is a guide to inspection and in-process control for a stencil in manufacture.
All stencils should be checked prior to release to the shop floor for production. The stencils should be checked with reference to the design data used to produce the foils when first received. It is recommended that selected apertures be measured and compared with the Gerber files to check for any aperture modification.
On receipt into Goods Inwards, the stencil should be marked with the reference number, stencil thickness and issue number. This information can of course be included on the surface of the stencil by the manufacturer if required, the same as is referenced on the route card or work instructions. A stencil log sheet will be attached to the stencil box for monitoring its use and age, any damage and requests to re-order.
The stencil should be inspected for any obvious faults: kinks, poor adhesion to the frame or mesh, missing or blocked apertures, correct orientation, missing fiducial marks, etc. The stencil should be checked against the artwork/design file (if available) or a PCB for any obvious defects. The stencil log should be completed to show condition of the stencil and used to record usage.
The stencil will be placed in store until needed for production and will be issued with the kit or taken from the stencil rack. When printing has been completed and the stencil cleaned, the operator will check visually and complete a log. It is recommended that apertures in the four corners and the centre of the stencil be checked for correct cleaning. If the stencil has a frame it should be checked for good adhesion of the bonding material to the foil and that no tension has been lost.
After cleaning, care should be taken to check the smallest apertures and any step-down areas for paste residues. If the stencil is in poor condition the supervisor should be informed and should make a decision to scrap the stencil. If the stencil is in good condition it should be returned to storage area.
If any stencils are defective, material control should be notified to enable a new one to be ordered promptly. The log should be removed from the box and given to material control to indicate that the stencil has been scrapped.
All stencils should be check prior to loading on the printer and before the application of paste. Again the apertures in the four corners and the centre of the stencil, as well as the finest pitch, should be checked for paste residue. Any paste blockage will require the stencil to be re-cleaned prior to use.
Bob Willis is a process engineer providing engineering support in conventional and surface mount assembly processes. He runs production lines for suppliers at exhibitions and also provides seminar and workshops world wide. Bob has one of the largest collection of training videos, interactive CD-ROMs and training material in the industry. Bob will be presenting four Master Classes at APEX in California, he will also be presenting classes at SMT Nuremberg in Germany for those engineers visiting the show. For further information on how Bob may be able to support your staff contact him via his web site www.leadfreesoldering.com
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Fortunately even the most conservative most prognosticators now see at least a subdued “recovery” in 2010. 
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Posted by: Oscar Vais () on 17 July 2009 at 09:54