|Ask the Experts|
November 20, 2019
Stencil Cleaning Procedure
Currently, as needed, our EMS contractor wipes the bottom side (board contact side) of each solder paste stencil using lint-free wipes saturated with IPA. Then once every 4 hours they remove each solder paste stencil from the screen printers and wipe both sides using IPA saturated lint-free wipes. They follow by blowing dry using air pressire.
Is this a proper procedure for cleaning solder paste stencils?
|Expert Panel Responses|
I would recommend reviewing the IPC-7526 document, Stencil and Misprinted Board Cleaning Handbook. It's a very good overview of stencil cleaning methods, available chemistry types, and process considerations. It will give you various options to consider when matching with your process and requirements.
The drying method you mention may need to be reviewed if you are using "shop" air, dryed or not. If you are using shop air, that's notorious for carrying compressor oil through the system, either by inadequate filtration or blow-by. Knockout separators rarely do an good job of removing the oil contaminants.
I would suggest either getting a dedicated cylinder of "5.0" compressed air or nitrogen if you feel you must dry the stencil after the IPA cleaning method. IPA has a fairly quick evaporation rate and the additional drying may not be necessary.
This is not an unusual process to see when cleaning a stencil. It is important to not abuse the use of IPA during the bottom side cleaning of the stencil.IPA if it gets in contact with the solder paste or permeates to the surface of the stencil may impact solder paste properties negatively.
Cleaning the stencil manually with a complete wipe down and a final compressed air blast is acceptable as long as all apertures are clear of any solder spheres and flux after cleaning. A close examination of the finer apertures is important to make sure all the paste or solder spheres are removed.
There are automated systems that clean the bottom-side of stencils and stencil cleaning machines to efficiently clean off paste residues but manual methods are still used in many places.
Some solder pastes depending on their chemistry may be better wiped down with a cleaner containing a combination of solvents. It is also easier to clean a stencil if cleaning is initiated after completion of the printing operation. Time allows solder paste to dry out making cleaning more difficult. Always a good idea to use a mask, protecting goggles and gloves when doing this operation.
Senior Market Development Engineer
In this process you have many variables that need to be optimized dependent upon the acceptable reliability level of the CCAs. The variables are (at the least):
Additionally, high reliability products require that the stencils are removed on a 4 hour frequency, and then washed in an automatic stencil washer or by hand using a hand-held ultrasonic cleaner to blow out the apertures. Your quality control group should be inspecting the solder flow after stenciling to ensure it is meeting your customer's requirements. If it is not meeting their requirements, then you need to clean more often. It all comes down to the reliability level that you are trying to achieve.
The practical answer is "it is if they can show that it does the job." What you really care about is that the print quality, board to board, is consistently good. This type of cleaning can be effective with good technique.It can be more effective if a moderately soft brush is used on the"double-sided cleaning" step. A lot depends on the paste and stencil as well.
Stencils with very small apertures will be more difficult to clean, and some pastes react more slowly to the solvent, making them more difficult to remove.Any paste that does remain will be "dried in" and may build up with time.Inspection of small apertures under a microscope can be helpful in identifying incomplete cleaning problems.
In the end, the EMS provider needs to understand how the cleaning process affects their printing results. If they can show that the quality of the prints is consistently good, then by definition the cleaning process is doing the job.
This is an excellent opportunity to bring the Stencil &Misprint Board Cleaning Handbook (IPC-7526) to your attention. A free copy is available on our website: http://www.smartsonic.com/article.html.
While the stencil is on the printer, the most practical method of cleaning is with a wet wipe. However, depending on the flux,alcohol may not be the chemistry of choice. Alcohol is a very reactive chemistry and may change the chemical composition of the flux. The resulting alcohol/flux compound may actually be more difficult to remove and/orcause other flux related issues.
When the stencil is removed from the printer, it should be cleaned using a stencil-cleaning machine specifically designed for the task. Manually wiping the paste will only push the paste into the apertures (similar to the action of a squeegee). Wiping may remove the majority of the paste from the stencil surface, but the apertures will remain contaminated.
Over time, this paste can dry hard as cement and become more difficult to remove and impede the performance of the stencil. For cleaning fine-pitch stencils, a machine incorporating ultrasonic technology and an appropriate cleaning chemistry should be used. Machines using spray technology may be adequate for stencils with larger apertures.
The use of compressed air to dry a stencil could potentially bend delicate landmass areas between fine-pitch apertures disrupting the coplanarity of the stencil. High pressure air nozzles are considered a hazard by OSHA as the nozzle can loosen and become a projectile. Low pressure dry compressed air (the same air used to operate a pick and place machine) is the fastest and safest way to dry a stencil.
The principal is not to "blow" the moisture off the stencil. The desiccated air, when applied to the surface of a stencil, will safely lift/evaporate the moisture. A 29-inch stencil can be dried in less than 2 minutes using low pressure dry compressed air.
Smart Sonic Corporation
Interesting question and wonder about what problem or problems exists by cleaning the stencils in this fashion. I imagine the intent of the initial IPA cleaning would be to prevent smearing on the bottom side of the stencil to prevent solder paste from being deposited all over the surface of the printed board.
The second cleaning would be to make sure any solder paste did not dry in the apertures of the stencil. By using a large amount of IPA it will soften any paste in the apertures and blowing it dry may clean out the solder paste from the apertures.
The other way of doing this cleaning is to use a stencil cleaner and place the stencil in the washing machine and cleaning off all the solder paste.
Either way I would document the process so some consistency exist in the way it is done. Secondly I would keep track of solder paste misses during the paste deposition process.If both of these are in place and there are no problems, my answer is if it works it's okay.
Vice President, Technical Director
Here are my thoughts on the process you are using:
The use of IPA remains common because it is cheap, has an acceptable efficiency and evaporates fast. Its flammability has to be taken into account.
Regarding the efficiency, IPA is able to dissolve most paste residue remaining in the apertures but some new generation solder pastes may contain products that IPA will not dissolve. So it is important to check the efficiency of IPA on the solder paste used.
Regarding the under-stencil cleaning method, (is it done manually?) I suggest to implement a cleaning frequency according to the type of PCB, and to wipe with a dry cloth after the wipe saturated with IPA. It is also important to be sure that the IPA does not get in contact with paste that will remain on the stencil, it could cause paste reaction, resulting in dry aspect.
Regarding the procedure of cleaning every 4 hours, the good point is the complete removal of the paste.I would say it is better than nothing to clean both sides this way, but a real cleaning in a machine would be better. To summarize, this procedure for cleaning solder pastes stencils can be upgraded by using automatic under-stencil cleaning, with preferably, dry wipe every x board and wet+dry every 2x board (depending on stencil apertures) and the use of nonflammable cleaner if possible.
Inventec Performance Chemicals
IPA wipes are very popular so they are used a lot. Being a manual process though, it depends so much on the operator's experience, judgment and dexterity. So,even the lint-free wipes can leave fibers on the stencils if not used properly.Every assembly is different so the rules have to be developed for each one of them.
I prefer automatic cleaning (happening on the printing machine) with dry, wet, vacuum cycle combinations based upon the complexity of the board, number of strokes, paste used, cycle time, etc. and a complete wash cycle in a stencil cleaner with an "as-necessary" frequency.
Also from a process perspective, look into applying nano-coating on the stencils (especially the ones with small apertures). The paste release is so much better and also you will find that the required stencil cleaning frequency will be way lower.
Engineering and Operations Management
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