|Ask the Experts|
October 19, 2020
Stencil Cleaning Frequency
What is the industry standard for the amount of time between stencil cleaning?
|Expert Panel Responses|
I would recommend reviewing the IPC-7526 document, Stencil and Misprinted Board Cleaning Handbook. It's give you an excellent 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.
Typically I would consider the following when setting up a cleaning cycle.
Note: This is derived from me the product that I use. You will need to inspect your post printing, and post reflow, to determine your optimum recipe.
Process Engineering Manager - Electronics
Altech UEC, South Africa
There is no established industry standard for a stencil cleaning interval. It depends on the complexity of the assembly, the solder paste you use,stencil design and printer settings.
Rule of thumb is to clean your stencil once you begin to experience non-repetitive results. For example,I worked on an assembly with a component mix so high that we needed to clean the stencil every print due to small aperture clogs while on other assemblies we cleaned the stencil very 10-15 prints.
Senior Manufacturing Engineer
There isn't any "standard" for the number of prints between wipes. It is up to you to determine how many prints you can undergo before it is necessary to clean the stencil. It depends on several factors. If the initial setup is off, then you will need to wipe more often.
If the humidity and/or room temperature are too high, the paste will tend to slump and smear more easily, and you will need to wipe more often. If there are excessive HASL deposits near the apertures that prevent good gasketing between the stencil and the PWB, you will have some paste squeeze-out and will need to wipe more often. If the squeegee pressure is too high, the same squeeze-out can occur.
If separation of the flux and solids happens (inadequate mixing or poor quality paste) the flux will squeeze out onto the stencil and prevent a clean snap off. The more apertures in the stencil, the more chances for issues; some boards will need to be cleaned at a higher frequency than others. Old paste will tend to be of a higher viscosity(dried out) and you will see more aperture clogs. On the other hand,excessively sheared paste (printed several times back and forth on the stencil without replenishment) will lose viscosity and be more likely to have squeeze-out issues.
There are a lot of reasons where you could find yourself having to clean after every print. With all of the variables under control, a good setup and good paste and a good environment (67 to 69 deg. F at 40-50%RH) you should be able to print at least 10 very complex PWBs before another wipe is required.
As I have pointed out in several previous posts, there are a large number of variables that must be controlled in both the paste handling and printing processes. If they are not controlled poor printing results, as well as more frequent cleaning of the stencil between prints.
This is why the controls over both the material(paste), the preparation (proper mixing to bring to room temperature and within the viscosity range on the manufacturer's Technical Data Sheet), and the printer setup are so critical; they can greatly affect the amount of touch up and rework required as well as the efficiency of the paste printing process itself.
Take the time to ensure you have control over the variables from the time the paste hits the dock all the way through the printing process; every minute invested in that endeavor will pay off at least 60 times in lost production time and rework hours and prematurely discarded solder paste.
It's not that hard to accomplish, but it takes a real paradigm shift or sea change in education, awareness, and attitudes to keep it going.
I don't know if there is an industry standard, but common sense would seem to dictate to clean the stencils as quickly as possible to prevent the paste from drying in the stencil apertures. Some of these apertures are so small, such as 01005 and 0201, you would want to the paste to still be viscous and not dried when being cleaned.
I would think that if the paste dried out, it would much more difficult to removed and in many cases would not be removed and this would be cumulative on subsequent usage of the stencil and completely plugged hole would be the result.
It would also recommend that all facilities have semi or automatic cleaning systems to clean the stencils to reduce the variable of cleaning them manually.
Vice President, Technical Director
Your question is an excellent one. There is a Grand Canyon-sized gulf between production engineers in the electronics world as they debate the optimal stencil printing process. Some engineers clean after every print; some clean after twenty.
Some supplement the cleaning with multiple passes of the cleaning rolls, with solvents, with interim-cleanings using pre-saturated wipes and even ultrasonic cleaning of the difficult areas of the stencil. Then there are the "outside" experts:the stencil printer makers, the stencil cutters, the solder paste companies, and even the customers. It's a mosh-pit of opinions; real data is hard to find.
Why do we work so hard? The SMT process, when fine-tuned, deliveries economies of scale and through-put which could be achieved by no other technology. The total annual value of the electronics assembly industry probably exceeded $1 trillion in 2014 - a staggering output by any measure. The vast majority of this output is in the form of surface-mount circuit boards, which means somebody,somewhere, is struggling with their stencil printing process.
At the heart of the SMT process, stencil printing is the weakest link in electronics production. The mere complexity of the process is causing costs to rise and yields to fall. One industry expert (se: Richard Clouthier, "The Complete Solder Paste Printing Process: Stencil Aperture Area Aspect Ratio," SMT Magazine,January 1999) found 39 process variables that must be controlled to get reasonable yields in today's stencil printing process.
In another perspective on the limits of current technology, experts estimate that more than 50% of today's production defects are caused by errors in the screen printing process , and another industry leader has privately confessed to this author the number actually is closer to 90%). As noted by Kamen, Goldstein, Asarangchai, et al. in "Analysis of Factors that Affect Yield in SMTAssembly".
I'm not aware of an "industry standard" as far as time between cleanings goes,usually it's set up based on frequency of prints. Obviously the stencil should be cleaned if the prints start looking poor. For No-Clean pastes, I recommend every 3 PCB's for fine pitch devices, 5-10 for normal.
You can usually go a little longer with water soluble paste, but no need to push it. After breaks,lunches and shift changes, or any time there is more than 10-15 minutes between prints. Try doing some experiments to dial in your needs based on your technology and pastes.
Esterline Interface Technologies
The time between stencil cleaning is going to depend on your type of solder paste and printing process. If you use an SPI machine, this will tell you whenyour stencil needs cleaning. Some SPI machines have closed loop feed back to the screen printer, signaling when to clean.
If post print inspection is Done manually, you will need to closely monitor your process to develop a screen cleaning frequency. Your environment also plays a big role in cleaning screens. If you do not have a good temperature and humidity control system, you may have a difficult time coming up with a consistent cleaning cycle.
Manufacturing Applications Specialist
Assuming that you are talking about the frequency of an under stencil wipe, there is no standard. It is very process dependent. A good practice is a dry wipe every 6 to 10 prints.
If you find that you have to wipe more frequently and/or use a solvent wipe followed by a vacuum cycle, that is a good indication that something is wrong with the printer set-up; like poor gasketing.
Technical Support Engineer
I would recommend you obtain, IPC-7526, which covers all aspects of stencil cleaning...
There are a lot of factors that will determine the stencil cleaning frequency
Engineering and Operations Management
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