With nano coatings on stencils evolving, what is the best method to measure the thickness? How do you know if the coating degrades over time? The Assembly Brothers, Jim Hall and Phil Zarrow, share their own experiences and recommendations. Board Talk
Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up CEM Selection/Qualification, SMT Training/Seminars, Legal Disputes
Phil Zarrow
With over 35 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis.
He has vast experience in SMT equipment, materials and processes.
Jim Hall
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics.
He is a pioneer in the science of reflow.
Transcript
Phil
Welcome to board talk with Jim Hall and Phil Zarrow of ITM Consulting, the Assembly Brothers. What's today's question Jim?
Jim
This question comes from R.W. With nano coatings on stencils evolving over the past few years, what is the best method to measure the nano coating thickness and how to determine if it degrades over time?
You just get out your nano ruler that has nanos on it and nanometers and you just stick it down on the edge of the board... nano coatings may be single molecules thick.
There is no practical way short of a very, very exotic materials lab to measure the thickness of a nano coating. Nano implies ten to the minus ninth meters. It's tiny. It is real, real thin.
And how it degrades over time. I guess my feeling is what you said, is evolving over the past few years. They are still evolving. New improvements are coming out monthly or semi-monthly.
You have to at this point rely on the manufacturers to tell you things like how long it's going to last, how to determine if it degrades.
The obvious point is do you start to get the defects that you were trying to eliminate when you applied the nano coating. You're trying to get better release, more consistent deposits, less smearing on the bottom so this can reduce your cleaning.
That is why you're doing it and so if it's degrading you should see the effects. You should be monitoring your solder paste printing in some way and you're using your SPC chart.
You should see the variation in height or volume as the nano coating degrades. Talk to the vendors, hopefully they can give you some references about other people's experience because it's new, it's evolving.
There is not a long track record and I'm sure with the different materials, the working life is going to vary, but definitely the performance of your printing should be the best indicator.
Phil
That basically sums it up. Beyond that I'll say you've been listening to board talk with Phil and Jim. Once you've printed that solder paste and you go to reflow it, whatever you do don't solder like my brother.
Jim
And don't solder like my brother.
Comments
Nanocoating thickness can be measured by Ellipsometry and similar optical measurements. Now-a-days Ellipsometers are not very expensive. You can get it for $ 50-60K. Another technique is to use FTIR to measure the intensity and do a calibration curve - which is typically used in HDD industry. For an end-customer/user, it's best to monitor the performance. Dyne pen, Marker, Contact Angle measurements are also easy techniques, but this would not give you the thickness of the coating.
Rajagopal
There should be some way to indirectly measure the thickness Nona coating, such as using volume and area to calculate the thickness?
To monitor degradation of Nano coating, surface energy measurement could help?I mean using chemical material or water on stencil and measure the contact angle, you may see the variation from contact angel.
Hong Wang
I am a combo SMT supplier. I specialize in both the SMT printer world and manufacture stencils for many CEMS in Latin America, from Mexico to South America. The concepts of each of the stencil technologies brings results based on individual needs, which work for basic applications but today the circuits are all far from basic.
Challenging all the printer parameters and applications. The stencil continues as a key element to this process. The emerging developments of the Nano/Polymer solutions seems to be very good except that the basics are not clear, the customer wants to know what "New" processes must be adapted to make sure that his process will not suffer, say at 0200 in the morning, when the normal staffing of engineers are not around....they cant afford the breakdown, given the restrictions now placed on the assembly houses, worldwide.
I have tested many Nano's, even developed a competitor to the Factory coated version of the nano's. The Dyne pen is a product which is effective but once again what to do if the test fails. We have to remember that the solution must be "FACTORY" adaptable.
The terms:
Ease of application
Homogenous process, from app to app.
Length of service
COST
Effectivity
Process support
Maintenance
ROI
Customer Support
The error factor.
I have reviewed many of the cases where the customer gets a Nano and also one of my stencils and says they work very well....in the beginning....then time tells all. I have been compared to Nano's, Efabs, Nickle and even the old famous PHD/FG types....I have improved printing results based on end of line yields, not just print results from an SPI. Remember the SPI only monitors the Print process, it is your line engineer alarm, but it is based on process.
The key here is developing the final product based on process, not just a blanket for all solutions. Everybody in the SMT printing world know that whatever solution you use must be adaptable to their factory world....not just a lab. The key word is Assembly Process.
Jim Villalvazo, InterLatin
It is great to see that nano coatings are being broadly adopted both here in the US and overseas. As the leading producer of nanocoatings for many industries, we understand a thing or two about how to coatings and how to measure the presence of a nanocoating as well as some best practices on how to extend the life of the coating.
Firstly with the industry's predominant coating for stencils , i.e. the wipe on version, sometimes referred to as a monolayer coating, the simplest way to measure the presence of the coating is the use of a dyne pen.
If the dyne fluid beads up then the stencil is still hydrophobic and the coating is still present. These dyne pens are sold on our website (aculon.com) for $10. The prior reader's comments about using a sharpie pen is not good advice as they are not as accurate as the dyne pens.
Relating to durability while many customers in the US have reported that monolayer coatings is a permanent solution, the real answer is it depends. You should check your stencil regularly with the dyne pen to ensure that the coating is still functioning as expected. It is also fair to say that many US operations are running 10,000-25,000 print cycles, as opposed to higher volume in Asia such as 50,000 cycles. Clearly the stencil takes more of a beating with 50,000 print cycles.
After 4 years of working with this technology on stencils and customers we have developed a series of best practices to improve Nanoclear durability that include the following:
1. Utilize Soft non Abrasive Understencil Wiper Paper E.g. DEK Eco Roll SC-ER360
2. Utilize a Solvent Wipe Rather than a Dry Wipe
3. Utilize pH Neutral Cleaners
4. Reduce Understencil Wipe Frequency
Now if you find that the monolayer coating wears off, one of the great benefits of a wipe on system, as opposed to the thicker polymer system, is that a wipe on coating can be reapplied thereby giving you all the benefits of improved yield, reduce rework, better print consistency via reduced print volume variation and reduce use of consumables due to reduced understencil wiping.
The retail price of a wipe on coating is $25 and it takes less than 10 minutes to apply at your facility. As opposed to a polymer system that cannot be reworked as it requires specialist equipment and a cure process. Even though it requires specialist equipment the thicker polymer coating is also likely to be less consistent due to the greater thickness variability of the coating, they have variability of 2-4 microns.
By the way a nanocoating is defined as less than 100 nanometers, so a polymer coating is not nano at all! Finally if a polymer coating wears it is likely to wear in the apertures and can chip off in the apertures. There is no good test to measure that.
Also one thing to think about is that the polymer coatings claim to dramatically increase transfer efficiency. I have seen claims saying they increase transfer efficiency by 20-40%. My question is that a good thing? Do you really want 20-40% more solder paste going through? That does not seem like a process that is in control. Finally clearly the aperture walls are treated with the wipe on coating, otherwise why would all these companies benefit from more accurate paste deposits, improved printing and keep buying it!
Edward Hughes, Aculon, USA
The trick is to use the right coating. There are mainly 2 types of coatings on the market today monolayer type and polymer type. There are several white papers out discussing the different types of coating and their performances.
The so called wipe on coatings are a monolayer coating that is only applied to the surface of the foil and not in the apertures and these coating are colorless. This type of coating tends to not last very long and in some instances degrades rapidly. Another drawback to this type of coating is the fact that it is hydrophobic while it last on the surface but it impedes the flow of paste through the apertures since the coating doesn't coat the aperture wall stopping at the surface and then resisting the flow of paste on to the intended pad resulting on lower paste volume. This type does aid in less underside cleaning, less bridging and better brick profile.
The polymer type coatings are cross linked making them a much more robust coating. With this type of coating they should last the life of the stencil. This type of coating gives better transfer efficiency, improved paste release, reduced underside cleaning, improved brick profile and less bridging. With the polymer coatings dye can be easily added to the coating making it easier for the operators to see the coating and be able to see if there is any degrading of the coating over time. With polymer type coatings the application of the coating to the foil is just as or more relevant than the type of coating is. Uniform coating of the foil surface and aperture walls is key to having the best coating.
Coating thickness varies from one supplier to the next. Typically you will see a thickness of 2-4 nm for the monolayer and 1-4 microns for the polymer type coatings. Contact your stencil supplier to see how they control thickness of their coating and they should be able to supply you with that information quickly. The concern by many is whether the thicker coating changes the surface area ratio which is true in its self but is made up for and exceeded by the amount of transfer efficiency that is gained from having the coating. There are a few ways to detect the thickness of the coatings but doing so in the field is not easily done and are expensive.
Easiest way to see if the coating is still effective is to take a permanent marker and try to mark on the coated area, the ink should roll into balls and easily lift from the surface with a cloth. Now this is only telling you that the surface tension in that area is good or bad not whether or not the coating is still left on the aperture walls. Some of the coatings have a trace dye that can be easily seen with a UV light. Ask your supplier whether their coating has the dye or not. Just keep in mind it isn't all about if the coating is still there or not, it also has to do with the surface tension of the coating as well. Surface tension can be measured using a goniometer to detect contact angle of a drop of dye that is placed on the surface. Some coatings drops in surface tension over time and others stand the test of time.
