For automatic under-stencil cleaning there are many cleaning methods including dry vacuum cleaning, wet dry vacuum cleaning. What factors should we consider to select wet based cleaning versus dry based cleaning? The Assembly Brothers, Jim Hall and Phil Zarrow, discuss this scenario and offer their suggestions. 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
And welcome to Board Talk with Phil Zarrow and Jim Hall, The Assembly Brothers. If you happen to be having a process problem or situation, you are actually in the right place at the right time. At least we hope so. Jim, today we have a letter from SA. SA writes, for automatic under-stencil cleaning there are many cleaning methods including dry vacuum cleaning, wet dry vacuum cleaning. What factors should we consider to select wet based cleaning versus dry based cleaning? Is it related to the amount of residual inside the aperture walls and the bleed-out under the stencil? What are the typical cleaning cycles for both cleaning methods?
I heard from some printer experts who say wet based cleaning removes the flux from the stencil aperture walls and this flux enhances the paste release through the stencil. Therefore, they want to minimize the usage of wet-based cleaning. I’ll start off by saying that in the beginning going back about 30 to 35 years, the printer companies came out with the idea of under-stencil wipe, up until then it was done manually. So, this was further automation.
The original ones did use a wet wipe followed by a dry wipe pulling vacuum. The problem was no matter where in the system they didn’t work really hot in a sense that it did leave some of the solvent in apertures, didn’t do a complete wipe. It is almost like an operator not doing his thing and it screws things up for the next introduction of solder paste on the next print cycle. A lot of people just said, I’m moving away from solvent. I’m going to do a dry wipe only. That became the situation.
Since then, the printer companies have improved the wet wipe portion of the cycle. But a lot of people had bad experiences, had their fingers burnt in the electric outlet or something and only do a dry wipe. Others do dry wipe to save on expenses, basically the wipes as well as the solvent. Take a look at the modern equipment, it is pretty good that way.
One important thing mentioning solvent is that it has to be matched to what you are trying to clean off. Isopropyl might work. First of all, remember isopropyl has a certain flash point and it may not be right for the solder paste. Consult the solder paste company, what they feel is the right solvent for wet wiping off the stencil. The other thing I’ll add, is the frequency is basically determined by the stencil geometry.
In a lot of cases, you have very small apertures, high density you might indeed have a more frequent wiping. Of course, other things are going to affect it, nano-coating seems to improve it to a certain degree. You want to have a good clean aperture so that you have good release. That is kind of what it is all about.
The best way to do this is by doing the design of an experiment where you wipe, look at it on a light table, how does it look, do another wipe. It is a little tedious but you will develop for each aperture design what the frequency should be. We have seen in some cases where people go 20 prints before wiping and we have seen people who are wiping after every print. What some of the newer printers have over the last couple of year, is they are using the cameras to actually inspect the apertures on the stencil.
If they see it is required, they will actually cycle it that way.
Jim
One thing I want to confirm from the question is cleaning related to the residual inside the aperture walls and the bleed-out under the stencil. The answer is yes, both of those can hurt you. Not getting good release if you are building up in the apertures and getting smearing, causing bridging or solder balls if you are getting a bleed-out that stays on the bottom of the stencil. All of these materials cost money and time. You want to minimize that. If you can do it with a dry wipe, great.
Fluxes, particularly no-clean fluxes that most people are using. Just dry wiping the bottom of the stencil can be problematic to get all of that residue off of there. Just to emphasize what Phil said, get the right solvent for the paste that you are using. If you are using wet cycle, that you dry wipe vacuum at the end gets all of the liquid solvent out of the apertures. Because if they are in there when you start your next print, you are just going to have problems.
Phil
We hope that we answered your question SA. A very important aspect of the soldering process, another aspect of printing. I just want to add that no matter how frequently you wipe the bottom of your stencil, please don’t solder like my brother.
Jim
And don’t solder like my brother.
Comments
Stencil apertures have gotten much smaller to accommodate new passive components such as 01005 and 201 parts which have solder paste pads that are smaller than the naked eye. So wiping and vacuuming the underside of a stencil designed for these parts is essential to good printing.
The question of wet- vs dry-wiping has been around for a long time and should be considered as part of the discussion, but care should be taken when choosing a solvent that will not damage expensive equipment. Plus, many stencil printing equipment manufacturers offer a list of acceptable solvents that can be used for wet-wiping. IPA is a flammable solvent and many manufacturers of solder printing machines have strict policies about use. Also, any wet solvent left in tight apertures can dilute subsequent printing of solder paste (the argument for dry-wiping, only), so the dry-wipe and vacuum after a wet-wipe is essential.
The key is to use a wipe material suited for the application, so a softer wipe is preferred when a stencil has been treated with a nanocoating, for example. But softer wipe material also has a higher absorption capacity and has a distinct advantage after a wet-wipe. So, part of the evaluation of wet versus dry wiping should take into account the wipe material being used.
A softer, more absorbent wipe material selection is critical to extending the number of printed circuit boards between wipe cycles whether doing a wet- or dry wipe. So a softer underside stencil wipe may perform better than a less expensive one.
