Every 4 hours we remove each solder paste stencil and wipe both sides using IPA lint free wipes and then blow drying. Is this the proper procedure? Jim Hall and Phil Zarrow, The Assembly Brothers, address this question. 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 the Assembly Brothers coming to you from the ITM Consulting failure analysis cave buried somewhere deep within Mount Rialto.
We're here to talk about electronic assembly, materials, equipment, components, practices and procedures and a lot of other things. But today our question deals with stencil printing.
Jim
Stencil cleaning is the topic and we're actually going to try to cover two questions. The first one is from P.M.
Once every four hours we remove each solder paste stencil from the stencil printer and wipe both sides using IPA saturated lint free wipes then follow by blow drying using air pressure. Is this the proper procedure for cleaning solder paste stencils?
Next question comes from M.K.
We are in the market for a stencil cleaning system. What are the advantages and disadvantages of standard spray cleaning or ultrasonic cleaning for the job? Are there cleaning solutions specifically designed for stencil cleaning?
Phil
Let's start with the second question first because we like to do things in reverse order.
There's a number of different stencil cleaning systems and certainly best practices are that you should always clean the stencil before putting it away in storage. Never put away a dirty stencil. While you're at it, inspect it for any damage.
Never continue using a damaged stencil and cleaning is a good time to look at it when you take it off the machine and send it over for cleaning.
As far as spray versus ultrasonic, they both work and Jim will talk a little bit about that solution but as far as the mechanical action they both work. I have had personally more experience with the ultrasonic.
It seems to work well particularly as we get into finer apertures.
Jim
This refers back to the first question, IPA is not a universal solvent for all solder pastes and flux systems.
Whatever method you're cleaning you should be using the proper solvent for the paste that was on that stencil. IPA may work for some pastes, but it's not going to work for everything.
Make sure in all cases you're going to use the proper solvent. I personally feel from my experience that either spray or ultrasonic is superior to hand wiping.
You get a better flushing action and it's a more comprehensive system that you know the whole surface is being mechanically cleaned, more uniformly than if somebody's doing it by hand with wipes.
Cleaning systems are available and I think they are superior to hand wiping.
Always inspect a stencil when you put it back on the machine to use again because it could have been damaged between the cleaning cycle and the time it got back to the machine.
Phil
The same principle applies to the squeegee. You want to clean the squeegee, store it, make sure it's clean before you put it on and always inspect it before and after you use it.
Well you've been listening to Board Talk with Phil and Jim, the Assembly Brothers and remember Board Talk has not been FDA approved and possible side effects may range from enlightenment to bewilderment.
But no matter how bewildered or bedazzled you are, don't solder like my brother.
Jim
And don't solder like my brother.
Comments
AIM has conducted studies comparing IPA to engineered solvents designed to be compatible with AIM solder pastes. We demonstrated that IPA degraded paste performance and stencil life and a compatible solvent extended the stencil life and improved transfer efficiency on fine pitch printing. Study results can be found at: https://aimsolder.com/technical-articles/effect-under-stencil-wipe-chemistry-print-performance
Timothy O'Neill, AIM
Cleaning your stencils with an automated system versus hand cleaning is always preferred for a consistent result. And as others have mentioned, hand cleaning runs the risk of damaging some of todays stencils and nano-coatings. Solvents definitely play an important part, so be sure you are using the correct product for your paste. Some require heat and some do not. Chemical companies can best guide you the best solvent for your application.
Spray in air does offer the flexibility to use aqueous, combustible, and flammable solvents when the equipment is configured correctly. Ultrasonic and spray in air are both great options. In my experience, I have witnessed issues with redepositing solder balls in small apertures when using ultrasonic cleaners. Therefore I prefer spray in air over ultrasonic. Some of the nano-coatings can also be damaged in ultrasonic, so best to consult your stencil manufacturer for their recommendation on method.
Todd Rountree, Austin American Technology
As always, when cleaning stencils either by hand, spray or ultrasonic bath, any solvent used should match the solvent used in the solder paste. IPA works on pastes with an alcohol-based solvent and aqueous-based solvents work well on aqueous-based pastes. Some of the more exotic solder pastes will require a chemistry booster such as ethyl acetate, so check with the paste supplier. For stencil hand cleaning, wipe quality selection is very important in avoiding damage to any stencil nano-coatings.
It is best to use a soft, clean wipe material and avoid wiping with high-cellulose content wipes, which can be too abrasive. While a stencil still is on a printer, it may be wiped with a pre-saturated wipe that has the correct solvent. When thoroughly cleaning a stencil that has been removed from a printer, a heated aqueous-based solvent in an ultrasonic bath will dislodge paste from tiny apertures more readily. Please note that some solvents are flammable and are not suitable for ultrasonic bath use.
Russell Claybrook, MicroCare, LLC
Having used both the spray cleaners with solution and the Ultrasonic cleaner with solution, the takeaway has to be, check with your solder paste representative and your stencil house to find the proper solvent solution that matches your combination.
Most solder pastes require elevated temperatures with specific solvent solutions with either spray or ultrasonic equipment. Both will clean your stencil, but I have always gravitated toward the ultrasonic bath for smaller apertures.
The caution is, regardless of which method you choose, make sure you talk with your stencil house as the adhesives used to adhere the foil to the frame may deteriorate in some solvent/heat combinations. You have to find the "happy medium" for your application and facility.
