How Long Solder Paste Can Be Left On a Stencil Without Activity
How long can we leave solder paste on a stencil without any activity? If activity means not printing, then we are talking about a pause or abandoned time. The Assembly Brothers, Phil Zarrow and Jim Hall, discuss this scenario and share their own experiences. 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 50 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 Jim Hall and Phil Zarrow, the Assembly Brothers Pick and Place. We are here to attempt to answer your questions, processes, materials, handling, and all that good stuff. Today, Jim, we have a question from S.K. He or she is asking, how long can we leave solder paste on a stencil without any activity?
Jim
Okay, I think we need to define activity. If activity means not printing, then we are talking about a pause or abandoned time or whatever. If you have a good solder paste manufacturer, they should give you some guidelines on what that should be. Our ITM recommendation is that you should have qualified that solder paste, and you should have measured your production environment with your boards. Perhaps you would like to elaborate, Phil.
Phil
Yeah, because the actual pot life of the solder paste on the stencil itself is going to be affected by relative humidity in your ambient atmosphere and temperature in your atmosphere. Again, when we do our evaluations, we don’t do beaker tests, we don’t do it in a lab, unless of course, the solder paste is going to be used in a lab.
We do it on the assembly floor with the actual equipment itself. So that is where you want to test it. Again, we are assuming for this discussion, by the activity, they are meaning the actual printing process, printing the solder paste. You are doing high volume and you are just shooting through boards and, accordingly, solder paste, probably not an issue. But I suspect that this question is from somebody who is doing lower volume, perhaps batch work, things along those lines.
So, the solder paste is going to reside on the stencil for a given amount of time. Of course, it is going to be replenished because you have to have at least a half-inch roll. But it is going to be residing there. In some of the tests that are used, there is print the pause test is probably the most common. The idea is you start off with your candidate paste you print three boards. You check the volume on all three. Then you let the solder paste sit on the stencil, again in your ambient environment, for an hour. Then, you print three boards again. Then you check the volume for all three prints as well, and it should be consistent.
You can do that as far out as you want, depending on your environment. I think when we do an evaluation, maybe three or four hours. It is seldom that we see solder paste sitting on boards any longer, but whatever you are doing. As Jim said, there is a spec usually given on the solder paste data sheet. Don’t forget to read your solder paste data sheet. It is probably a general guideline, but again, they don’t know your environment, etc. As Jim said, it is something you have to evaluate yourself. That is the best practice.
Jim
Phil, one thing you didn’t mention about our tests, when you run this test and print a board, you should use your most difficult stencil with the smallest apertures because that can affect the pause time also. Obviously, a stencil with big apertures is going to be less subject to a slight variation in the paste, whereas a stencil with small apertures would probably start showing a reduction in volume or a consistency of printing sooner.
Phil
My brother has made a very good point. We definitely state that emphatically. Well, good, I think that we covered the topic. We look forward to whatever additional input our listeners have. You have been listening to Board Talk with Phil Zarrow and Jim Hall, The Assembly Brothers. No matter how long that solder has been sitting on your stencil, whatever you do, don’t solder like my brother.
Jim
And don’t solder like my brother.
Comments
Testing for Environmental factors should include seasonal changes. Most larger manufacturing operations I have worked in have big temperature and humidity swings between the middle of winter and the middle of summer, while still falling in "acceptable" ranges for ESD controls and operator comfort.
But while your solder paste may work great at 72F and 40% humidity, it may be crap at 85F and 65% humidity. Some paste data sheets give you the ranges that the paste has been tested to, but as the brothers say, you need to test in real world conditions with your assemblies on your equipment and in your environment.
Alan Woodford, Zentech
The stand-time is the factor to look at. I contacted the paste manufacturer, and, with our particular paste, 45 minutes was declared as the absolute limit. Empirical issues prompted the need to contact the paste manufacturer. One of the issues was extended down times with the screen process sitting for 2 hours at a time then compounded with some screened boards sitting for extended time (days, in a refrigerator).
By just enforcing the stand time limit, the solder defect rate went for several percent (sometimes double digit) down to around 0.04% In a two-step process, get rid of the large refrigerator and use a 'mini' refrigerator in which solder paste containers fit but circuit boards would not fit, then relentlessly monitoring the process to the point where the offending personnel complied just to be rid of the constant monitoring.
Jaye Waas, Renkus-Heinz
You really have to do your own testing, with your most challenging stencil. I had a supplier that wrote into a procedure that 4 hours was the max time on a stencil, yet they had no data to support the process. In fact, they were having trouble with BGA's after an extended shutdown of the SMT line but refused to look at the paste because their process said it was ok to do that. SMT machines are finicky and are prone to problems. They do go down from time to time and for changeovers. Unless you have data, it's safer to throw away old paste.
