Our solder paste has a specified exposure life of ten hours. We run continuously for forty hours, adding 250 grams of new paste every two hours. After the first ten hours of operation do we need to remove all of the paste on the stencil and scrap it?
Phil Zarrow, ITM Consulting
With over 35 years experience in PCB assembly, Phil is expert in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes.
Jim Hall, ITM Consulting
Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.
Welcome to Board Talk with Jim Hall and Phil Zarrow of ITM Consulting, otherwise known as the Assembly Brothers. Today we are coming to you from high atop Mount Rialto, at the ITM printer extravaganza lab.
We are here to talk about all kinds of things related to electronics assembly, materials, equipment, components, practices and procedures. Jim, what are we talking about today?
Well, it is solder paste life on the stencil and it comes from S.S. Our solder paste has a specified life for exposure on the stencil at ten hours.
We run our stencil operation continuously for forty hours, adding 250 grams of new paste every two hours. The additional paste becomes mixed with the existing paste.
After the first ten hours of operation do we need to remove all of the paste on the stencil and scrap it? Currently the practice is to keep adding new paste until the end of the forty hours of continuous production.
First off I want to say, S.S. thank you for giving such a clear, concise description of what is really going on. That makes it much easier to give a reasonable answer.
Well, as reasonable as you would expect from us.
Well for me, putting on my lean six sigma hat, I would say what are your defect rates? If you are currently continuously running your process, adding your 250 grams every two hours, if there was a problem with that you should see it, in an increase in defects.
The most obvious we think about when leaving paste on the stencil is that it is going to dry out or degrade in other ways and it is not going to print well.
So my question is, are you inspecting your printing post print, either 2D on the machine or 3D post print. If you are, are you seeing your defects increase, the number of boards you have to wash and redo over that forty hour period?
If you are, then it may benefit you to completely change the paste and clean the stencil every ten hours, or at some other interval. Then further down at the end of the line the flux can be compromised and you don’t get as good wetting.
Well, what are your results from final inspection and test over that forty hour period? Are you seeing an increase in that defect level?
If not, then your current practice of adding the paste every two hours is probably adequate for, as my brother said, the conditions and the specific paste you are using, the level of difficulty of your printing, and your soldering.
Right, the operation, your procedures and your materials, you’ve got a good solder paste. So again, that is one of the other variables.
Some other solder paste might not give you as optimum, or some might give better. So that is another variable in the mix as well.
Yeah as Jim said, you’ve basically got to take hold of what you have there and go with that. The end result is what kind of yields are you getting.
Basic statistical process control, continuously measure what you are doing and look for trends. And even if you’re not getting bad defects, can you see some trends in there that are indicating there is a degradation going on.
We know what you are looking for. You are looking for reduced printability and reducing wetting, more solder balls, maybe even bridges if you are getting slump problems.
But those should be measurable. You should be measuring them in your process and using them to improve your process through feedback.
Right, as the saying goes “In God we trust, all others bring data.” Well good, you just squandered another five or so minutes listening to Board Talk.
We appreciate it and we look forward to your other questions and inquiries and comments. And in the meantime whatever you do, wherever you go.
Don’t solder like my brother.
Please don’t solder like my brother.