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Should We Measure Solder Paste Thickness?

Should We Measure Solder Paste Thickness?
On average, how many locations should we measure our PCBs to confirm our solder paste thickness? Our PCBs are approximately 10" by 10".
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
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
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.


And welcome to Board Talk with Jim Hall and Phil Zarrow, the Assembly Brothers, here to answer your SMT and electronic assembly questions and issues, and things like that. 

Jim, today we have a question from M.S. who asks, On average, how many locations should we measure our PCBs to confirm our solder paste thickness? Our PCBs are approximately 10" by 10".  

I'll start off by saying that the answer is as many as you possibly can. But, let's qualify that, shall we Jim? First of all, if you're using - it depends on what vintage SPI system and whether it's online or offline. If it's an online system, again, what vintage is it? 

We're seeing more and more machines, more modern machines now that are capable of speeds where something like that, again depending on how many sights you have there, but pretty much being able to digest the entire board, not become a bottleneck in your line, which is something you don't want your SPI to do.  

And then if you're doing it on a AQL basis, where you're sharing your machine, you're doing it offline, I guess, it almost depends on how much time you have to spend on it, your patience and what frequency you're doing it.  

In either case, if you have to limit, because of time, or the bottleneck issue, I would recommend that you give priority to the most important areas of interest, shall we say. Want to elaborate on that, Jim?  


Sure, if you had to do one, I would pick the pad counter deposit, which came from the aperture with the smallest area ratio. That would be the most likely to be non-repeatable and to give you problems. If you have a very big apertures, you are worried about getting too much paste, you might want to look at them.  
So that would mean your smallest component, like a .5 or .4mm BGA or CSP ultra miniature chip component, like an 0201 or 01005, those are the types of components that are typically more likely have very small apertures that have restrictive area ratios, lower than .66 to use the traditional number.  

Beyond that, looking across the board, opposite corners, opposite sides, to make sure there's no variation, and so forth, and with all of this you want to ideally with any measurement, putting on my lead six sigma master black-belt hat, all inspection is non-value added. You don't want to do any more than you have to.  

Obviously, as Phil said, if you have one of these new high speed SPI systems and it can look at all of them without slowing down your line do it. If you're not, you want to focus on those points where you have a tendency to cause mistakes.  

So you should be correlating your defect data on this specific board and relating that to what sites has solder deposits. Are you getting defects at any kind of frequency and then you might want to check those?  

Likewise, if you're measuring a specific aperture, a pad on the board, as part of an every board SPI strategy, and over time you find you never have any defects on that pad, do you need to measure it anymore? That would be the common strategy. If you're SPI is, for whatever reason, is time limiting, your overall production.  

I'm disappointed in you, Phil. You didn't start off by saying, M.S., we really applaud you for realizing the importance of measuring solder paste thickness.  


Oh, I would think by now, that they heard our soapbox so often it's practically worn into the ground.   

But I do want to add one other thing is, another area going back to soapboxes, my original battle cry if you will for incorporating SPI is other areas of interest are the joints that you can't see, visually or AOI after they're soldered. In other words, get them right in the first place.  

And of course we're talking about area arrays. BGA's, BTC, and certainly chip scale flip chip, because now is the time to get it right.  

To me, that's one of the most powerful uses of SPI. If you get it right now, then I think most of us have had the wonderful opportunity of having to repair an area array device. Isn't fun, and sometimes it makes things even worse. So, get it right in the first place and that certainly includes proper solder paste volume.  


Taking it one step farther, my brother, once you get beyond the stencil printing stage the next stage is placement, and visual observation of those sites goes away. If you don't catch it at SPI, you're going to have to wait till you X-ray it.  


Exactly. That's a bad time. It so much value, and so much labor to correct any problems. Yeah, it just increases exponentially.  

With that, I will say that regardless of how accurate your measurement of your solder volume is - well, don't solder like my brother.  


Don't solder like my brother.


Spot on with your feedback on the need for SPI to get the job done "right in the first place." The need for some level of SPI is critical to the overall quality of the end product produced. I look at it this way when trying to promote the use of SPI within the production process - anyone can build a good board, but how many desire to build the best board possible? Without SPI and the means to fully qualify the overall print quality prior to placement, your ability to deliver the best board possible becomes simply...Impossible.
Steve Arneson, ASC International

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