Solder Paste Inspection - When and Why

Solder Paste Inspection - When and Why
Our facility is considering a 3D solder paste inspection system. Are these systems common and under what condition should we consider buying one?
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.


Welcome to Board Talk, this is Phil and Jim, the Assembly Brothers pick and place.

Today's question comes from P.P.

Some folks at our facility are pushing us to purchase a 3D solder paste inspection machine or system, better known as SPI. Are these systems common and under what condition should we consider buying one of these fangled machines or systems?

There are a plethora of these machines available these days and you picked one of our favorite soap boxes. I don't know what else is going on in your facility, but as far as equipment goes, I can't think of a better investment going forward.

Especially with the component technologies and everything else that we have today. Solder paste inspection, certainly it's come a long time ago, certainly it is a process development tool, but an in-process tool as well.

Whether you are choosing a solder paste, or evaluating/optimizing your print parameters, how do you judge between what's a better quality or more repeatable print if you can't quantify the process. There is no better way to quantify the printing process than to measure the volumes of the individual solder deposits under comparable conditions or repeatability conditions.

To be specific, I'm looking at two solder pastes, I want to say which one gives me more reliable printing, well, you're going to print some tough boards, how do you compare? With a solder paste inspection system, you measure the volumes, see how consistent they are, see what the transfer ratio is and you can make a much, much more accurate comparison and use statistical analysis to make a choice.

And as far as an in-process tool, look at the number of components we have today where you cannot use visual inspection of the reflow joint.

In some cases, you can make a partial inspection, but not a thorough inspection of bottom terminated leadless components.

But with others including area arrays... I can tell you that in our consulting business, since we do a lot of process failure analysis, that many cases the problem has been inadequate solder particularly on area arrays. When you do the root cause analysis, basically this could have been averted with properly implemented solder paste inspection.

Properly implemented means, establishing realistic spec limits for your rejections coming out of the system. It's nice to have these systems, but if you don't use them correctly, use them effectively, based upon real feedback from quality results -- Garbage In Garbage Out.

We've seen a lot of that, these things have become very, very expensive pass through conveyors and that is like a sin against the nature of electronic assembly. But there are a number of these things available, whether you are doing it on a AQL basis off line with a really rudimentary per se, table top solder paste height inspection, if you got some inline volume inspection, anywhere in between, you are better off than not doing it.

There are some very good quality bench-top systems, granted they do one point at a time, but for an AQL level, for process development, at least to get your feet wet.

To get a feel for it, and to help you make a decision investing in an line system, I think it's really worthwhile, rent one, borrow one, check it out, go have a demo, but remember, at least 50 percent of the defects in all assembly come from solder paste printing.

Most test show more like 70 or 80 percent of the defects. So that's where you want to put your process development, that's where you want to put your inspection, at the post printing process in post print 3D inspection where you get height and volume just being shown to the -- in my opinion hands down the best way to do that.

Basically a fantastic investment, so it's going to work and it's going to be one of the things you look back with and wonder how you got along without it.

And if I might say this about the commercial market today, it's another one of those capitalistic successes you've got a number of companies out there, competing for a market for an ever expanding market, giving you a lot of features, a lot of capability, money, data outputs, setting up feedback to your printer, a feed forward to your other inspections machines, there's all kinds of exciting things that can be done, in overall process improvement process control, process optimization but with this, your SPI system is the building block.

So take a look what's out there and find the one that is best suited for you and beyond that this is Phil and Jim, the Assembly Brothers saying there's probably a good reason we're not in the IPC Hall of Fame, and probably don't want to solder like my brother.

And definitely don't solder like my brother.


SMT inspection was a $500M+ market in 2019. So SPI has become a common part of the SMT process. You should consider contacting the multiple vendors to find a right equipment and partner fit.
Allen Phung, Koh Young
Agree 100%. While most companies invest initially in AOI to catch defects post reflow, it should be imperative to use SPI to define and refine your printing process. If the printing process was perfect...there wouldn't be a market for SPI systems. Reduce a majority of your solder defects here vs catching them later. Use the tool and define the issues...but make the process changes. As the brothers say, don't turn it into an expensive conveyor by opening up tolerances and not using the data it can provide. Good Luck!
Sy Creed, CyberOptics Corporation
I would insure that whatever system you purchase you insure it will pass an Gage R & R study. I would make this a requirement prior to buying a system.
Steve Vargas, Polaris Contract Mfg. Inc.
An online SPI system is the perfect solution for most of high volume products. However, an offline SPI system is a very good solution for solder paste printing process troubleshooting. You can investigate issues on your whole printing processes. You can check anomalies of solder paste printing height or paste volume printing distribution, or even, misalignment of solder paste on PCB surface, caused by a bad PCB holding system inside your printer. Also, by inspecting not printed PCBs (as they were supplied) you can learn a lot about your PCB suppliers.

If you have two or three different PCB suppliers you can do a comparison among them, which is better in terms of flatness, for example, and explain why some printing problems never happen with certain supplier. For instance, you can perform a relative comparison between PCB solder mask heights with PCB pads heights. In some cases, higher pads work as step-up for nearest lower pads and these ones always get much more solder paste deposition, causing solder bridges or excessive solder, and you never detect this without a proper inspection by a SPI system.

Another advantage of an offline system, in case you have many SMT lines, you can develop an inspection plan for different boards being produced in these SMT lines. For example, four PCB panels per line per hour must be collected in each SMT line and, by using a specific sample plan, you can prepare a statistical process control for solder paste based on Cp or Cpk.

Another use of offline system as troubleshooter tool, you can follow up the conditions of squeegees, stencils, and also PCB holder system to decide when it is necessary change them, or to generate statistics for printer maintenance. In my experience I was able to use an offline system to perform a virtual x-section on soldered chip components (yes, after reflow) by using virtual x-section tool available at machine to check wetting angle for small components (as 0402, 0201, sot23).
Glayson Figueiredo, Philips Medical Systems, Brazil
Great article! Yes, manual bench-top systems are an excellent and inexpensive way to see the value SPI provides.
Allegra McNeally, VisionMaster, Inc., USA
I agree with the Assembly Brothers 100% on SPI. It is by far one of the best investments you can make on an assembly line. Regardless of your technology level, properly implemented SPI will improve yields and save money. Even if the budget only allows for a used or rudimentary system, it will still pay itself back faster than any other machine in the factory.
Chrys Shea, Shea Engineering Services, USA
I would suggest starting with a manual benchtop system to get your feet wet, as the system will serve you well now and in the future.
Bill Neuenfeldt, ASC International, USA

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