Electronics Assembly Knowledge, Vision & Wisdom
Solder Paste Transfer Efficiency - What/Why
Solder Paste Transfer Efficiency - What/Why
I've heard the term transfer efficiency relating to printing. What does this term refer to? What actions impact solder paste transfer efficiency?
Board Talk
Board Talk is presented by ITM Consulting

Phil Zarrow
Phil Zarrow, ITM Consulting
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, ITM Consulting
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.

ITM Consulting
* EMS Qualification, Evaluation and Selection
* SMT Process Consulting and Troubleshooting
* SMT Process Development and Set-up
* SMT Process Audits
* Lead-free Process Readiness Audits
* SMT Process Optimization
* On-Site Workshops
Submit A Comment
Comments are reviewed prior to posting. You must include your full name to have your comments posted. We will not post your email address.

Your Name

Your Company

Your E-mail

Your Country

Your Comment

Welcome to Board Talk and greetings from Mount Rialto, the Board Talk recording studio where we ponder the questions on process and materials and equipment that our wonderful audience sends in. Jim, what have got for today's question?  

It comes from B.Z. I've heard the term transfer efficiency relating to printing. What does this term refer to?  

Funny thing about solder paste. You have your stencil, your aperture, dimensions of the aperture, your thickness of the stencil.  So then you can calculate volume.  

Yes, printing is three-dimensional. Theoretically you have a volume of solder that you are placing in the aperture and that you are transferring to the board.  However solder paste does not necessarily want to go everywhere you want it to go.  

The bottom line is, what you're filling in the aperture with is not necessarily going onto the circuit board which is where you desire it. And there are things that can impact what we call the transfer efficiency to various degrees.  

So the transfer efficient is simply the ratio, usually expressed as a percentage, of the volume of the paste you actually get on the board divided by the actual theoretical volume of the stencil.

If you're counting on the full volume of the aperture to give you enough solder to give get a good joint and your transfer efficiency is low it's going to affect the quality.

Also, putting on my Lean Six Sigma hat, is the repeatability of transfer efficiency. You print one time and you get 90 percent transfer efficiency and the next time you get 70 percent transfer from the same aperture. So now you have a variable volume that you're getting on the board for a particular joint and that could be really problematic and tough to debug.  

So the question in stencil engineering over a number of years has been what we need to improve to help assure better transfer efficiency. One of the things that almost everybody's aware of is the efforts made to make sure the aperture walls are as smooth as possible.

And that's one of the things that led to changing from chemical cutting of apertures to laser cutting, and finally, to electro-formed stencils.

We've gone into things like fine-grained stainless steel and high grade stainless steel materials.  

And polishing the apertures after laser cutting. Polishing through plasma etching and micro-machining and other techniques, to make them smoother so there's less tendency of that solder paste to stick.

And this is what's happening. It's strictly a function of tack in the area, thus the use of area ratio and giving a prediction of this. But now aren't there even newer things from the fifth dimension? The magic word: nano.  

Yes, these are coatings that come in basically two forms:
  • The coating can be applied by the stencil manufacturer when they're putting their stencil together for you.

  • Coatings that you can put on after market. I guess they go on with something that looks like a wipe.
There have been some interesting studies. Most recently, by our good friend Chrys Shea, a study that she did; a very extensive and exhaustive study. It's also on her website: http://www.sheaengineering.com. She's presented some material and she did this in conjunction with Vicor. It's a really interesting study. You'll learn a lot about transfer efficiency from this paper alone.  

We killed that subject.  

This is Phil Zarrow and Jim Hall.  

Don't solder like my brother.  

Please don't solder like my brother.

Hi Phil and Jim,

What improvement if any does vibrating squeegee use have on solder paste transfer efficiency?
Don Evans, EvansSMT
An overlooked variable in transfer efficiency is the PCB support tooling. Dedicated vacuum produces the most repeatable results and diminishes subjective set-up parameters. If a process requires ultra-thin stencils and has a wide range of pad sizes, dedicated PCB fixtures may be the only method to achieve high yields.
Dee Claybrook, Rapid Tooling, Inc.
Free Newsletter Subscription
Every issue of the Circuit Insight email newsletter will bring you the latest information on the issues affecting you and your company.

Insert Your Email Address

Directory Search

Program Search
Related Programs
bullet Printing of Solder Paste - A Quality Assurance Methodology
bullet Taking the LED Pick and Place Challenge
bullet Challenges for Selecting Appropriate TIM2 Material for CPU
bullet Double Print Stencils Systems
bullet Assembly of POP with Novel Epoxy Flux on Solder Paste
bullet Big Ideas on Miniaturization
bullet Generalizations About Component Flatness at Elevated Temperature
bullet Enclosed Media Printing as an Alternative to Metal Blades
bullet Printing and Assembly Challenges for QFN Devices
bullet Profiled Squeegee Blade Review
More Related Programs