Solder Paste Transfer Efficiency - What/Why

Phil
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?  

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

Phil
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.  

Jim
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.  

Phil
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.  

Jim
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.  

Phil
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.  

Jim
We killed that subject.  

Phil
This is Phil Zarrow and Jim Hall.  

Jim
Don't solder like my brother.  

Phil
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?

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.