Screen Print Stepped Stencil Review

Phil
Welcome to Board Talk. We are the Assembly Brothers, Phil Zarrow and Jim Hall of ITM Consulting. We're here to answer your questions on SMT processes, equipment, materials, procedures, or anything else you have on your mind. 

Today's question comes to us from Gerald M. of Tampa, Florida. Gerald writes, "I've heard you guys advocate the use of step stencils as a means of getting the proper volume of solder paste down when you have a wide range of quoted lead densities. So can you use a metal squeegee with step stencils or am I stuck using a rubber squeegee?"

Jim
The idea that you need to use a rubber squeegee so that it can deflect down into the step is just not correct. The squeegee is not pushing the solder paste into the aperture.

The squeegee is causing the solder paste to roll on the stencil, which reduces its viscosity because it's a thixotropic or sheer sensitive material and that allows the paste at the lower viscosity to flow down into the step and adequately fill the apertures within the step, even though that squeegee material never comes down to the top surface of the bottom of the step. So that's a misnomer.

Phil
We'll kill two myths at one time. The idea that you have to use a rubber squeegee, and that the squeegee pushes the solder paste in the apertures. The real question here and the answer is, yes, you must be using or should be using a metal squeegee. In fact, we don't see polyurethane squeegees used much for except maybe adhesives.

Jim
Very few people are using the urethane squeegees to print solder paste.

Phil
Basically the two real questions here are how far down can you step and when you do step down, how close can you have adjacent apertures with a step in between?

Jim
Most people who are using step stencils feel comfortable with a 1-mil step. They feel that that gives them good results and some people will go to a mil and a half. Above that, you get into varying opinions. I've seen people stepping down 2 mils.

Bill Coleman, of Photo Stencil, who makes the stencils, says that they step as far as 3 mils. Now that's a lot.

Of course, they would have a lot of benefits for a very complex assembly but I honestly have never seen an application using that big a step and certainly the degree of difficulty would be increased in terms of getting consistent fill and release, and getting your whole process balanced. Certainly, if you're starting at 1 mil, you'll probably have a very high success rate even starting out.

But what about this spacing, Phil, how close can I get my apertures to the edge of the step? The bottom line being how much space do I have to allow my board, because in many applications, space is money and I want to put the components as close together as I can.

Phil
Well, we've heard all kinds of numbers thrown out. Blank numbers saying 125 thousandths is as close as you can get.

Jim
That's a nice conservative number. You'll have no problems at that.

Phil
Generally, you shouldn't.

Jim
But what do some of the experts say?

Phil
Our good friend Lewis from Transition Automation, who are the squeegee people and he certainly has a heavy duty background in printing, the guys over there have basically come up with about .062" as a proximity. It's about 1/16^th of an inch, for you guys who have rulers.

Jim
That's half the space of .125, so you picked up a lot there. Again, Photo Stencils uses a formula that's based upon how tall the step is, and they say they want 35 mils of spacing for every 1 mil of step height. So they're saying if you have a 1-mil step, you could put the apertures as close as 35 mils to that step. Now that's pretty aggressive.

But I would consider that the range. 125 would be very conservative, 35 mils per millistep.

Phil
That's in line because if you're doing a 2 mil step, which is generally what I like to do, it's about 70 mils and that's pretty close to the .062. So we're in the ballpark.

Jim
Remember, this is not an exact science. Many people have differing opinions of what the limit should be and what works best for them. You have to make the step parameters work with your paste, your printer, and all the other parameters for your stencil printing operation. Obviously if you're more conservative, take shorter steps with more spacing, you'll probably have less problems and less difficulty optimizing your process.

Phil
You definitely need to do a design of experiments and bear this out with your application and your boards, your stencil source, your printer and your solder paste.

This has been Board Talk with Phil and Jim of ITM Consulting and the Assembly Brothers. And remember, don't solder like my brother.

Jim
And don't solder like my brother and keep those kids away from the solder pot.

Does this apply to step-up stencils as well? I would imagine the impact to the squeegee blade would be significantly greater if it has to bump into something like a raised section of a screen.

I had an application where we had a nineties designed board with mixed technology, including 4 fine-pitch QFP's as well as 8 .050" pitch J-leaded devices. We used a 5 mil stencil and constantly got shorting on the fine-pitch and insufficient solder on the J-leads (quad 16-pin).

I ordered a 6 mil stencil etched down to 4 mil, leaving islands of 6 mil for the J-leads. I also designed laser cut squeegees that would lift over the J-lead islands. The squeegees had .003" slits that terminated at the top, just under the holder with .100" holes to prevent stress risers.

We suspected that the squeegees would leave lines of paste from the splits, but the splits clogged with paste and never left any features. It cured the shorting of the fine pitch as well as the insufficient J-leads. Both components came from PhotoEtch in Lowell, Ma.

It's worth it to use dedicated squeegee holders in this case, but the blades don't work-harden and distort or crack like they would if they weren't laser cut.

Either a rubber or metal squeegee can be used with a stepped solder paste stencil, but most use metal squeegee blades. However, the usable life of a metal squeegee blade is reduced due to accelerated wear caused by the metal squeegee blade impacting the edges of the step areas.

The proper depth of a step is determined by the stencil aperture sizes within the step areas. The foil thickness within the step areas must be thin enough to meet, or exceed, the minimum aspect ratio and/or surface area ratio requirements for the stencil aperture sizes. Most stepped solder paste stencils have step depths in 0.001" increments.