What is the Ideal Stencil Thickness?

Phil
Welcome to Board Talk. This is Phil Zarrow and Jim Hall, sometimes known in certain corners of the world as the Assembly Brothers. We're here to answer the questions you sent in on surface mount process, assembly, materials, equipment, anything that may be bothering or perplexing you.

Today's question is from a Mr. or Ms. V.F. from Dallas, Texas, who writes, I have several PC boards with mixed density components. In other words, I have some of what you guys call coarse pitch, 50 mil big capacitors and the like, but I also have some difficult fine pitch, including some .5 millimeter pitch CSP's. If I use a 6-mil stencil, I have problems with my CSP's and if I go down to a 4 or 3 mil stencil, how do I get sufficient volume for the big boys? So what do I do? I'm all mixed up. Help me.

Jim
Before we get started, let me talk about the rampant use of the term mixed. This questioner is using mixed densities talking about having different sized components on the board with different pitch or size leads.

Think of the other times mixed has been used. I defined mixed technology 30 years ago as printed circuit boards that have both surface bound components and through hole components on the same board.

But more recently, we've talked about mixed assembly being backward and forward compatibility with lead-free assemblies, in particular backwards compatibility with lead-free components being introduced unknowingly or inadvertently into a tin lead assembly.

So be careful when somebody says mixed. It's a very common word. I would have to say most boards assembled in the world today fall into this category where you have some 50-mil pitch IC's and some big capacitors which require a relatively large volume of solder. If we consider the solder fillet on the size of the leads, to get a good solder joint, you need a relatively large volume relative to the size of the pad.

On the other hand, for fine pitch 20-mil pitch IC's or in this case, .5 mm pitch CSP's, they're very small leads, very small balls on the bottom of the CSP's. They need a relatively small volume of solder and solder paste to be applied in order to get the appropriate joint.

What are some of our options here, Phil?

Phil
The obvious is we could do a reduced aperture and that would to a certain degree help out the little guys.

Jim
That's where we'd stay with the thicker, in this case the 6-mil stencil. Normal apertures for the big components, reduced apertures for the small components.

Phil
But the liability here is what we like to call too-tall apertures.

Jim
For a thick stencil, as we start making the aperture smaller and smaller, we reach a point where the aperture becomes so tall and narrow that it's difficult to get repeatable flow of solder paste in and out of the aperture. So therefore, when we go to print, we don't get consistent results because the solder paste just won't fill and release repeatedly from this tall, narrow aperture.

What other options might we have?

Phil
Well, of course we can go to the other extreme. We can go down and reduce the thickness of our stencil. Let's go down to say a 4-mil stencil maybe even that much and that would certainly help out those little CSP guys.

Jim
Right, but then what do we do about the big guys?

Phil
That's right. Yeah, what do you do? If you go into an overprint, you're going to risk things like bridging and of course the ever present, ever ominous solder balls, so that may or may not work.

And finally, another solution, and it goes under that category of everything old is new again, the old step stencil. You want to explain what a step stencil is?

Jim
A step stencil is really the most robust solution for this problem. It's where you actually create different thicknesses in the stencil in different areas.

The most common technique would be start with a thicker, in this case, 6-mil stencil, and then in the areas where you have my fine pitched parts reduce the area, usually by milling or etching the thickness of the stencil to what's required for the fine pitched part. You don't have to do any aperture reduction. You can get good fill and release and get the proper volume for the parts.

Unfortunately, when you go to a step stencil, you open a whole bunch of other potential problems for design, process variables, limitations, and so forth. I think as much as we can say now. We're gonna have to come back on another session and talk about the details and trade offs for step stencils.

Basically that's the problem, you have three options: thick stencil aperture reduction, thin stencil overprint, or a step stencil.

Phil
So Jim Hall and Phil Zarrow of ITM for Board Talk. We are otherwise known as the Assembly Brothers and we remind you, don't solder like my brother.

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

In Asia cost is main concern. Double print is something rare here. Is step which the norm done on squeegee side can it be done on PCB side? I know some company doing it but there is no feedback what issue they encounter.

Don't forget - a number of premium stencil suppliers are now offering precision welded step stencils with none of the downsides of etch or milled steps. Welded steps have the same accuracy as the normal stainless material so can give exact paste high deposits, and they can be placed much more accurately if you have a really tight placement issue.

For speed and accuracy, you can't beat the step stencil option. I would also recommend using nano coating. Jet printing is slow and the equipment expensive. Two print process is possible, but why do it if you can print everything at one time?

All of this skirts around what seems like the ideal solution: solder paste jet printing. The right amount of paste at every pad every time.

These brothers by different mothers didn't even bring up the most robust solder paste print solution, i.e., a two pass screen print process. The 1st pass print would use a 4 mil thick stencil for the CSPs and ultra fine pitch ICs, etc.

The second pass print would use a 6 mil thick stencil for all other components with bottom side etch out areas over the 1st pass print component footprints. This two pass screen print process is utilized in high volume production for the 1st pass RoHS exempt high-Pb Flip Chip assembly with eutectic Sn63Pb37 solder paste, followed by the 2nd pass Pb-Free solder paste print for all other packages.

On what basis is the stencil thickness decided in case of BGA's or fine chip components.