Solder Paste Printing From the Stencil’s Perspective

Estimated reading time: 22 minutes

Matties: In your career spanning over 20 years, you have seen a lot, for sure. What are some of the most common errors you see people have in their printing process?

Schake: If a printing issue is escalated to my attention, then, typically, the checklist of the most common errors has already been probed, and more advanced troubleshooting is required. My troubleshooting proficiency extends beyond the mechanics of the printing machine to consider the various materials that are required to run a robust printing process. I explore and seek to understand the properties of those materials and to verify the design of those materials is suited for producing the best print outcome. The definition of materials includes a host of things like solder paste, stencil, squeegee, tooling, cleaning consumables, and circuit boards. One common aspect of the errors I encounter relates to process design.

Matties: Are you saying to look at it from the total process point of view?

Schake: Yes, we look broadly at what makes a printing process successful and consider anything that can corrupt it. Happy, you referred to a multi-thickness stencil and said you’d put the non-flat side on the circuit board. I said that’s not a good process design choice. We’re talking about bad stencil gasketing there, which, singularly, can really foul the print result. But there can also be a combination of less severe things going on that, if left unchecked, can resonate to produce a similar negative print consequence.

As a metaphorical example, in a card game, you may be dealt a bad hand that requires you to adapt and change your play strategy accordingly. The cards matter. Performance of stencil printing is similarly dependent on the characteristics of the PCB “cards” supplied and influenced by obscure details concerning design and quality aspects such as pad size, solder mask openings, warpage, etc. Again, the cards matter! One of our top challenges is handling and fixturing highly routed thin and flimsy PCB cards designed to be populated with miniature components at extreme density and to position these inside the printing machine in a consistently co-planar fashion to the stencil. As opposed to a playing card, a PCB card has two sides to deal with.

Matties: What do you think about the move toward jet printing?

Schake: My view on it is that there are opportunities to improve assembly capability and performance based on the utilization of jet printing. It complements stencil printing, but won’t replace it. There are good places for it. For example, jet printing is an excellent tool to be used for prototyping and new product introduction. It may also serve well in some low-volume manufacturing and high mix environments. It’s very flexible, customizable, and configurable.

Matties: It addresses the issues you’re talking about with respect to the surface level and different requirements for components.

Schake: In theory, yes, but in practice, the actual process controlled result might be more difficult to achieve. I don’t claim to be a jet printing expert, but I think it’s fair to speculate on what could be challenging. From my casual observation, I’m aware of solder materials that are used in jet printing applications are very formulation sensitive and specific. There’s not a great variety of approved materials for those systems. Managing clogging may be a challenge, particularly when faced with unpredictable and intermittent line stoppages, but these things will be worked out and improved. Where I see those systems fitting in, at least in the scale of a high-speed process, is the utilization of a jet printer working together alongside a stencil printer supplying solder in hard to print locations or locations that require a boost in solder volume. In fact, there’s already jet print head options available inside an SPI tool. I think you’re going to continue to see new versions of this technology and capability emerging.

Matties: The high-mix low-volume players are going to be jumping on the jet printing. That makes a lot of sense for them. One of the things that another expert told us is that a bigger problem is not bridging, but opens and not enough material. Does that ring true to you as well?

Schake: You were saying opens were the concern?

Johnson: The temptation is to use as little solder paste as possible. The tolerances are tight now, where you used to have a couple of mils of tolerance, but now you don’t because that’s your entire pin spacing. The temptation has been, in the field, to use less paste and make a thinner covering of paste. Suddenly, there isn’t enough. The trickier field failure to find is the intermittent open as opposed to the short.

Schake: It’s an interesting statement. I don’t dispute any of that. The trend is to use thinner stencils. I associate your reference to opens occurring with ball array components. The combination of low print deposit volume and warpage can wreak havoc to cause head-in-pillow defects. But for other component types, the bigger problem could be a different defect, like bridging that leads to shorts. Here’s an example. A recent challenge that I’m facing requires assembling 01005 size capacitors, but the spacing between placed parts is one-quarter of the component width. That’s a tall order.

If the paste deposit is too large or shifted and then you start pressing the component down into the printed paste, the solder squeezes out. And then there’s the risk to bridge with the neighboring deposit. Now, you have to minimize the amount of paste printed while still ensuring enough is printed to form a joint. How do you do that? One of the things being looked at is printing through a 75- or 80-micron thick stencil, so it’s less than the standard four mils. You can do that for the 01005s, but you have a lot of other parts that you must consider the challenges of being able to print enough paste volume for. I see the future of where people are going and trying to push the boundaries of density, but by using a thinner stencil on some components, like the warp sensitive ball arrays, this can instigate head-in-pillow opens, as you alluded.

Johnson: What are the challenges you see from the stencil perspective for the solder paste manufacturer?

Schake: As I’ve described, one of the things recently that I’ve been doing is printing small deposits. It’s not necessarily a problem with printing that I have; it’s more the problem I’ve been having with reflowing because I’m getting a bit of graping. I didn’t want it to turn into a solder paste evaluation, but it has turned into something like that because these small deposits are difficult to reflow.

Johnson: Practically speaking, if we need to have less solder paste and a thinner coat, that means the solder paste and soldering process itself needs to be more effective.

Schake: Yes. Every solder ball counts. We’re getting to the point where we’re counting solder balls. You can quantify pass-fail based on the number of solder balls you can see. That’s a scary proposition.

Johnson: That’s precise.

Schake: Yes (laughs). Unfortunately, in my work environment at RIT, we don’t have a nitrogen-inerted reflow oven, and I tend to think this capability would improve my fine feature print soldering results. One of the things that I don’t completely have a good grasp of is a survey of the industry to know who is using nitrogen reflow and how common is it. Dare I say we are heading in a direction where the print requirement will compel people to be incorporating nitrogen in future assembly processes?

Johnson: For the sake of any of our readers who aren’t familiar with why nitrogen is important to the process, could you give a quick explanation?

Schake: Purging reflow ovens with nitrogen effectively creates an atmosphere in the oven where heating occurs without oxygen. The purity of this heating environment reduces exposure to oxidation. For very small printed deposits, like the ones I’ve been having trouble reflowing, these have a large ratio of solder ball surface area exposed to the oven environment compared to their bulk solder volume. In this case, the flux chemistry is working hard to preserve the solderability on all of the exposed solder ball surfaces. And without nitrogen, the chemical activity of the flux has to work harder to keep all the solder balls pure. In a worst-case scenario, flux activity can be prematurely spent, and the solder spheres don’t all coalesce together, leading to a reflow defect known as “graping.”

Johnson: The implication here is that with these very small, thin requirements, we’re starting to get to a point where nitrogen is now a requirement, not a “nice to have.”

Schake: These solder paste deposits can be so small that the surface area of the solder spheres is at a dangerously high level, where you don’t have enough flux to be able to fight off the elements of oxidation.

Johnson: That showed up in an earlier conversation as well. With less solder paste down, there is less flux to do its job, meaning that the flux now has to work harder and be better than ever before to get a good solder joint.

Schake: Correct. But keep in mind also that the chemistries that assist in the flux performance are not necessarily critical ingredients that have an impact on printability. You could have a paste that has a terrible reflow process and has wonderful printing or vice versa.

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