Circuits Assembly Online Magazine - Forsake Not Thy Stencil

New technology makes reusable stencil frames attractive (finally!).

For every complex problem, there is a solution that is simple, neat, and wrong. – H. L. Mencken

Everything should be made as simple as possible, but not simpler. – Albert Einstein

I have often pointed out that the solder paste deposition process is responsible for 50-60% of assembly-related defects. In other words, when defects resulting from poor design for manufacturability (“forgive the design engineer for he knows not what he doeth”) and from component and PCB issues are excluded, at least 50% of defects originate with a printing or paste parameter out of control limits. Even in assembly operations running at 3 sigma defect levels (few and far between), 50% of defects originate in printing.

A fishbone analysis of the printing process reveals many variables, most involving operators: squeege pressure, speed and wear; snap-off angle; machine wear; machine creep (this does not necessarily refer to the operator); separation speed; stencil tension, accuracy and thickness; PCBA support; wipe frequency and methodology; plus numerous paste, operator and stencil variables. Even in highly automatic printers, a lot can go wrong. This makes a good argument for post-print inspection. It also illustrates that since defects result from the sum of all tolerances, the system is only as good as its weakest component.

One printer component common to the entire range of automation – manual to highly automatic – is the stencil. Sadly, as is the case with solder paste, it usually wants for attention. Most of the time proper aperture reduction is incorporated and we all have our favorite aperture shapes for certain applications. What I am talking about is treating the stencil as a commodity piece of tooling. For example, how many users inspect their stencils at incoming for proper thickness, pattern orientation, etc.? I rest my case.

Stencil accuracy will become paramount as we weave our way into lead-free. Wetting forces are lower on almost all lead-free alloys, meaning that paste-on-pad accuracy has to be dead-on. At liquidus, these alloys are not self-centering, at least with current pad geometries. The paste versions of these lead-free alloys have also exhibited reduced transfer efficiency. Ultimately, we can expect to see enhanced aperture design rules and possibly different stencil alloys for optimized lead-free printing. In the meantime, current stencils will work but regardless of whether printing lead or lead-free paste, parameters have to be controlled.

One seemingly good idea, with regard to stencil mounting technology, is the reusable stencil frame. After all, frames with a stencil foil permanently attached to the aluminum frame take up a bit of space in storage. Not having to spend money on an individual frame for each stencil seems to make sense, too.

Years ago, the concept of the reusable stencil frame was conceived. A great idea, but harder to execute than one imagined. On most of these, the foil is mounted to multiple tooling pins along the inner perimeter. Some use mechanical methods including springs, others use pneumatic methodology in the form of an air bladder mounted within the frame to apply tension to a stencil once it is affixed.

Again, great idea, but it does not work that well. Sealing problems occur, as does unevenness in the tension forces acting upon the foil. Insufficient tension across the stencil creates negative acting variables affecting solder paste deposition accuracy – in all dimensions. Handling the stencil foil out of the frame takes considerable care to avoid damaging the stencil – and your hands. (If you think a paper cut is bad, you should see what 0.006" stainless steel can do to fingers.) Field performance revealed the best solution was to stick with permanently fixed stencil frames and leave tension to the pros – the folks who make the stencils. They know what they are doing and you eliminate another costly variable from the mix. In addition, at the end of the stencil life (wear or, most likely, obsolescence due to an engineering change on the PCBA), you could recycle the frame. Common wisdom was to avoid reusable stencil frames since they did not really work.

Times have changed. Most stencil shops no longer give a significant trade-in value for the aluminum frames. Shipping them back costs often more than their redemption value. Fortunately reusable stencil frame technology has advanced as well. I have had satisfactory experience with two of the available “systems.”

QTS’ Wizard Frame System, now a few years old, has a container frame with 28 contact pins for securing the foil. The foil itself is surrounded by a plastic edge, making handling the foil component safer. The system uses pneumatic methodology to maintain tension. Unlike its predecessors, this system appears to work well. A number of shops are licensed to fabricate stencils for this system.

The latest generation VectorGuard system from DEK represents the leading edge in reusable stencil frame technology. The foil is mounted in an extruded aluminum sub-frame that ensures rigidity and safe handling. The sub-frame mounts in the reusable frame easily, without pins, slots or teeth and without special tools or a mounting base. Most important, once the system achieves proper foil tension, it mechanically locks in place – automatically with no fear of loss of tension due to pneumatic leakage. Moreover, the sub-frame system permits quick and easy cleaning of the stencil. All the sought-after advantages of the reusable frame are here, at a fraction of the storage space of permanent stencils. They come in standard and special thicknesses for most SMT applications and are compatible with most printers from a number of vendors. Several stencil fabricators are licensed to produce VectorGuard stencils. This is the hot setup and those shops are worth seeking out.

Sometimes it takes a while for a good idea to become viable and evolve to a must-have methodology. But the wait is well worthwhile. Remember, we’re all in this together.

Author’s Note: Special thanks to my friend, Barry “Mr. Stencil” Goukler of Metal Etch Technologies, for insights and ongoing discussion on stencil technology.

Phil Zarrow is president and SMT process consultant with ITM Consulting (itmconsulting.org); phil_zarrow@itmconsulting.org.