When it comes to understencil cleaning, the devil’s in the details.

Most would consider screen printing a single process: the first step in the assembly line. However, understencil cleaning is a process itself: a screen-printing sub-process. Not only must the production engineer be adept at printing techniques, but cleaning parameters and routines as well.

Understanding software features such as cleaning rates (how often should you clean) and modes (wet, dry, vacuum and their various combinations) is critical and, of course, highly application-dependent. The industry default mode is generally wet/dry/vac – not that it is necessarily the right choice always, but that is what most operators are used to. Aside from determining rates and modes, cleaning materials selection is also tantamount to a robust cleaning process. The understencil fabric and solvents used to remove solder paste from today’s ever-smaller apertures play a large part in the success of the cleaning sub-process, and the overall print process.

As an example, let’s take a look at the fabric. Obviously, the main goal with any cleaning fabric – even those used at home – is to clean the object free of debris at the end of the process. Unfortunately, quite a few understencil cleaning materials don’t do that. Instead, they put lint back onto the stencil, which may in turn block apertures. I’ve heard many engineers debunk the lint effect, arguing a 10 µm thread would have little to no impact. That may have been true five years ago when, by comparison, apertures were fairly big. Today it’s a whole new ballgame.

In fact, during some recent stencil testing using various understencil fabrics, our company discovered lint is, indeed, quite problematic. With 0.3 mm CSPs that required aperture sizes of approximately 180 µm, our AOI results and analysis turned up some strange trends. When we evaluated the stencils in question, we found lint contamination. In most instances, we were finding low paste volumes, which were directly attributable to lint blockage of the apertures. In one case, a lint particle had become drenched in paste, and the inspection system actually viewed it as a bridged deposit. Keep in mind the lint issue is primarily a problem for the dry cycle, but the fabric selection impacts wet and vacuum cycles too.

Ever tried to breathe through a paper towel? How about a piece of printer paper? I’d argue the former is more beneficial for your oxygen intake. The same theory holds true for the vacuum on the printer’s understencil cleaning system. You’re trying to pull vacuum through the fabric to clean out the apertures and, hopefully, dislodge and then capture all the residual paste into the open weave of the fabric. If the weave is too dense, you are blocking the vacuum, thus compromising its power and your cleaning process. You want a fabric constructed of porous material that permits air flow, yet captures material into the pores of the fabric.

Then, of course, there is the wet bit. The solvent selected can most certainly have an impact on print performance (a topic for another column). Consider the importance of how the fabric wicks the solvent across the exposed area of the material. The solvent breaks down solder paste that remains on the outer edges of and inside the aperture walls, so a fabric that absorbs the solvent uniformly is desired. This must be a stable and highly repeatable process to be effective. The material should be fast and absorbent, and should almost instantly start wicking and create one solid bar across the fabric. What you definitely don’t want is a quick wick that keeps on wicking all the way through the paper because then, when you index the paper along for a dry wipe, it would still be wet. A good solvent process is primarily about the absorption rate of the fabric and also a little bit about the ability of the machine’s solvent bar to dispense the solvent at consistent volumes. If the fabric doesn’t wick properly, a thorough clean is unlikely. This introduces the potential for blocked apertures and, therefore, defects.

The bottom line: Treat the understencil cleaning process with as much rigor and attention as the other parameters of the screen-printing operation. Don’t skimp on the fabric because, in the end, a few more cents for a high quality material will likely save big bucks.

Clive Ashmore is global applied process engineering manager at DEK (dek.com); cashmore@dek.com. His column appears bimonthly.

SIDEBAR

DIY Fabric Tests

Here are three easy, do-it-yourself fabric performance tests:

• Lint: Take a piece of Scotch tape, adhere to fabric, press down and then rip it off. You will quickly see if a so-called “lint-free” material actually lives up to its claim.
• Vacuum: Simply turn the vacuum on and feel the flow to sense the power and volume of the air coming through the weave. More elaborate air-flow measurement mechanisms can be used, but a simple “touch and feel” should do the trick.
• Wicking: Cut a strip of fabric, place a line of solvent across it with a basic dropper and evaluate the speed and uniformity of the wicking. 

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