Aug 19, 2009 Do you have trouble with contaminated PC boards? Where do you think it may be coming from? Did you ever think that it may be coming from the stencil wiping roll? The very thing that is supposed to clean your stencil and prevent contamination may be causing it.
To find out, look under a microscope and if you see solder joints growing whiskers (Figure 1) then you need to perform the “TAPE TEST” (Figure 2).
The TAPE TEST is simple. Take a piece of clear scotch tape and place it on the stencil wiping fabric. Apply the same amount of pressure to both strips, then remove the tape and apply it to a surface that has a black background. An observation can be made with the naked eye, but a magnifying source is best. If many fibers are observed, you should be concerned.
The TAPE TEST is an excellent way to compare stencil wiping fabric and its ability to maintain its integrity when it comes in contact with sticky solder paste and the scraping action of the stencil’s apertures. The far left of Figure 2 shows the results after tape has been applied, and then removed from GREEN MONSTER. As you can see, far fewer fibers were pulled off of GREEN MONSTER than that of the two white paper/polyester-based materials.
Products like High-Tech Conversion’s GREEN MONSTER use ESD polyester and absorbent rayon fibers that are bonded by a highly cross-linked copolymer. This copolymer is virtually “bullet-proof” against abrasion and solvents that typically are used in printing machines. In fact, it works so well that many companies do not even use a solvent.
Beware of smooth paper/polyester-based products. The static-producing polyester is held together only by entangling the fibers. In between the polyester lie tiny linty paper fibers that are held together by weak hydrogen bonds — the same bonds that a newspaper uses. The newspaper may seem strong but what happens when it gets wet? What happens when these weak paper fibers get wet under the grating effect of the stencil?
Benefits of GREEN MONSTER include a very aggressive wiping action, the porosity that optimizes the vacuum action and, because no paper is used, no trees are destroyed. GREEN MONSTER really is green!
Furthermore, Figure 3 illustrates that there are numerous nooks and crannies in which solder balls can collected, hence the aggressive cleaning action and the optimum porosity. Therefore, to avoid contamination, use a polymer bonded fabric as opposed to a paper/polyester-based fabric.
About High-Tech Conversions Inc.
High-Tech Conversions manufactures a variety of wiping products and can supply virtually any type of consumable item used in cleanrooms, assembly lines, laboratories and manufacturing. Visit the Web site or contact the company directly to learn more. High-Tech Conversions is pro-active and able to respond quickly and accurately to customers' needs. The company offers high-quality products at reasonable prices and strives to be ultra-efficient to keep overhead costs low.
