By Pär Gester
“Very strong, great cleaning characteristics, high level absorbent and extremely low in lint. “
Have you heard it before? Almost all stencil wiper roll manufacturers say the same. But are these parameters important and are they backed up by evidence or is it b....it?
STENCIL CLEANING ROLL PARAMETERS
Let’s have a closer look at SMT stencil roll parameters. When you understand them, you can make an informed decision on what to use and what to avoid.
What is “very strong”? Is it needed? No, you don’t need a very “strong” fabric. There are relatively small forces on the fabric in most printers. The fabric will not break, even if it’s not very “strong”. Elongation is more important. This is expressed in per cent and tells you, simply put, how flexible the fabric is. The larger elongation at the break, the more deformation (stretching) of the fabric when used. This is something we would like to avoid. We don’t want the fabric to behave like a rubber band.
The craving to brag about high absorbency comes from the original use of the fabric used for making a majority of stencil rolls – wipes for mopping up a liquid spill. In the under-stencil cleaning process, there is very little liquid to absorb, right? You don’t absorb the metal in solder paste and the ability to catch the metal particles is certainly not measured as absorbency.
The only liquid that needs to be absorbed is the cleaning agent for the purpose of wetting the stencil and dissolving solder paste residues. There is no need to absorb more than the fabric can convey to the stencil (with some margin). The amount of liquid that is transferred from the fabric to the stencil is basically only dependent on the stencil and cleaning agent properties.
Only a few grams per square meter can be transferred from the fabric to the stencil. No matter how thick or soaked or whatever the fabric is. Some math will tell us that you theoretically need around 80 grams per square meter absorbency or 200% for a 40-gram fabric.
Very high absorbency is achieved by using cellulose fibres. Fine for mopping up your coffee spill. In your screen printer, it only means that you soak the fabric with a solvent that is not used to clean the stencil. Waste of money.
Of course, low lint is desirable. Cellulose (paper) based fabric can only be made low lint by hard fibre compression during manufacturing, leaving a glossy and flat surface that is anything but good for stencil cleaning. A good example is the Sontara type of fabric. It’s low lint but not a good material for stencil cleaning.
A simple way to test whether your stencil roll is linting is to take a piece of adhesive tape, put it gently on the fabric, peel it off and look under a microscope if you see any loose fibres on the tape. In the pictures below, you can see the difference.
"Great cleaning characteristics"
Really? How was it measured and achieved? Is there any evidence?
Air permeability is a key parameter for cleaning performance. Air permeability describes how much air flows through the fabric. You rarely see this in numbers of datasheets, other than in the form of claims such as "great vacuum performance". The reason again is that it’s not an important parameter for the off-the-shelf mopping paper that is used in most products on the market. The data is not available to the wiper roll manufacturer. Or they don't care or understand.
You want as much air as possible to flow through the fabric in order to make the vacuum cycle effective. High air permeability can, of course, be achieved by making the fabric very sparse. But this simple approach will result in a fabric that with bad cleaning performance and it will cause rapid contamination of the vacuum system in your printer.
Fibre properties and mix must be engineered for SMT Stencil cleaning. All SMT stencil rolls on the market are made from nonwoven fabrics. Nonwovens can literary be made in millions of combinations. There are thousands of fibres on the market for the textile engineer to choose from and endless process parameters to master. You will find nonwovens all around you in car seats, hygiene products, cosmetic wipes, industrial wipes, medical products, packaging material, filters, geotextiles, membranes... it's obvious that each product must be designed for its intended use.
Finishing of fibres and fabric is commonplace in the textile industry. There is a whole world of finishing agents to choose from and mix. The properties of the fabric can be dramatically modified with a finishing process. Of course, the finishing used for SMT roll fabrics must be formulated for the application.
Taking a standard fabric and offering it as a stencil wiper is not good enough any longer. It will be a "cheap" roll, satisfying the short-term chase for cost reductions. But the actual cost for the "cheap" roll should be calculated from the cost for soldering defects, not the purchase price per roll.
Ask your supplier for data that counts and demands claim to be backed up by evidence! It’s that simple. Choose a stencil cleaning roll that is designed for SMT stencil cleaning.