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Understanding SMT Under Stencil Wiper Rolls
January 7, 2014 |Estimated reading time: 1 minute
IntroductionThis article provides solder paste stencil printing engineers and purchasing professionals of stencil cleaning products a clear understanding of the different types of stencil cleaning paper/fabrics that are currently available. As fine-pitch applications and small apertures have become the norm, the performance of stencil cleaning rolls is more critical than ever.
Main Products in the Market
- Cellulose/polyester paper;
- Cellulose/polyester hydro-entangled non-woven;
- Rayon (viscose)/polyester paper/fabric;
- Rayon (viscose)/polyester resin bonded; and
- 100% polypropylene non-woven.
Thicknesses of these products vary from 40 grams per m2 to 65 grams per m2.
Manufacturing Process and Environmental Impact
All cellulose products start with the same process, reacting the cellulose with sodium hydroxide (NaOH), which is a very strong alkali (corrosive). This alkali cellulose is then reacted with methyl chloride, chloroacetic acid, ethylene oxide, and/or propylene oxide depending on the type of cellulose ether product to be made. These chemicals are all classified as hazardous air pollutants by the U.S. Environmental Protection Agency.
Rayon (viscose) starts the same as cellulose and then the alkali cellulose is allowed to age before it is reacted with carbon disulphide (CS2), a highly toxic substance and also classified as a hazardous pollutant. Typically, only 50% of the carbon disulphide can be recovered, leaving 50% into the environment. Extrusion and then immersion in a bath of sulphuric acid (H2SO4) generates hydrogen sulphide gas (H2S), also classified as a hazardous air pollutant. Rayon is a particularly “dirty” product to make and has been associated with deforestation in developing countries.Read the full article here.Editor's Note: This article originally appeared in the December 2013 issue of SMT Magazine.