engineers that saw a fl ame in their printer (IPA fl ash point is 12°C/54°F). Nowadays, most printer manufacturers (if not all)
recommend the use of higher fl ash point or non- fl ammable solvents. Cleaning agents manufacturers have been attempting to introduce water-based chemistries over the past few years, but most engineers are very reluctant to use due to concerns about drying speed, layer separation (due to lack of agitation of the chemistry) and lack of a proven record. The key characteristics for a cleaning agent for underside wipe of stencils in printers are • - Solubility characteristics. The cleaning agent needs to match well with the soil(s) being cleaned to have an effi cient cleaning process.
• - Compatibility. There is no rinsing so any cleaning agent remaining on the stencil/printer has to be harmless. Most tend to be pH neutral.
• - Quick evaporation. Drying has to be fast. Otherwise, the fi rst print(s) could be affected if any cleaning agent is left on the apertures.
• - Health & Safety. The cleaning agent needs to have a high fl ash point (preferably be non-fl ammable (fl ash point ≥60°C/140°F)), low emissions and mild odor.
The Cleaning Process Most printer manufacturers will have some recommended parameters for cleaning the underside of the stencil. Most processes will follow this profi le a) Wet wipe with solvent Solvent delivery systems have improved dramatically in recent years but not all delivery systems provide the same uniform and repeatable solvent application on to the lint free paper roll.
b) Vacuum Vacuuming is supposed to help remove solder paste from stencil openings and improve the clearance of partially clogged apertures so there is no problem depositing the correct volume of solder paste.
c) Dry wipe This fi nal wipe is supposed to dry any remnants of the solvent and remove any leftover paste residues. For printing, we have focused
on solder paste but everything discussed applies to the removal of SMT adhesives. The main difference would be the solubility characteristics of SMT adhesives. Because these
January 14th 2012
characteristics are so different, we would need to consider a different type of chemistry to be effective cleaning these soils. However, it doesn’t mean that we need to have two separate chemistries, as the chemistries that clean SMT adhesives tend to match very well with solder pastes. Once again, since printer technology varies from one
manufacturer to another, I encourage you to follow the printer manufacturer’s recommendations to implement the underside cleaning of stencils in your printing process. I also encourage you to use engineered solvents in your printers as they tend to be safe and show excellent compatibility with printers and stencils.
2. Offl ine Stencil Cleaning Cleaning is required both, in the printer (while in process) and at the end of a printing job when the stencil is removed from the printer to remove solder paste accumulation from the apertures and stencil surfaces before it dries and it gets hard. Automatic & Semi- Automatic cleaning equipment can be used for cleaning stencils after the stencil is removed from the printer. Manual cleaning is also an option, although less
desirable due to the poor repeatability and user exposure to chemicals, which in many cases tend to be very volatile (i.e., IPA), thus becoming a concern in terms of health and safety. There are different types of automatic stencil cleaning equipment. The most common types use spray- in-air or ultrasonic agitation. Much less common, but still found, is the cleaning equipment that uses spray- under-immersion agitation. Semi-automatic equipment manufacturers tend to use ultrasonics as the main source of agitation (mechanical energy), followed by a manual rinse (sometimes no rinse) and manual or forced air drying. As with any other cleaning process, we need to consider
the key elements I. Soils II. Substrates III. Chemistry IV. Equipment V. Process Validation
I. Soils Common soils, as discussed previously, are solder paste and SMT adhesives. Solder paste cleaning is the most common soil when dealing with stencil cleaning. However, some processes need to clean both, solder paste and SMT adhesives.
www.electronicscomponentsworld.com /
www.electronicsproductionworld.com EIU 23
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40