Optimizing equipment can reduce defects; catching a defect before the joint is made at reflow reduces rework, but 100% inspection after printing or dispensing and post-placement is not always possible when running fast tact times. Careful selection of process materials, such as paste, underscreen cleaning rolls and wipes will all help reduce contamination.
SMT lines are not located in cleanroom environment. Trap floor-mats may be positioned on entry to the production area to reduce contamination. A nominal usage of protective coats and hair nets does not always provide an environment around the bare substrate that is conducive to a particle-free environment. Typically, PCBs are supplied in sealed plastic wrappers or individually wrapped in tissue paper. Opening or separating the substrates will generate a small electrostatic charge, enough to attract dust, fibers and hairs from operators and surrounding area. Add to this the surface particles from sawn or snapped-out substrates that have been stacked together, and the inherent fibrous nature of the material, and you have a board with loose surface contamination. Examination of the exhaust duct in any reflow oven will show these fibers present within the residue deposits.
If the contaminants don’t impede pad areas or the profile of a device, they have no effect and will end up in the oven exhaust. However, if they do contaminate pad areas, the integrity of the joint can be compromised. Hairs and fibers will sit within the reflow joint and can pass from pad to pad. Lint, tissue or even loose resist can obscure a total or partial pad, resulting in in-sufficient solder or no joint. Epoxy dust and snap out particles on small chips will cause stand off, in some cases resulting in voids and causing tombstoning.
Contamination from one board can be passed to another during the printing process as it attaches to the stencil, obscuring an aperture or affecting the board to stencil gasketting. This results in an increase in the number of underscreen cleaning operations required. Dispensing contaminants when passed to the needle can result in blockages or stray adhesive droplets and trails.
Hybrid production has witnessed contaminants affecting mass deposition printing. Early solutions saw the incorporation of angled air jets to gently blow contaminants off the surface. Although successful in achieving an improved print, the contaminants remained within the working environment. As the hybrid areas are typically “clean room”, a solution was required that contained the contaminants. Using technologies from other industries, systems have been developed to remove loose surface contaminants down to a 1µm particle size. Utilizing specially selected elastomer rollers, surface contaminants are removed. These rollers in turn contact an adhesive roller to collect and remove the contaminants from the working environment. Typically, the adhesive surface is replenished on a daily basis.
A range of systems now exist to support SMT production, ranging from a simple hand roller to automated in-line solutions that sit independently with a printer or dispenser occupying just 150mm of footprint space. The typical cost to install and run a full in-line system on a volume line is less than euro 15 per day. Bald operators may reduce some contaminants, but for a total solution and peace of mind put your faith in elastomer.
