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Source: MYDATA SMT & Inspection | May 12, 2009

Tackling SMT Enemy Number One

Is screen printing technology able to keep pace with rising quality demands and increasingly complex board layouts or is new jet printing technology ready to fill the gap?
A comparison study between the two methods reveals some interesting differences. Screen printers offer some possibilities for optimizing solder paste deposits, but optimization is far easier and quicker with the jet printer. At the same time, the ability to print individualized deposits on every single pcb pad may be the ultimate answer to the growing quality challenge.

The role of the solder joint is all important in ensuring the final quality of any printed circuit board assembly. As well as providing a robust electrical interconnection it also needs to ensure a reliable mechani­cal interconnection. The application of solder paste is key to ensuring high quality solder joints but it is affected by a great many parameters. Currently, printing issues are the most commonly cited cause of error during the surface mount assembly process and account for some­thing like 70% of all soldering errors – mainly opens, insufficients and bridges.
Industry trends are making the situation ever more challenging. Boards are becoming denser, use smaller and finer pitch components and require small and large components to be placed very close to­gether. Modern manufacturers are faced with more complex produc­tion, higher quality demands and the need to maximize automation in order to improve efficiency and to remain price competitive. In today’s ultra-competitive environment there is no longer any place for errors!

Screen printing with a squeegee blade and metal foil stencil represents the standard method of depositing solder paste in surface mount as­sembly operations. Despite advances in automation and increasingly sophisticated solutions stencil printing remains a key area of concern. What causes that concern is the sensitivity of the printing process and the fact that so many parameters contribute to the final result. Pa­rameters include: printing speed; squeegee type; angle and pressure; gasketing between the stencil and pcb; separation speed; underside wiping of the stencil; pcb support (especially for second-side printing); and stencil thickness and aperture design. Each of these parameters needs to be carefully optimized to ensure quality. And a new set of parameters needs to be assigned for each new job that is undertaken.

One of the limitations of stencil screen printing is the fact that the volume deposited is, to a large extent, determined by stencil thickness. While stepped stencils go some way to overcoming this issue they also add further costs and complexity. They also mean certain restrictions are imposed on pcb design in order to maintain ‘keep-out distances’ (the distance required between an aperture in a ‘stepped’ area and the nearest ‘normal’ area).
By far the most common situation is for manufacturers to use regular metal foil stencils in their production.

Automated jet printing is a relatively new technique that uses a unique ejector mechanism to deposit solder paste onto printed circuit boards at high speed. The non contact printing technology applies no force to the pcb and builds up solder paste deposits in three dimen­sions (i.e. solder paste droplets can be deposited on top of each other).
The process is completely software controlled and default settings are provided for each component based on CAD data. However, the user has the freedom to fine-tune the volume, area coverage, height and layers of solder paste for each individual pad, component or package.

The stencil-free technology provides much faster response times com­pared to screen printing. There is no lost time ordering, waiting for or cleaning stencils and, as print programs are prepared off-line, setup and changeover times are also minimized. In addition, layout changes or print adjustments are extremely quick and simple to implement.

From MYDATA´s "Tackling SMT enemy number one – Raising the standard of solder paste application."

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