DEK Printing Machines, Weymouth (UK)
Visitors to productronica will be exploring the exhibition looking for technological solutions to a number of challenges. One of these is to deliver smaller components within heterogeneous assemblies and in a cost-effective manner. Consumer trends are pushing towards greater miniaturisation, with smartphones emerging as the primary computing device of choice, with global sales outstripping the PC market.
Today’s electronics manufacturers are increasingly looking for solutions to questions such as: How small can we print? What are the tightest pitches? Can we print small deposits next to large ones for high mix technology assemblies? How closely can we place components for high density products? In addition, how can we satisfy some of the cost pressures through the whole supply chain and improve the production process?
But miniaturisation is nothing new. The 1.27mm pitch Ball Grid Array (BGA) packages that appeared in the mid-nineties represented a significant progression. Today, 0.3mm pitch packages are on the horizon. The feature sizes that are involved would, only a decade ago, have been considered in the Semicon domain. To add to this challenge, the smaller assemblies are also more complex.
For an assembly community already operating close to the limits of SMT capabilities, scaling stencil feature sizes for shrinking component geometries poses a significant challenge. Stencil aperture area ratio dictates the size of deposit printed. The area ratio rule is the relationship between stencil aperture opening and the aperture surface area. To deliver the next generation of miniaturised SMT devices, this rule has to be broken and doing this will require new processes and tools.
Research has already discovered stencil printing techniques able to address some of the challenges with miniaturisation. For example, active squeegees such as DEK’s ProActiv, have been shown to push area ratio limits to new boundaries, in particular these have been shown to be useful for heterogeneous assembly. This enables the industry’s smallest components to be printed alongside its largest and most complex, using a conventional printing process and single thickness stencil. An increasingly important part of the overall equation is stencil aperture shape and design. This is key if electronic manufacturers are to achieve economic repeatability when delivering smaller components. With this in mind, DEK recently completed a project, resulting in greater effectiveness of a square-designed aperture in delivering paste, and a preference towards an active squeegee process.
DEK’s project looked at opportunities to move the area ratio curve by investigating the impact of various combinations of aperture design, solder paste and squeegee assembly. Theoretically, a square aperture of a given size provides the opportunity to deposit 21.5% more than its circular counterpart and the company’s experiments demonstrated this is also the case in practice. On the question of solder paste particle size, net gain was seen to be at a fairly low level. It is worth pointing out, however, that only one solder paste formulation was tested. The ultrasonically activated squeegee also delivered significant increases in solder paste transfer efficiency, supporting a number of other published investigations. These results tell us that marrying effective stencil design with an active squeegee process will deliver the highest levels of accuracy.
