Jeff Schake and Mark Whitmore
ASM Assembly Systems
GA, USA, Weymouth, UK
Continued demand to miniaturize consumer electronic products compels the use of smaller components to satisfy more stringent assembled package dimensioning requirements. Along these lines, implementation of metric 0201 or M0201 size surface mount passives (the imperial designation is 008004) will help enable the next generation form factor electronic packaging. Upon further identification of the M0201 solder joint geometries that form acceptable attachment profiles to the bonding pads, it is realized their stencil printed paste volumes correlate to mere countable quantities of solder particles. A non-stepped stencil used to print all device pads on the board is expected to be at minimum 80µm foil thickness, which should still permit enough printed solder alloy to produce sufficient joints on larger component types contained in the assembly. However, this stencil thickness constraint obligates the utilization of traditionally hazardous aperture area ratios well below 0.5 for the M0201s. The results of the printing investigation discussed in this paper revealed unexpectedly stable and adequate paste transfer levels for demonstrated successful M0201 component assembly.
We have identified a specific challenging mobile product-like application containing M0201 component pads and imposed the restriction to use a minimum 80µm thickness non-stepped stencil foil to satisfy heterogeneous assembly. Upon exhausting more rational process options the direction of this research took a surprising path to pursue investigation of stencil printing tiny stencil aperture dimensions largely considered impossible to print. This work featured using stencil apertures designed below an area ratio value of 0.4 that were demonstrated to provide appropriate solder volume accommodating successful assembly of M0201 capacitor components in a highly controlled limited batch assembly process. While this success is ground breaking insofar as implementing critically low area ratio apertures, the practical use of such stencil designs in formal manufacturing processes is still highly discouraged. Further assembly investigation utilizing critically low aperture area ratios should include more rigorous validation testing with scope to capture process boundary conditions.
Interest in pushing the limits of stencil printing is expected to grow as miniaturization continues to evolve and expand. As we’ve learned from this work, such demands oblige exploration of all (even counter intuitive) options, including reconsideration of best working practice guidelines (i.e., stencil aperture design) leading to either reinforce the recognized rules or realize new process capability potential.
Initially Published in the SMTA Proceedings