Authored By:
Mark Whitmore
ASM Assembly Systems
Weymouth, UK
Jeff Schake
ASM Assembly Systems
Suwanee, GA, USA
Summary
The challenges associated with stencil printing of miniaturized components in heterogeneous assembly are well documented with proven printing solutions [1]. Now with the reality that the ultra-small Metric 0201 passive component is being introduced to market, printing capability is once again not assured. A series of focused Metric 0201 experiments investigating printing process sensitivity to circuit board properties, including pad dimensional accuracy and landscape topography, identified stencil gasket as significantly important on achieving print process control.
The effects of pad size influencing print quality were found to be less using a nano-coated stencil. The print quality produced on undersized Cu pads was significantly degraded with an un-coated stencil, whereas full Cu pads printed better. The stencil with an applied nano-coating improved print volume uniformity on all Cu pad sizes more significantly than improving overall paste transfer efficiency. Based on this test, the recommended print process used an 80 microns thick nano-coated foil, which outperformed a 50 microns thick stepped stencil of equivalent aperture size.coating, miniaturization
Conclusions
The results of this printing research largely support the printing capability benefits offered by using nano-coated stencils. The largest benefit observed from the nano-coating was reduced print volume distribution scatter, particularly on the circuit boards with small undersized Cu pads. The discovery of one unique test condition combination that produced poorer printing performance with the nano-coated stencil is currently unexplainable and warrants further review to confirm consistency of this behavior.
This work has also identified the effect of pad structure to have profound influence on printing results, with the planar Bare Cu board surface performing stand-alone best. However, demonstrated printing capability on bare board surfaces does not guarantee the same success on real patterned PCBs. Board design and manufacturing quality can significantly influence the printing outcome. While average print volume transfer efficiency results were similar comparing large pads against small pads, the print volume uniformity was noted better on large pads.
M0201 printing capability on proper circuit board pads proved to be best controlled using the 80 microns thick nanocoated stencil despite the unfavorable area ratio compared to the 50 microns thick step stencil results. The 50 microns thick step stencil apertures printed much larger than expected paste volume and produced a wider scatter in the print volume distribution attributed to poor squeegee wipe efficiency inside the step area.
Initially Published in the SMTA Proceedings
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