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May/June, 2012 Stencil Printing Challenges for Small Devices By William E. Coleman Ph.D., Vice President Technology, Photo Stencil, Colorado Springs, CO T
he very small form factor of today’s QFNs (quad flatpack, no leads), DFNs (dual flatpack, no
leads), and µBGAs allows for smaller packages, and thus smaller electronic devices with more overall functionali- ty, especially in the newer mobile devices. QFNs stand out because they provide better grounding and better heat sink thermal properties com- pared to other SMT packages. Most QFNs have a metal pad on the under- side for grounding and heat conduc- tion. DFNs have a similar center metal pad but have leads on only two sides. Typical thickness of the QFN devices is 0.85mm and the X and Y body dimensions range from 3 up to 12mm, so the packages are very small and very light. The QFN leads and ground plane conductor are flat and in the same plane on the bottom of the package. Micro BGAs with 196 I/O with 0.200mm balls on 0.300mm pitch are now available in a 14 x 14mm package.
Printing solder paste 1-1 with
the ground plane can cause the QFN to float during reflow, thus mis-regis-
solder paste printed on the ground plane. Typically a 50 to 60 percent reduction will solve the QFN float problem. However, the aperture
the stencil design and stencil tech- nology for the SMT printing process. The stencil printing process is a two phase process; first the solder paste must fill the stencil aperture and sec- ond the solder paste must transfer from the stencil aperture to the pad on the PCB. The paste transfer process can be pictured as a tug of war; the pad on the PCB is pulling the paste out of the stencil aperture while the aperture walls are holding the paste from releasing to the pad. Paste transfer depends on two major factors: the smoothness and lubricity of the aperture walls and the area ratio, which is the area of the aper- ture wall contacting the paste to the area of the opening underneath the aperture. The smaller this area ratio the more aperture wall there is impeding paste transfer. Two stencil technologies have
Solder mask designs.
reduction must be done judicially. A window pane aperture is recom- mended for most cases. This allows the solder paste volatiles to easily
There are several difficult challenges to overcome when screen printing solder paste on PCBs, mainly getting the paste to go where you want it and in the proper quantity.
tering the leads on the QFN and the pads on the PCB. QFN float can be controlled by reducing the amount of
escape during reflow without moving the QFN device. The next challenge is choosing
very smooth aperture walls: electro- form stencils and NicAlloy stencils. The electroform stencil aperture walls are very smooth as a result of the manufacturing process; nickel is plated up around photo resist pillars defining the aperture, basically a molecule at a time. NicAlloy is a laser-cut stencil with special post processing including electropolish and a special nickel-plating process. Both of these stencil technologies provide very smooth aperture walls. There is an additional process which can aid in paste transfer: nanocoat-
ing. In this process a nanocoating is applied to the aperture walls and the contact (non-squeegee) side of the stencil. The coating is very thin, a molecular monolayer, but provides lubricity for solder paste. This lubric- ity promotes paste release from the smooth aperture walls and also pro-
NSMD window with mask to pad gap of 0.03mm.
motes anti-smearing of the paste underneath the stencil since the paste does not wet or stick well to the stencil underside surface.
AMTX and NicAlloy Stencils AMTX electroform stencils and
NicAlloy stencils are available from Photo Stencil. Photo Stencil has established a general area ratio design guideline: laser-cut stencils >0.66, electroform and NicAlloy sten- cils >0.55, electroform and NicAlloy
Continued on page 57
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