May/June, 2012
www.us-tech.com Stencil Printing Challenges for Small Devices Continued from page 54
with nanocoat >0.5 , electroform with nanocoat >0.43. Typical aperture widths as low
as 0.175mm and aperture lengths as low as 0.4mm present a challenge to the printing process as far as percent paste transfer. The other challenge is the solder mask employed on the PCB. There are three types of solder
mask designs which are: l
defined by the solder mask. l
SMD (solder mask defined), where the pad opening on the board is
ically 0.05 to 0.075mm per side). l
NSMD (non solder mask defined), where the pad itself defines the boundary of the pad and the solder mask is pulled back off the pad (typ-
NSMD-Window. In the last case there is no sol-
der mask between the pads so bridg- ing between pads is more likely than with solder mask between pads.
Stencil and PCB Design Table 1 shows stencil design
guidelines for the three solder mask cases. This table shows the package size, the lead pitch, the number of I/Os, the package lead dimensions, the recommended PCB pad dimen- sions, the recommended stencil aper- ture dimension, recommended sten- cil thickness, and resulting area ratio. For NSMD the stencil aperture is 1-1 with the PCB pad dimension. It should be noted that the recommend- ed length of the pad on the PCB com- pared to the length of the lead on the QFN is 0.2mm larger. As seen, the area ratio for a 0.125mm thick sten- cil is >0.66 for all the examples list-
are recommended. The final example in Table 1 is
the NSMD-Window. The pitch is 0.4mm, leaving little room to put sol- der mask between pads on the PCB. Aperture size is also small giving a challenging area ratio for 0.125mm thick stencils; therefore, 0.100mm thick stencils are normally recom- mended to provide a more robust stencil printing process window.
Higher Paste Deposits Another problem arises when
using a NSMD-Window when the solder mask is higher than the pad on the PCB. In this case the solder paste is extruded through the stencil since the stencil is not in contact with the PCB pads during printing. This extruded paste will make con- tact with the bottom side of the sten- cil causing potential bridging during successive prints since there is no solder mask between neighboring pads. Stencil wiping after every print may help reduce this problem.
A special electroform stencil which is 0.08mm
Table 1. Stencil design guideline.
ed. Aperture size for the SMD is 0.05mm smaller than the PCB pad. There are typically two reasons for this reduction. If the stencil is slight- ly misaligned to the PCB, paste could be printed on the solder mask. Also,
there might be high stress points if solder contacts the mask. The reduc- tion in aperture size has reduced the area ratio making paste transfer more difficult. For area ratios below 0.66, electroform or NicAlloy stencils
thick everywhere except in the QFN area inside the solder mask can solve problems of differing solder mask heights.
One possible solution suggested
by a customer is a PCB side step stencil. This is an electroform stencil which is 0.08mm thick everywhere
Continued on next page.
Page 57
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