Monday, June 18, 2018
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Stencil Printing Challenges for Small Devices
Solder mask designs.

The 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 functionality, especially in the newer mobile devices. QFNs stand out because they provide better grounding and better heat sink thermal properties compared to other SMT packages. Most QFNs have a metal pad on the underside for grounding and heat conduction. 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-registering the leads on the QFN and the pads on the PCB. QFN float can be controlled by reducing the amount of solder paste printed on the ground plane. Typically a 50 to 60 percent reduction will solve the QFN float problem. However, the aperture reduction must be done judicially. A window pane aperture is recommended for most cases. This allows the solder paste volatiles to easily escape during reflow without moving the QFN device.

Choosing Stencil Design
The next challenge is choosing the stencil design and stencil technology for the SMT printing process. The stencil printing process is a two phase process; first the solder paste must fill the stencil aperture and second 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 aperture 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.
NSMD window with mask to pad gap of 0.03mm.

Two stencil technologies have very smooth aperture walls: electroform 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: nanocoating. 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 lubricity promotes paste release from the smooth aperture walls and also promotes 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 stencils >0.55, electroform and NicAlloy 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:

  • SMD (solder mask defined), where the pad opening on the board is defined by the solder mask.
  • 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 (typically 0.05 to 0.075mm per side).
  • NSMD-Window.

In the last case there is no solder mask between the pads so bridging 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 dimensions, the recommended stencil aperture dimension, recommended stencil thickness, and resulting area ratio. For NSMD the stencil aperture is 1-1 with the PCB pad dimension. It should be noted that the recommended length of the pad on the PCB compared to the length of the lead on the QFN is 0.2mm larger. As seen, the area ratio for a 0.125mm thick stencil is >0.66 for all the examples listed. Aperture size for the SMD is 0.05mm smaller than the PCB pad.
Table 1. Stencil design guideline.

There are typically two reasons for this reduction. If the stencil is slightly 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 reduction in aperture size has reduced the area ratio making paste transfer more difficult. For area ratios below 0.66, electroform or NicAlloy stencils are recommended.

The final example in Table 1 is the NSMD-Window. The pitch is 0.4mm, leaving little room to put solder 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 recommended 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 contact with the bottom side of the stencil 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.
Step electroform stencil on PCB side, 0.1mm thick around QFN apertures and 0.08mm elsewhere for all other apertures.

One possible solution suggested by a customer is a PCB side step stencil. This is an electroform stencil which is 0.08mm thick everywhere except in the QFN area inside the solder mask where it is 0.01mm thick. In this case the mask opening was of the order of 0.125mm per side except on the ends of the pad rows where it was less. There are several limitations to this approach. First, the spacing between the step and the solder mask is extremely small allowing for little misregistration. Also, the stencil is thinner for all other components except the QFNs, which may yield insufficient paste. The first limitation could be addressed at the PCB design level by making the mask-to-pad clearance much larger, of the order of 0.25mm per side, as well as leaving the ground plane without any solder mask surrounding.

Stencil without a Step
Another possible solution is a single level stencil without step, but with nanocoating on the aperture walls as well as on the bottom (PCB) side of the stencil. Nano coatings have a property called fluxophobicity. Quite simply, it is the stencil's ability to resist the spread of flux on its surface. It is measured in the form of the "flux contact angle". This is the angle that the flux will form when a drop is placed on the surface of the stencil. Nanocoating not only increases the paste's ability to release from the apertures, but also to resist spreading on the bottom side of the stencil when the paste is extruded into a cavity created by the NSMD-Window. This property not only eliminates the need for frequent under-board wiping, but also reduces the occurrences of pad-to-pad bridging. Stencil design for micro BGAs depends on the solder mask design on the PCB. For non-solder mask pads overprinting is recommended. For example, for a 0.3mm pitch µBGA with a 0.2mm pad and a 0.25mm mask, overprinting with a 0.175 aperture is recommended. For a 0.100mm stencil thickness, the area ratio for this configuration is 0.43. For a 0.075mm stencil thickness the area ratio is 0.58. A 0.100mm thick stencil would require an electroformed stencil with nanocoat, whereas a 0.075mm thick stencil would require an electroform or a NicAlloy with nanocoat.

Although QFN and µBGA devices present a challenge to the SMT assembly process, with proper stencil design, proper stencil technology selection, and proper PCB solder mask layout these challenges can be overcome.

Contact: Photo Stencil, 4725 Centennial Blvd., Colorado Springs, CO 80919 719-599-4305 fax: 719-599-4334 Web:

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