Stencil Printing for 0201 Applications

Challenges:

The basic defects that impact assembly yield are bridging, satellite solder balls, and opens (i.e. tombstones / draw bridges). There are many variables with the stencil printing process that can impact these; several of them are listed next.

Web Spacing � As a general rule, apertures that are spaced at least 10 mils apart will inhibit bridging.

Aperture Gasket � Aperture openings that are the same size as the pads, unless perfectly aligned to one another, will not seal consistently well. For tightly spaced components where bridging may be of concern, aperture size reduction across the critical dimension is recommended (by 0.5 mil or so) to tolerate subtle changes in aperture to pad registration, while still maintaining a good seal. Printing the substrate with contact against the bottom of the stencil is also recommended in order to produce the best gasket results. Board support will also impact the aperture gasket. The previous onboard forum discussed flexible pin array tooling modules, which are a good alternative to standard posts, box, or dedicated vacuum tower. A unique feature of the flexible pin array tooling is the additional support provided to the stencil outside the edges of the substrate. The major defect attributed to the poor aperture gasket is bridging, although it is also possible to produce solder balls.

Aperture Size � Aperture size reduction relative to the pad size will affect the aperture area ratio, leading to a larger proportion of aperture wall surface area in contact with the solder and making it more challenging for the solder to release during stencil separation. On some 0201 pad designs, it may not be possible to design for aperture size reduction due to violation of area ratio rules. Area ratio, as I define it, is the aperture wall surface area divided by the aperture opening area. Some define this as the inverse proportion. I suggest that the area ratio be smaller than 1.65 to achieve stable and consistent paste transfer, such that the deposit volumes across 0201 pads are relatively balanced. Balanced volume deposition is important in order to maintain equal wetting surface tension forces on 0201 terminations during reflow to inhibit tombstone defects.

Aperture Shape/Position � There are many opinions about the best aperture shape to use. I have found rectangles to work just as well as home-plates, bow-ties, etc. My recommendation is to use the same shape as the pad. If solder balling is of concern, position the apertures slightly offset (2 mils or so, depending on the pad design) from pad center so that the deposits are not so far underneath the component. Be aware that the gasket between the aperture and pad will not be as good on the overhanging aperture edge.

Pressure � On tightly spaced components, bridging may be sensitive to print pressure. This will vary depending on size of substrate, solder paste rheology, squeegees/pressure printhead, etc.

Stencil Maintenance � Stencil cleaning at regular intervals is recommended. The automatic under stencil wiping system on advanced stencil printing machines will provide adequate hands-off bridging deterrence, and will remove debris that may impact aperture gasketing and paste release consistency. I find that a dry wipe works quite well every few prints. If using a wet cleaning process, it is advised to ensure the solvent has completely evaporated before continuing with the next print. Over time, solder residue may build up on the stencil bottomside as a result of repeated wiping, requiring a more thorough cleaning technique to be used.

Slump � Cold slump is not too common and should not be a concern for bridging as long as the solder paste is produced by a reputable vendor, it has been stored at vendor recommended conditions, and contains a valid non-expired date code. The material I typically use is 900-1000 kcps viscosity and have never experienced cold slump.

Process Settings:

Speed � Some solder pastes print better than others, and it is difficult and frustrating to understand why this is. This said, there is at least one commercially available material out there that prints 0201s well at high speed, which I have tested (100 to 150 mm/sec) producing repeatable full deposits. I would expect other products out there will work well too. Paste material, squeegee condition, and aperture gasketing quality will be the limiting factors.

Pressure � As a general rule 0.5 to 0.75 kg of print pressure per inch of squeegee length is recommended to start. See also �pressure� discussion above.

Separation Speed � From experiments, data suggests that faster is better.

Snapoff � On-contact is recommended (i.e. zero snapoff) to produce best aperture to pad gasket consistency.

Tooling � As long as the substrate is fully supported, the use of standard posts, box, vacuum tower, or flexible pin array tooling options are all effective.

Stencil Cleaning � See �Stencil Maintenance� discussion above.

-Jeff

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