Screen Printing

Clive AshmoreAdding just enough material for a set amount of prints can ensure good outcomes.

Although no-clean solder pastes are the most prevalent materials used in electronics assembly today, water-soluble pastes are still in the game. In market sectors like aerospace, military, automotive and industrial, water-soluble materials are frequently the specified-in, legacy product – often because of the reliability requirements to remove flux residues. Printing water-soluble solder pastes, however, is quite a different process than printing no-clean materials. Assembly specialists take note!

Back in the day, no-clean pastes were the more fickle materials, with delicate operating windows and strict storage requirements. Over the years, massive amounts of development and a focus on maximizing process efficiency (i.e., eliminating an unnecessary cleaning step) put no-clean in the processability fast lane, while water-soluble material R&D got lapped. Although new water-soluble pastes have been released in recent years, they are still generally more difficult to print than no-clean pastes, and the finesse required to successfully print them isn’t always well understood. Put simply, the primary challenge with water-soluble pastes is they are hydroscopic (absorb water) in their function, making them a bit sponge-like.

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Clive AshmoreSelf-adjusting paste deflectors save solder paste and simplify cleaning.

Regular readers of my column are certainly well aware of the challenges around print process variability and the impact even small changes can have on printing results. With a procedure as dynamic as printing, ensuring every input is spot-on is critical to a good outcome, especially in the age of miniaturization. This holds true for what many would consider even the most minor of details: the paste deflector.

Every squeegee our company supplies to a manufacturer comes with a set of paste deflectors. These very simple pieces of formed metal are mechanically connected to the squeegee body with two bolts, and their job is to keep the solder paste material from moving outside the print area. They act like dams to keep material within the angle of the squeegee. While paste deflectors play an important role in reducing material waste and maintaining material integrity, they don’t come without challenges. Setting the height of the paste deflectors is a manual operation, and getting them just right can be tricky. If the deflector is set a bit too low, when the squeegee comes down to meet the stencil and pressure is applied, the deflectors will actually grind into the stencil. At worst, the deflector can punch a hole through the stencil, but it will most certainly leave a trail or coin the stencil if set even slightly too low (FIGURE 1). To avoid damage, operators often set the deflectors a little higher to permit a margin of error. This approach, however, lets paste run underneath the gap, and enables material to run up the outside of the squeegee, which can also introduce process problems.

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Clive AshmorePredictions on material transfer get a wrinkle.

When you’re pushing the limits of area ratio rules and trying to realize an acceptable result out of a shrinking window, even the slightest dimensional alterations can make a big difference. That was the topic of my last column, as we looked into the impact of challenging area ratios of 0.4 (the accepted standard is 0.66), as well as the aspect ratio influence as the geometric shape of the aperture moved from a square to a rectangle. The work revealed that at an aspect ratio range of 1.6 to 1.8, the Cp values were noticeably higher than for those apertures with a 1.2 or 2.0 aspect ratio (each end of the square to rectangle spectrum). Intrigued by these results, our team pushed on with the miniaturized aperture (0.4 area ratio) shape analysis, this time to understand the effect of moving from a square to a circle.

I should note a few years back, our company also evaluated the printing performance of pure squares versus pure circles when using activated squeegee technology. Ultimately, the square apertures produced better transfer efficiency rates, which improved dramatically with the use of an activated squeegee versus a standard squeegee. However, what this previous study didn’t assess, and what our team set out to investigate this time, were gradual changes in aperture shape when moving from a square to a circle. For example, taking the Y dimension and slowly changing the radius by slight percentage increases to round the square edges and eventually move to full circle (FIGURE 1).

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Clive Ashmore

 Do aperture area ratio and aspect ratio play an equal role?

Miniaturization has been a central theme of this column for a long time. The enduring relevance of the topic speaks to the ever-moving goal posts when it comes to component sizes and printing dimensions. Today the industry is facing the reality of passive metric 0201s, which has prompted another look at the influence of stencil architecture on the printing process. The impact of introducing sub-150µm apertures means heterogeneous assemblies will include next-generation surface-mount devices that push area ratios below 0.5. To understand the bearing this will have on the print process, our team evaluated not only at area ratio – which has been the historic measure of printability – but also at the area ratio’s associated aspect ratios. The results were intriguing, to say the least.

By definition, the aspect ratio relates to the measurement of the shape of the aperture, and the area ratio corresponds to the aperture opening and side wall area. The formulas for both follow:

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Jerome Rousval

Has its time finally come?

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Clive Ashmore

When print results are out of spec, board thickness could be to blame.

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