Screen Printing

Clive AshmoreWhy print offsets occur and how to correct them.

Printing offsets – the degree to which a material deposit is off center from the pad – can occur due to three primary elements of printing: the printed circuit board (substrate), the stencil and the printer. Each has to be manufactured and is surrounded by a process bandwidth, each with its own tolerances that can accumulate. Add to this the variables from different manufacturing methods, sites and base materials and, well, offset inevitability becomes obvious.

Let’s begin with the board and stencil. Gerber data is king; it’s where the designs begin and is the blueprint for PCB and stencil manufacture. Simply put, Gerber is an x, y coordinate and angle for a certain feature size and shape. When an offset occurs, it is the difference between what the Gerber says and what is actually produced. At the PCB level, the offsets derive from the artwork, the subtractive chemical process and the FR-4 laminate. Each of these has the potential for variability, as in the case of FR-4 that can stretch and move during temperature processing, because the coefficient of thermal expansion (CTE) is relatively poor, especially considering today’s dimensions. Given these realities, the board could be off in one corner, or it could be a gradual movement from the left corner to the right, from the center outward or just focused in one area of the board.

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Clive AshmoreOne approach: Aperture designs that reduce paste volume from the inside wall.

While the primary theme of this column over the past few years has been miniaturization’s impact on stencil printing, standard SMT process challenges are alive and well. In many applications, where average device dimensions are large compared to the latest package footprints, reducing traditional defects is still a battle. One such frustration: mid-chip solder balls. To understand how to correct this well-known defect, one must appreciate the origin.

Mid-chip solder balls are most often observed with passive devices, specifically, resistors and capacitors. Since these chips have terminations on each end, solder paste material is deposited on two corresponding pads on the board; an open space of solder mask-defined PCB lies between them.

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Clive AshmoreA new dispensing system facilitates loading 500g containers.

Your mother always told you not to drink from the milk carton or eat from the peanut butter jar. But, as far as you were concerned, consuming directly from the container was a far more efficient approach. The germ factor notwithstanding, turns out you might have been right! Who knew your insight might lead to innovation in material management for screen printing?

All joking aside, eliminating process steps and reducing the chance for error introduced with manual operations generally results in more efficiency and higher quality output. This is most certainly the situation when supplying material for the printing process. As you are aware, my mantra of “good inputs = good outputs” is the basis for high-yield printing. In the case of paste management, ensuring proper volumes of paste in front of the squeegee blade at all times exponentially increases productivity, optimizes output and reduces defects related to insufficient material. Putting down material automatically, as opposed to manually, saves cost, reduces line downtime and eliminates errors.

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With understencil cleaning now a more frequent part of the print process, quickness really counts.

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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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