Screen Printing

Clive AshmoreA poor stencil aperture seal may lead to solder bridging.

When we think of a gasket, we often relate it to something experienced in our home, perhaps a plumbing gasket or a gas pipe. A tight seal – or gasket – ensures no leaks. In SMT printing, the same theory applies: A good gasket means no solder paste leaks and a better printing result. With stencil printing, gasketing occurs when the aperture cut into the stencil is pressed firmly against the corresponding pad so it forms a seal. To achieve a good gasket, the aperture should be at least 1:1 with the pad size. If the aperture is larger than the pad, a poor gasket will result. Our company generally advises a one mil (25µm) reduction of aperture size to ensure a tight seal. So, for example, if the pad is 125µm x 125µm, an aperture of 100µm x 100µm is recommended. All things being equal, this approach will help form a good, tight gasket.

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Clive AshmoreWhen diagnosing print problems, don’t overlook substrate support.

Substrate support is an important, yet often overlooked, element of the screen printing process. Sure, tooling is top of mind when all necessary parts of a new PCB assembly are being developed. But once it’s in the printer way down in the print nest on the table, we tend to forget about the tooling block. I suppose this out-of-sight, out-of-mind mentality is why manufacturers often point to more obvious, visible components of the printing operation as the culprit when the process shows inconsistency. Surely it must be an issue with the squeegee, the stencil or the board fabrication, right? Well, sometimes that’s the case. In many instances, however, a resolution may be as simple as a look at your tooling; something could have changed, or maybe it’s been incorrect from the start.

If the tooling isn’t manufactured properly or has been altered during production, printing inconsistency is the result – either across the board or from board to board. Paste volumes may be noticeably different in various areas of the PCB or panel; one corner may be just fine, and another has too much or too little material. This dynamic could be the first clue tooling is the cause. If the problem were an improperly manufactured stencil, for example, the issues would more likely be consistent across the entire board. Tooling errors can be extremely focused.

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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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