Fiducials in Focus Print E-mail
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Written by Clive Ashmore   
Friday, 29 March 2013 16:22

Good board and stencil alignment makes printing a snap.

While the latest tools and technologies, advances in materials and improvements in speed are all massively important to the printing process, there is a case for getting back to basics. Not basics as in simple technology, but basics as in utilizing proven techniques to ensure good outcomes for today’s miniaturized assemblies. So this month, I want to take the opportunity to focus on fiducials.

Fiducials – the small circles, squares, crosses or other shapes etched on the printed circuit board and also on the accompanying stencil – are really the fundamental tools for depositing material precisely where you want it. Simply put, fiducials are registration marks that permit the printer to align the stencil and board to ensure proper paste deposition location. It all sounds easy enough, but there is a science behind both fiducial creation and use. Proper execution – or not – of both of these factors can make or break a printing process.

Let’s start with the stencil. Historically, fiducials were etched into the stencil and filled with a black epoxy. This was brilliant and provided an absolutely beautiful contrast that a printer vision system could easily see. But etching and manual filling were expensive, and stencil manufacturers were under pressure to reduce costs.
So, the industry moved largely toward scoring the stencils with a defocused laser to create the fiducial during the stencil cutting phase. With this technique, it is critical that the stencil manufacturer creates a fiducial with a clear circumference and a well-defined edge. And here’s where the fun starts.

If the fiducial isn’t “cut” well, it’s possible to get quite hazy edges, making it difficult for the machine to register the edges and find the absolute center for alignment. So, first, it is very important that the stencil manufacturer have a robust fiducial process. Second, the printer needs to incorporate very good lighting technology on the camera. When a camera looks at the etched fiducial on the stencil, it’s often the case that the direct lighting needs to be reduced, while the oblique lighting is increased. This adjustment enables improved contrast so that the vision system can clearly find the division between the white background of the stencil and the start of the fiducial. Once the edge is located, the vision system can calculate the center. Good software capabilities permit the printer to calculate where the fiducial should be, and several platforms offer video capture to teach pads and apertures as a video model in cases where the fiducials haven’t been placed on the boards or stencils.

Of course, it takes two to tango. Good fiducial creation on the board is just as critical as it is on the stencil. Usually, the fiducials on the board are etched at the same time as the pads, so any artwork offset is the same for the fiducial as it is for the pads. But, with boards there are all sorts of finishes to contend with. From OSP (organic solder preservative) to ENIG (electroplated nickel gold) to HASL (hot air solder leveling) to immersion tin and more, various board finishes tend to react differently to alignment module lighting. By far, the worst finish for lighting compatibility is HASL. With this finish, direct light often hits the top of the solder and refracts, making it very challenging to find the fiducial center. Just as with stencils, this is where good lighting capability is essential. Increasing the oblique lighting will provide some illumination on the outside of the fiducial. Turning down the direct lighting will enable the background – the FR-4 substrate – to become very dark so that the software can analyze the white ring registered by the vision system and find the center. It’s very important that printing systems provide this flexibility – not just lighting flexibility, but software controls that can be applied to account for different scenarios.

When aligning the board, most print operators use the panel fiducials. Although boards can have as many as 20 fiducials, most of these are used for local adjustments during the placement process. For printing, the panel fiducials are those that are generally captured. These fiducials provide the largest offset and the biggest distance between fiducials, which is good for triangulation and general purpose alignment. But take note: If panel fiducials are used for the printer, they must also be used for the SPI machine. I’ve seen one too many instances where that was not the case, and the SPI registers unfounded print errors. A word to the wise: Stick to the same fiducials all the way down the line.

With miniaturization, alignment is now becoming ultra-critical. Pads are often at the edge of their tolerances, making alignment non-negotiable and forcing many production specialists to revisit alignment strategies. Years ago, we never thought we’d go beyond 25µm accuracy. Now, alignment accuracy has reached 2 Cpk at +/- 12.5µm.

Good printing starts with fiducials. Good fiducial creation during board and stencil production, along with robust print platform lighting, vision system and software capability, will help ensure a positive outcome. Electronics assembly has changed. The world is much smaller now; it’s time to go back and refresh those fiducial strategies to safeguard success in the age of miniaturization.

Clive Ashmore is global applied process engineering manager at DEK International (dek.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it . His column appears bimonthly.

Last Updated on Monday, 01 April 2013 10:51
 

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