Staying Healthy: The Print Process Checkup, Part 2 Print E-mail
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Written by Clive Ashmore   
Thursday, 03 June 2010 16:42

  

Assessing location and timeliness/repeatability.

I’m sure since my last column readers have all their material volume in control, so let’s move to the two other areas central to a healthy print process: location and timeliness/repeatability.

Proper alignment of the board and stencil are essential for high-yield printing. So, if experiencing problems with location, evaluate the following inputs:

Board and stencil fiducials. When fiducial placement on the board is incorrect, it’s generally an etching issue or a plating issue. With etching, if the board is solder mask defined, it comes down to the quality of how the mask was defined and the technique used. Plating errors generally are more common with HASL boards.

While printers have intelligence and certain tolerances, the more guessing a machine has to do, the more issues with alignment. Likewise, the stencil fiducial quality is critical. Etching the fiducial onto the stencil requires a skilled laser operator and a stencil manufacturing process completely in control, to ensure good fiducial size and shape. Proper stencil cleaning and maintenance are equally important. Over time, paste particles may find their way into the fiducial marks and become welded into the fiducial, making machine centering more challenging. In short, if you give the machine perfect circles on both board and stencil, it will move the stencil to within +/-12.5 μm of where it should be. If you give the system imperfect fiducials, the printer software will guess the perimeter and center, making it more challenging to achieve perfect alignment.

Rail system. Simply put, the printer rail system moves the board in and takes it out. But, if the rail system is set up incorrectly, the board will move. Here’s why:

Operators often overcompensate the board width. As board dimensions may vary slightly, it’s possible that a couple of boards may be slightly bigger than the others, and instead of rejecting them, the operator makes the rail width wider. The boards are flying through, but the large, wide track could also mean you aren’t clamping correctly. The board then moves during the print stroke or the movement of the table, which can cause incorrect deposit location.

Tooling. If using vacuum tooling, ensure the vacuum is in the right place, that it works and is not blocked, so board stability is optimized. In some instances, the milled tooling may have been improperly machined, so the board moves in the tooling plate either during or after the print, which will illicit an alignment error message from the printer.

Calibration and maintenance. For proper performance, maintain and service the printer annually. Calibration is an important component of this maintenance. The vision/alignment calibration is the heart of the machine, really, and should be performed once per year. Shockingly, only about half of all printer owners follow this schedule and at their own peril, I might add. If the alignment isn’t right, you’re wasting your time – especially in the age of miniaturization.

Finally, controlling the print process to achieve high-yield assemblies in a timely, repeatable manner is the last piece of the puzzle. And, while cycle time is an excellent headline figure, there is much more to printing than the cycle time credentials. Let’s take a closer look.

Software. Cycle time evaluates board in/board out speed. While relevant, there is much more to the print process than this, and there are two primary reasons users don’t get the most out of their machine in relation to throughput and cycle time. First, the throughput optimization software still can be difficult to navigate through. Second, there is the tendency to rely on older-generation programs, with adjustments only for board dimensions; thus, any throughput inefficiencies are duplicated. Fortunately, next-generation touch screen software offers intuitive and descriptive selections so that operators can easily find programs and print parameters to ensure maximum throughput.

Setup. In a high-mix environment, being able to change over product quickly also impacts the timeliness of the process. Software certainly plays into this, but so do things like fast changeover of consumables (understencil cleaning paper cassettes, for example) and automatic configurable tooling that helps eliminate setup.

The timeliness and speed of the equipment, however, are only relevant if fast and repeatable. This is where the rubber meets the road, and where machine mechanics aret tantamount. What makes a repeatable system?

Mechanical elements. A printer is, after all, a machine. It has metal, bearings, striker plates and a host of mechanical parts to maintain or replace. The machine has to be repeatable for its lifetime and between service visits. Let’s face it: 12.5 μm of repeatability is just as much about good stability and engineering from the frame all the way up as anything else. And, as we creep toward 300 μm ptich, it becomes critical that printers are aligning and repeatable. Equally important: a credible third-party system to measure against the manufacturing spec.

Alignment repeatability. Dry alignment Cpk is one thing, but running that Cpk in process is quite another. We’re not making alignment machines; we’re making printers, and the actual print stroke, process, and movement of the print head should be included in the alignment repeatability statement.

Clamping system. Having a good and versatile clamping system on board is another factor that impacts repeatability. Some systems permit one clamping system, which is fine when producing only a single kind of board. But most firms need process flexibility and the ability to change clamping systems as the product comes in. And, all the clamping tools have to be equally good and perform well under the influence of the print stroke.

In quite an oversized nutshell, that’s it. There are so many inputs and variables to the printing process, but systematically evaluating all of these on a regular basis will nearly guarantee a robust, healthy print operation. Stay well!

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, 07 June 2010 15:13
 

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