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Written by Clive Ashmore   
Wednesday, 01 April 2009 11:59
Screen Printing

There is one process control item that is remarkably simple, yet often forgotten.

For those of you who regularly follow this column, no doubt you have read many of my rants about good process control. When it comes to printing, the Ashmore motto is: good inputs = good outputs. Just how those good inputs are achieved has been the subject of many of my writings. Indeed, good process control is a broad term and encompasses many aspects of the printing operation, including machine accuracy, software capability, squeegee angle, squeegee material, stencil quality and material, solder paste particle size, squeegee length, inspection capability…well, you get the idea; the list goes on and on. Of course, some of these are easy to achieve and others are more difficult to master. But there is one remarkably simple process control item often forgotten: ensuring there is paste in front of the squeegee before printing.

Yes, it’s true. You do actually need paste to print. And, even truer still is the fact that operators often forget to make sure there is enough paste – or any, for that matter – in front of the blade. It’s an honest mistake, I suppose. Imagine a factory where there are numerous lines and an operator is responsible for monitoring several sub-processes. It gets busy; another problem needs troubleshooting, and then, uh-oh, before you know it boards are going down the line with no solder paste! In fact, you’d be surprised how often this happens. (This sheds some light on why insufficients – not enough paste on the pad – are the biggest in terms of failures at nearly all factories, doesn’t it?)

While printer technology has come a long way and now provides incredible alignment capability, inspection technology, intuitive software, high speeds and more, the truth is that it still is not a totally “lights out” process. Often, product quality comes down to the operator making the most basic decision: add solder paste! Unless certain tools are in place on the printer, it will just keep going regardless of paste presence, and if paste is not present, the end-of-the-line result can be disastrous. But, like most production challenges, a remedy is available for those firms that have operators who are paste-memory challenged. By using an easily-installed tool for monitoring the height of the paste roll, remembering to put down some paste is now aided by a reminder from the printer, or better yet, the printer takes care of everything including paste dispensing.

Fig. 1

The paste roll height monitor is a relatively simple, yet effective tool that helps eliminate insufficient defects and improves end-of-the-line yield. The technology consists of two sensors that sit on either side of the squeegee blade framework support and are fully integrated into most printer software, so they are easily controlled and monitored with today’s intuitive user interface technologies (Figure 1). Here’s how it works: Using laser technology, a light curtain detects solder paste presence or absence, sending a signal to the printer. If paste crosses the light threshold and breaks the light beam, then the printer reads that as a signal that paste is present and will continue to process boards as normal. If, however, the light curtain is not disturbed, indicating a low paste height, the printer will alert the operator to add solder paste. As the paste roll starts to recede, the printer issues a warning so that operators are given time to react before the solder paste is completely gone. Once the warning is received, the printer can be programmed to stop and await manual paste replenishment, or the paste dispenser can automatically engage to deposit paste. In extremely high-volume applications, employing the automatic paste dispenser is recommended, as it facilitates a more fully automated process and saves operators from having “paste depositor” as their chief job description, freeing them to manage other critical components of the process.

So, operators can lay off the memory-enhancing vitamin supplements. Adding a paste height monitor system to the printer platform can greatly enhance quality and yield and add a new level of process – and memory – control to your operation.

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



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