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Written by Clive Ashmore   
Sunday, 30 November 2008 19:00

Inspection can reveal post-print problems or help analyze DoEs.

Screen PrintingIt’s as predictable as the sunrise. When the economy struggles, efforts to optimize line yields and reduce manufacturing costs become, well, let’s just say a bit more intense. I’ve been through many a slow cycle, and the cost-cutting pattern repeats with each economic swing. This behavior, while understandable and necessary, is also a little curious, as many of these measures should probably be employed as normal practice regardless of the macro economy. Process optimization tenets are something I shout about constantly and, apparently, sometimes my enthusiastic ramblings have fallen on deaf ears. But, these auditory channels are now remarkably keen to hear what can be done to maximize resources. In the case of screen-printing, that means getting the absolute most out of your machine.

For the past five or six years, electronics assemblers have installed a plethora of new SMT lines to achieve required volumes. (Don’t get me wrong; I’m not complaining, as we have all benefited.) What happens, unfortunately, is that in an effort to reach certain volumes, sometimes quantity – not necessarily quality – becomes the driving force. It’s not to say manufacturers are putting out sub-par products. They’re not. But what has been happening is first-pass yields have suffered, with some firms running at sub-80%. That’s a lot of money wasted and that’s where screen-print optimization can pay dividends. If you consider the cost of an assembly increases logarithmically as the board travels down the line, when you start putting on chips, reflowing and testing, then printer investments include the cost of the bare board, solder paste and stencil. Relatively speaking, the assembly value at the printing phase is at its lowest. If you can capture any problems coming in before or after the print, then it will save you a heap of money down the line where, at best, any mistakes will be found by test and, at worst, discovered by a consumer.

Taking some time upfront to fully analyze a new product run – or even an existing one that’s not up to par – can go a long way toward saving significant costs. It’s a fairly simple proposition when employing productivity enhancing tools available with many screen printers. One such tool is 2-D inspection, which offers a two-fold solution: use as a production tool (its common function), or as a useful aide for Design of Experiment (DoE) exercises to optimize the process. The DoE route is what we recently took with a manufacturing operation that was experiencing some issues. A customer was manufacturing an assembly that had a CSP placed right next to a large RF shield. As we’ve discussed in this column (“Heterogeneous Assembly,” September 2007), optimizing heterogeneous assemblies is essential in today’s miniaturized world. Because of the large component, a goodly amount of solder paste had to be deposited to ensure a robust solder joint. But, when placed next to the fine-pitch CSP, this customer’s current process wasn’t cutting the mustard, and the small CSP deposits were struggling to release from the stencil. The customer’s inspection equipment was indicating major fails on the CSP. Using 2-D technology, we took the customer’s multi-board panel and created a few different stencil/paste volume scenarios to analyze which architecture yielded the better result. After about an hour’s time running that board, we had enough data to predict which stencil design would produce the best results. By slightly enlarging the CSP apertures so as to take them to the aspect ratio limit, we were able to solve the problem, and this customer’s process was suddenly running at much higher yields, and therefore much more cost-effectively.

While inspection can output production data on post-print problems or help analyze DoEs, it can also shed light on and accurately set other critical printing inputs such as cleaning frequency, print speeds and pressures. If you’re cleaning too early, you’re wasting money. If you clean too late, you may potentially be sending defective boards down the line. 2-D inspection can indicate where certain areas of the print are out of control, help determine inadequate or excessive tooling, evaluate whether print pressures are too high or too low and so on.

Driving your printing equipment to reduce production costs isn’t difficult. The tools are there and, if you just use them, you’ll realize a very fast financial return on your investment of time. It’s a lesson particularly relevant for today’s economic situation, but one that should be employed even when the economy is humming.

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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