CircuiTree - August 2008 - (Page 22)

[ Flexible Thinking ] By Joe Fjelstad The Importance of Clean Room Space in Flex Circuit Manufacturing W hile John Wesley is credited with saying “cleanliness is next to godliness,” the proverbial concept has reportedly been used since biblical times to extol the importance of this fundamental virtue. Today, with ﬂexible circuit features continuing to shrink to meet demands for ever-smaller electronics with ever-increasing number of features, ﬂexible circuit manufacturing technology is reaching (or has perhaps already passed) an important inﬂection point where the need for attention to detail with respect to operating area cleanliness is of the highest importance. While in earlier facilities and ﬁxtures. In typical standard PCB facilities of today, there is unfortunately often very limited clean room space and those areas that have been established as clean areas (normally only in the imaging processing of the facility) are often limited to Class 10,000. The fact is that today processing ﬁne pitch interconnections with feature sizes of less than 100 µm pitch and hoping to do so at any kind of acceptable yield will require clean room processing areas with more stringent cleanliness levels of at least Class 1,000 and perhaps as low as Class 100. Clean room space is admittedly not free but then neither is scrap product. Clean rooms operate on a principle of having air pumped into the times careful processing could allow one to get away with some less than fastidious habits, slipshod manufacturing practices today will rapidly cause one to depart the business. As a result, the ability to produce large-area, ﬂexible circuit interconnection structures with ﬁne circuit features on thin materials will require some real attention to detail and cleanliness. The concern of cleanliness is transcendent and covers all aspects of the manufacturing process including not just facilities but also circuit design, materials, processes, and the manufacturing equipment used. Each of these areas of concern can inﬂuence and either positively or negatively impact circuit quality and yield. Astute manufacturers know well that to create large-area, ﬁne-featured ﬂexible circuits they need clean process 22 August 2008 • circuitree.com room though high-efﬁciency particulate air (HEPA) ﬁlters, keeping a positive pressure on the room at all times. These HEPA ﬁlters can remove at least 99.97 percent of airborne particles 0.3 µm in size, which is not good enough for today’s nanometer size features but sufﬁcient for today’s bleeding edge 25 to 50 µm circuit features. While clean room facilities are important, they are only as good as the people who use and maintain them. Thus, to achieve the desired low-defect densities that both manufacturers and customers hope to have, clean room facilities must have good tools, work practices and processes, and appropriately garbed and attentive operators with a full understanding of operational controls and practices. Not every clean space requires that all operators wear what have been affection- ately called bunny suits, but static-resistant/ dissipating lab type coats, shoe covers, face masks, head covers, and gloves are generally needed at a minimum. Clean room concerns are not limited only to the imaging areas. Coverlayer and covercoat processing areas must also be given attention. One of the big concerns in processing organic materials, of almost any sort, is the potential for a buildup of static electricity on the surfaces. We have all seen how a rubber balloon can be made to stick to the ceiling due to static charge buildup by rubbing it against a wool sweater. The same effect can cause a plastic ﬁlm to be charged up and attract particles to its surface. With ever-smaller circuit features, those particles do not have to be large to create big concerns in the process. This is because anything that bridges conductors is likely to be a cause for rejection. As a result, a partner technology for any successful clean space is a good static control system that can include conductive ﬂoors and other room surfaces and ion generators to neutralize any charges that might have built up on surfaces. Anyone who has ever peeled a protective polyester ﬁlm from a polyimide coverlayer ﬁlm can attest to the charge generated by that action. Ultimately, slow and deliberate movement within the clean room is an important practice required to keep things under control. In summary, as the ﬂex circuit industry transitions to ever-smaller circuit features on ever-ﬁner pitch and thinner substrates, much more attention to the matters of cleanliness and static control will be required. Such adjustments will not come without cost but they will allow the manufacturer to stay in the game. ■ Joseph Fjelstad, founder and president of Verdant Electronics and co-founder of SiliconPipe, is an author and innovator in electronic interconnection and packaging technologies. Download his flex circuit book free at www.flexiblecircuittechnology.com. E-mail: jfjelstad@siliconpipe.com http://www.flexiblecircuittechnology.com http://circuitree.com

Table of Contents Feed for the Digital Edition of CircuiTree - August 2008

CirciuTree - August 2008 Contents My Line Industry Review Tech Talk Flexible Thinking Creating Connections Between Electronics Design and Manufacturing Happy Thoughts View From the Middle Market Outlook Fein Lines IPCA Showcase Technical Product Spotlights Classified Ads Upcoming Events Ad Index Buyers Guide

CircuiTree - August 2008

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