CircuiTree - July 2008 - (Page 20)

[ Flexible Thinking ] By Joe Fjelstad Back to Basics Part 22 – Design Features for Flexible Circuit Coatings hose who read the last installment of this series will recall it was focused on ﬂexible circuit coatings, which included: cover layers, cover costs, and cover ﬁlms. There is inevitably a need to access features through the coating regardless of the type used, and this topic was discussed earlier in some detail for both surface mount land design and through-hole component terminations. The sizing of coverlayer openings will, of course, vary in accord with pad design features and the presence or absence of a plated through-hole structure and beyond. Thus, single metal layer ﬂex designs may be better served by omitting land hold-down features but retaining the use of ﬁlleting of the transition from trace to hole. In such cases, the openings in a coverlayer should desirably be roughly 250 µm (approximately 0.010 in) less than pad diameter. This will provide adequate clamping to hold the component land in place. Coverlayer design features vary according to the structure of the ﬂex circuit and can impact electrical and mechanical requirements of the ﬁnal product; these concepts are illustrated in Figure 1. making circuit design connections; it should come without surprise that no opening is required unless the via needs to be accessed for electrical test purposes. Another area of concern relative to polymer coatings over ﬂexible circuits is the matter of distance from circuit features to the edge of the part or the trace to cut line, as it is also often called. Best design practice in this arena has been moving with gains in technology and manufacturing prowess. Conservative design practice for ﬂex circuits generally recommends that the edge of the part to the edge of conductor spacing be greater than C Coverlayer design features vary according to the structure of the ﬂex circuit and can impact electrical and mechanical requirements of the ﬁnal product. Figure 1 General Guidelines for Design and Manufacture associated with the termination. As before, the key area of concern relative to termination access design features in the ﬂex circuit is with single metal layer ﬂex circuits. The primary concern is related to the possibility of pad lift during assembly or use, and, with the elevated temperatures of lead free, the problem is a more urgent concern than just a few short years ago. There are some long-established guidelines in this regard that provide general guidelines for sizing of coverlayer openings. For example, for a single metal layer ﬂex circuit with land hold-down features (i.e., tiedown tabs or anchoring spurs), the opening in the coverlayer can be roughly equal to pad diameter; however, keep in mind that with rising frequencies, the tie-down features represent electrical stubs that can cause undesirable signal reﬂections at high speeds but which can also act as antenna, broadcasting electronic noise into the local environment 20 July 2008 • circuitree.com That said, if very small holes and component lands are used, the guidelines might prove challenging or perhaps even impossible. One caveat in this regard relates to the phenomena known in the industry as adhesive squeeze out, a situation where the bonding adhesive encroaches on the solderable surface. The concern is for the most part conﬁned to lamination processing where heat and pressure of lamination can manifest the problem. For double-sided ﬂexible printed circuits and for multilayer ﬂex circuits as well, the rules are a bit different. In either of these cases, the ﬂex circuit, having plated through-holes and ﬁlleted lands, can be ﬁtted with a coverlayer opening that can be roughly equal to or even slightly larger than the pad. This simple design approach has proven effective in minimizing squeeze out for many years. The ﬁnal case is, of course, related to noncomponent plated through-holes that are used as simple vias for 1.25 mm (0.050 in). However, it has been hb shown in practice that ﬂexible circuits can be reliably made with edge to conductor spacing of less than 250 µm (0.010 in), but this normally comes at an increased cost that will vary depending on the tooling system used. In summary, ﬂexible circuit coatings have some very speciﬁc design guidelines to ensure that the design features can be properly and reliably accessed. Adherence to some simple rules will help assure design success. In the next installment, the discussion will turn to the design features that will help make the circuit more robust and less prone to damage in handling. ■ 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 - July 2008

CircuiTree - July 2008 Contents My Line Industry Review Tech Talk Flexible Thinking Toward a PCB Production Floor Metric for Go/No Go Testing of Lossy High-Speed Transmission Lines Intelligent Design 20-Year Retrospective Ask the Flexperts Environmentally Speaking BPA Growth Curves Considering Design Variants to Maximize Process Efficiency Market Outlook Technical Product Spotlights Classified Ads Upcoming Events Ad Index

CircuiTree - July 2008

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