Flex Bridges Print E-mail
Written by Mark Verbrugge   
Tuesday, 03 January 2012 20:17

Minor design tweaks can improve performance and save money.

An old engineering expression goes like this: Anyone can build a bridge that is strong enough, but it takes a real engineer to build one that is “just” strong enough. The new version of this axiom could well be, Anyone can build it with enough money, but it takes a talented designer to build it with just enough money. All the latest materials and processes are useless if there isn’t a viable market for your product. Flex is no exception.

We see dozens of applications cost way more than they should. Let’s revisit what drives cost, not from the viewpoint of required materials and processes, but with an eye toward needless cost. We’ll review where we should spend a little more, and where we can save without impacting performance. This is simple, but all too often simply forgotten.

Materials. Some materials cost more than others. Polyimide hardboard is usually more than a similar FR-4 clad, but did you realize that FR-4 can vary widely from the same supplier, even when the difference is only a few mils in thickness? One example: I recently had a request for a 0.012" (0.3mm) FR-4 stiffener, with a tolerance of +/-0.003" (0.076mm). There was no “real” need for the tolerance; the designer selected this thickness because it seemed reasonable. A quick review of the materials available showed a price per sheet of $24.30 for 0.012" FR-4. The same supplier listed a 0.015" (0.381mm) at $18.12 per sheet. More material, less cost! By adding a paltry 0.003" (0.076mm) to our nominal stiffener thickness, we reduced our material costs by 25%! Granted, this is one material, a relatively inexpensive one at that. In this instance it saved a few pennies a board, but when multiplied by the 100,000 boards expected over the following year, you just paid for your tooling.

This applies to more than just FR-4; many common materials vary widely with minor changes in specification. Every designer/engineer should have current price lists available for reference when building the stackup. Make no mistake, the potential for savings is significant. If total demand is 10 pieces, you have little to worry about. But 100,000?

Layer count. Basic math says more layers, more cost. True … sort of. Flex circuits are not made in the exact manner as the typical rigid board. Whereas a rigid manufacturer would create outer layers using a “cap” of 1 oz. copper over a prepreg adhesive, a flex manufacturer would use a prebonded copper/polyimide film. What does this mean to the flex designer? For starters, a three-layer board costs as much to produce as a four-layer. If you want a three-layer, the flex supplier needs to “etch off” one side of a double-sided clad film. You paid for that copper, and you just threw it away! True, foils are available commercially in single-sided clads, but again the price can be higher than its double-sided counterpart. The difference in total thickness of a three-layer flex is nearly identical to a four-layer. In short, don’t knock yourself out trying to route everything in three layers when you have four to work with. This is true of any even-numbered layered part (5 vs. 6, 7 vs. 8, etc.).

Cover type. The traditional cover, a polyimide film with an adhesive, has been around since the beginning of the modern flex industry. An excellent choice for strength and durability, but as feature size has shrunk with tighter-pitched components, its limits begin to cause trouble. Small feature openings give rise for the need to punch or laser cover openings. Consider the “new” photoimageable covers (liquid polyimide, photo solder resist, etc.). These materials are significantly less expensive on a per panel basis than a traditional cover, where tight cover openings are required. Greater accuracy and finer opening features are also possible. Many designers more accustomed to rigid think this is an “all or nothing” option. Not true. “Hybrid” combinations of traditional polyimide and liquid photoimageable (LPI) are commonplace today. It is not at all uncommon to use an LPI or liquid polyimide in an area supported by a stiffener, while covering the flex “arms” with a traditional coverlay. The advantage here is higher yields, equaling lower costs.

The “poor man’s rigid-flex.” A recent design requested a six-layer rigid-flex. SMT components were on one side only, but there was a concern for overall thickness and CTE issues. Figure 1 shows the original design. The designer was clearly used to doing things in the rigid world. By taking advantage of double-sided adhesiveless polyimide cores, we significantly reduced the overall CTE concerns by reducing the amount of adhesive in the stackup. Eliminating the need to drill through a thicker stackup by using a traditional stiffener greatly reduced the PTH depth, further reducing CTE concerns (Figure 2). The result? A board at roughly 70% of the original cost for a similar rigid-flex.



Failure of a device is not just limited to its physical performance as intended. A device that cannot be sold due to cost is a failure in its own right. A project needs to be both sound in design and economy. Watch the pennies. A product that prices itself out of the market is a bridge to nowhere.

Mark Verbrugge is a field applications engineer at PICA Manufacturing Solutions (picasales.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it . He and co-“Flexpert” Mark Finstad ( This e-mail address is being protected from spambots. You need JavaScript enabled to view it ) welcome your questions.

Last Updated on Wednesday, 04 January 2012 00:05
 

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