With today’s ever smaller packaging requirements, electrical, mechanical and industrial designers are continuously searching for any solution that allows them to squeeze that extra feature into their product, makes that product slightly smaller ahead of the competition, or provides flexibility to create a new and interesting shape for the product. Typically, advances in technologies enable early adopters to reduce packaging benefits, and increase the overall cost of the product due to recovery of the R&D expenditures necessary in creation of the technology. Occasionally a solution is found using existing technologies in new and creative ways to solve problems--flexible FR-4 PCBs are one such solution.

Three Dimensional Stacking

Advertisement

As product designers run out of PCB real estate in the X and Y axis, they look to the Z axis and stacking technologies to solve their problems. Current solutions include board-to-board connectors, polyimide-based flexible circuits, polyimide-based rigid-flex circuits, and cabled interconnects. Board-to-board connectors are the most common method of three dimensional (3D) stacking due to pricing and a myriad choice of solutions based on size, density, industry standard and signal integrity (SI) issues. While the board-to-board interconnect choices are seemingly endless, they compound the initial problem they attempt to solve by consuming valuable PCB real estate. Additionally, the daughter card assembly is also required to contain the mating half of the connector set, using valuable PCB real estate on that card as well.

Polyimide Flex Circuits

Polyimide based flexible (flex) circuits offer a connectorless interconnect solution. Flex circuit 3D solutions are endless, and offer the most creative freedom to packaging designers. Flex circuit technology enjoys strong popularity in certain markets and “according to the latest report from printed circuit Market Intelligence company BPA Consulting, the 2003 worldwide market for flexible circuits was $4.4 billion (less than the 2000 peak of $4.6 billion).” The technology allows for static and dynamic applications allowing one-time folds and those with hundreds of thousands of cycles. Flex technology allows multiple boards to be combined into one PCB yielding a singular part number, bill of materials (BOM), stencil, assembly operation and testing solution prior to folding into the final configuration. All of this comes with a price to your budget. Due to limited suppliers, high material cost, elaborate tooling, long lead times and lower yields, per piece pricing is typically three to four times that of FR-4 based rigid options.

Rigid Flex Circuits

Rigid-flex circuits combine the best from both worlds providing a rigid portion for component and mechanical mounting, and a flexible portion for transitioning signals from one plane to another in either a static or dynamic application. Unfortunately, they suffer from the same drawbacks as flexible circuits--limited suppliers, high material cost, elaborate tooling, long lead times and lower yields--but the addition of material mismatches between FR-4 and polyimide further increases the difficulty in successfully building these circuits. This is reflected in a similar three to four times price modifier vs. rigid PCBs. While there are other flexible materials with similar properties to FR-4, these are further limited in suppliers and more costly than polyimide based products.

Cabled interconnects are the lowest technology solution for 3D problems. They offer maximum design flexibility, are simple to build, offer unlimited length and quick-turn capability but are manually intensive, difficult to rework and offer limited SI solutions.

Flexible FR4

Those familiar with rigid PCB manufacturing know that raw thin core materials are very flexible right out of the package. This “feature” has been exploited by a small handful of companies for almost two decades. The problem has been in developing a consistent and repeatable manufacturing process that could be exploited to take advantage of the low material cost, the well known processing technology and then tested to industry accepted standards.

Typical FR-4 PCB construction uses fully cured (C stage) laminate cores with Copper (Cu) foil bonded to the outside interleaved with layers of partially cured or tack cured (B stage) pre-preg. When placed under the appropriate temperature and pressure in a lamination press, the pre-preg flows, cures and bonds the layers to each other forming the “rigid” PCB that we all know and love. For the purposes of simplicity, clarity and understanding of the technology, creative license has been used on stack up details and we will build the finished product from the center outwards.

Figure A.

We begin the process of building a four-layer flexible FR-4 PCB with standard off-the-shelf FR-4 materials (see Figure A).

Figure B.

Remembering that the center core is very flexible fully cured C stage, we print and etch transitional traces from the left side PCB to the right side PCB (Figure B). Typically a solid sheet of pre-preg is then laid into the stack to bond the next core or foil.

Figure C.

In our case, the processed core is moved ahead in the manufacturing flow to solder mask where we apply a custom blend LPI flexible solder mask across the transition region, extending slightly into the future rigid portion of our stack-up (Figure C).

Figure D.

After the solder mask has been imaged and cured, it is returned for lay up. While the core has been in LPI, the pre-preg material was sent to the router to open a window over the now-soldermasked transitional traces, removing the bonding material from this region. This “windowed” sheet of pre-preg is purposely routed short of the edge of the final rigid portion of the PCB allowing an overlap onto the flexible mask and extending slightly into the flexible FR-4 region.

Figure E.

On a six-or-more layer PCB that has only one flexible core, all non-transitional cores are also routed with a similar window (Figure D). The pre-preg is then placed into the stack-up (Figure E).

Figure F.

Last into the four-layer stack-up is the Cu foil which is placed on the outer layers and the stack-up is ready for lamination. We are now left with an air pocket, from the previously routed pre-preg, separating the flexible mask from the foil outer layer (Figure F).

Figure G.

After lamination, the left and right hand images are fully cured rigid PCBs. The transition section with no pre-preg and its supplier cured C stage core remains as flexible as it was just out of the package (Figure G). The laminated panel is returned for print and etch of the outer layers as any typical four-layer PCB.

Figure H.

After outer layer print, the copper foil covering the transition region dissolves into the etch solution revealing the flexible FR-4 region below (Figure H). The panel is then finished as normal with solder mask and silkscreen processes on the rigid portions. Similar to flex and rigid flex PCBs, flexible FR-4 PCBs must be held in position by rigid rails or sub-panels for assembly.

Figure I.

By modifying two events in the typical rigid PCB manufacturing process, the addition of flexible solder mask to target locations and the routing of pre-preg prior to lamination, we are able to retain the initial flexibility of an off-the-shelf FR-4 core. This process is easily repeatable for multi-core applications on 6- and 8-layer PCBs (Figure I). We have delivered 20-layer backplanes with 9 cores (18 layers) of flexible FR-4 interconnect.

Testing to IPC-TM-650

Figure J.

Fabrication is the first piece of the puzzle, but for market acceptance the final product needs to be thoroughly tested to accepted industry standards. As flexible FR-4 is designed as a replacement for polyimide and acrylic flexible circuits, testing to IPC-TM-650 is required (Figure J). This testing requires that the flexible substrate be repeatedly folded over a mandrel, a metal rod of specified diameter, until failure. The published data verifies a high level of reliability for product applications with 77-634 lifetime flexures, greatly exceeding the requirements of most static applications.

The flexible FR-4 technology was also subjected to accelerated lifecycle testing at an independent customer laboratory for electrical failure during temperature cycling. A side-by-side comparison was made using the customer’s existing polyimide based rigid-flex solution and the flexible FR-4 solution. The test samples provided were actual customer product. The PCB was subjected to 25 cycles from -25^oC to +145^oC where the result was “no electrical opens or shorts….” The conclusion of the laboratory’s test engineer was “there was no major electrical/mechanical difference in the (flexible FR-4) board versus the polyimide board.”

Summary

Flexible FR-4 is a valid replacement technology for applications with low- to medium-flexure requirements and where per piece cost savings is critical. Currently, greater than 20% of new orders are from product designers that had not even considered polyimide solutions due to cost and delivery, verifying the technology as a replacement for rigid PCB connectorized solutions.

By using existing rigid PCB manufacturing processes and technologies with a couple of twists, we are able to provide a solution that saves customers 30-70% of their current per piece cost vs. polyimide-based flex and rigid-flex solutions, while providing a quality product on rigid PCB manufacturing schedules and without the exorbitant tooling charges. To date, over 1,000 orders have been completed using this high volume rigid flex (HVRFlex™) flexible FR-4 technology.

Author’s Note

Author’s Note: The described process was patented by Coesen Inc. in 1996, trademarked under HVRFlex™ and is currently licensed to Titan PCB with manufacturing locations in Amesbury, MA and Fremont, CA.