Multi-board Etching: Managing Rigid-Flex Designs and Conductivity

October 15, 2019 Cadence PCB Solutions

Robot in a traditional bicep flexing pose

 

When I was a very young boy, my dad would delight and tease me by flexing his muscles. But, he did this with a very special flourish. Rather than simply flexing, he would put his thumb in his mouth, blow, and flex. All this gave the appearance that huffing and puffing on his thumb made his bicep pop out. This was a great show for a little boy.

In PCB design, we work with a different type of flex. Advances in consumer and industrial electronic devices have allowed manufacturers to produce smaller devices that have more functionality.

Those devices use rigid-flex designs that provide the form factors needed for portability and—many times--include multiple interconnected boards. With all the interconnections between the boards, PCB designers pay equal attention to signal paths and to the electrical connectivity between boards. On the mechanical design side of things, each board must fit within an enclosure that meets the product specifications. As a result, every multi-board design combines electronic design automation (EDA) with mechanical engineering design (MCAD).

The Sum is Greater Than the Whole

Good troubleshooting techniques involve considering a system as individual parts rather than as the whole. The same techniques apply to your work with multi-board PCB designs. Each board consists of a single unit that has its own lifecycle. 

Some product designs may use a single PCB design for multiple functions or for multiple devices. Others may interconnect multiple PCB designs to produce a complete, fully functional system. Depending on the product design, a single PCB design may have multiple uses. No matter the method, the multi-design represents an overall approach to system design.

Multi-board designs feature rigid-flex PCB technologies. Combining flexible circuits and rigid circuits extends the benefits of the individual technologies. While rigid PCBs carry most of the components, flexible PCBs serve as interconnecting pieces between the rigid boards. In addition, the combination of rigid and flex reduces complexity and cost, saves weight, and improves reliability.

Let’s Be Flexible Here

While some applications use static flexible circuits that have minimal movement, other products use dynamic flexible circuits that move frequently. The two types of flexible circuits use different types of materials and require different methods for construction. For example, dynamic flexible circuits require a much longer flexing lifespan and require elongated grains in the copper. 

As we consider the construction of flexible circuits, we begin with a stack-up of flexible substrate material and copper. A combination of adhesives, heat, and pressure laminates the materials together.

The substrate consists of a polyimide—or a strong, flexibly thermosetting polymer. PCB manufacturers use different types of polyimides based on the application requirements. Given the need for flexibility, the circuits rely on rolled and annealed copper along with acrylic adhesives that combine softness with higher coefficients of thermal expansion. 

 

Thermal conductive wiring under a floor

While thermal conductive layering under floors may differ in size to your boards, considerations are similar

 

The different types of flex circuits include the Type 1 single layer with one conductive layer sandwiched between two layers of polyimide, the Type 2 double layer that has two conductive layers, and the Type 3 multilayer flexible PCB that uses three or more conductive layers.

The rigid part of a rigid-flex circuit uses standard PCB design and manufacturing processes. 

Along with prepreg, the board typically consists of an FR-4 substrate, conductive copper layers, the solder mask, and identifying information. Type 4 multilayer rigid-flex boards have three or more conductive layers. Because most designs feature rigid sections that have a different set of layers than the flexible sections, multilayer rigid-flex designs do not have a consistent set of layers across the entire design. The circuit consists of the inner layers of multiple circuits attached together with adhesive.

Etched in Memory Forever

A  multi-board circuit combines the attributes of rigid and flexible circuits. Given these attributes, the circuit offers the advantage of allowing circuits and components to fit into limited spaces. In addition, the use of rigid- flex technologies reduces plug and connector components. Because  multi-board circuits have applications across a wide range of industries and applications, the circuits often feature dense, fine line trace width and spacing. The circuits also have complex layout options that meet the design requirements of specialized applications.

Satisfying those requirements requires routing software that provides contour routing for curved, flexible circuits. Contour routing eliminates any imprecise spacing and allows design teams to specify uniform spacing for traces.

The unique attributes of multi-board circuits also require adjustments in traditional manufacturing processes. While rigid-flex PCBs follow most of the traditional PCB design processes in terms of the use of design documentation, the printing of outer and inner layers, the substrate, and the removal of copper, the manufacturing processes can become more complex. For example, etching the conductive layer of a flexible circuit can cause the polyimide core to shrink and may force manufacturers to use materials that compensate for the shrinkage.

 

Multilayer printed circuit board with plenty of vias on its surface

Packaging your PCB properly will ensure accurate multi-board production

 

Traditional etching practices also offer challenges for the production of the fine lines and line width tolerances seen with multi-board designs. The need for precision requires greater attention to etch rates, the reduction of pools, and higher etching efficiency. With the growth in multi-board applications and the use of rigid-flex technologies, manufacturers have incorporated features into standard mechanical PCB operations that accommodate etching of traces that have spacing as low as 4 mils. As processes and technologies emerge, PCB manufacturers have begun using other technologies such as laser machines for small flexible circuit etching.

Laser technologies offer the precise positional accuracy and depth-of-cut calibration needed for etching complex designs and or working with unique flexible polyimide materials. A 20µm diameter focused laser beam easily handles smaller traces and spacing requirements. Laser systems use hatch and controlled delamination processes for removing copper from double-sided designs. While laser etching systems place less stress on flexible boards, the adhesive layer of a flexible circuit absorbs the laser energy.

With Cadence’s suite of design tools and analysis functions, you’ll be able to account for any conductivity, enclosure, or circuit needs in a multi-board system. Allegro PCB Designer, particularly, will work with smart DRCs and impeccable analytical tools alongside its efficient production tools to get your design out the door with less headache than ever. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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