Flexible circuits built with High Density Interconnect (HDI) technology offer significant design, layout, and constructions benefits over regular flexible circuits. HDI technology involves incorporation of microvias and fine features for achieving highly dense flex circuitry, and offers increased functionality with smaller form factors. Use of HDI technology offers improved electrical performance, allows use of advanced integrated circuit packages, along with better reliability using thinner materials and microvias. Some advantages of HDI flex circuits are:
Working in Harsh Environments
Fabricators cover HDI flex circuits with Polyimide. Although this is a standard practice, other cover and base materials are also available to suit a broad range of harsh ambient conditions. Compared to regular circuits covered with soldermask, the Polyimide dielectric layer is flexible, and protects the circuit far beyond the capabilities of the brittle soldermask.
Repeatable Installation with Flexibility
Compared to ribbon cables or discrete wiring, an HDI flex circuit offers a repeatable routing path, which you can customize within your assembly. Not only does this give dependability where necessary, but also the longer lifespan of the HDI flex circuitry drastically reduces service calls.
Capability to Withstand High Vibration
Along with flexibility, the ductility and low mass of HDI circuits allows it to withstand high amounts of vibration much better than conventional circuits can, reducing the impact upon itself and its solder joints. The higher mass of regular circuits imposes additional stress upon itself, the components soldered on it, and its solder joints.
Working with Longer Duty Cycles
The design of HDI flex circuits allows them to be very thin, but adequately robust to withstand a high number of flexing cycles. In fact, HDI flex circuits are capable of flexing thousands to millions of cycles while carrying power and signal without a break.
Packaging Options with HDI Flex Circuits
Designers can shape HDI flex circuits to fit where no other circuit can. As HDI circuits are a hybrid combination of an ordinary flex circuit and a bunch of wires, they exhibit the benefits of each and more. In reality, you get unrestricted freedom of packaging ability with HDI flex circuits, all the time retaining the repeatability and precision necessary. HDI flex circuits replace a few major components in equipment—the hard board, usually called the printed circuit board (PCB), and the connectors and wiring harness that bridge multiple PCBs. This offers several packaging options such as:
- Lower Mass
- Versatile Shaping
- Stiffeners for Component Mounting
- Vibration Resistance
- Robust Connections
- Repeatable Wire Routing
- Faster Assembly Times
- Reduction in Weight and Space
As the HDI flex circuit is made of thin material, it can often save up to 75% of the weight and space required by conventional circuit boards and wires. Designers feel compelled to adopt HDI flex circuit technology because they can form it into three-dimensional configurations. However, the flexibility often makes it difficult to mount large surface mount components on HDI circuits and engineers surmount the problem by selectively bonding stiffeners where required.
Some equipment have multiple boards interconnected with wire harnesses. Shock and vibration plays a large part in failure of these harnesses resulting in recurring costs. In most cases, a single HDI flex circuit can replace all the boards including their wire harnesses. As the HDI flex circuit is lighter, it is more resistant to the effects of shock and vibration, resulting in huge reductions to the recurring costs, Elimination of wire harnesses leads to lower routing errors, ultimately reducing test times, rework, and rejections.
Moreover, HDI flex circuits also replace the connectors at each end of the wire harness. Flat foil connectors may have to replace some connectors. This is an advantage over the use of round wires, as flat conductors with their larger surface area dissipate heat better, and thereby, carry more current. Conductor patterns in HDI flex circuits have more uniform characteristics, leading to a better prediction and control over impedance, crosstalk, and noise.
Use of HDI flex circuits reduces several assembly processes such as color-coding and wrapping bundles of wire. In volume production, this not only reduces the chances of assembly rejects and in-service failures, it saves assembly time, and lowers the total installation costs.
Benefits to the Designers
Designers build up HDI flex circuits with microvias as this offers them several advantages. Drilled by lasers, microvias are extremely small, and their effective use opens up more space for routing. Combined with the use of thinner traces, this leads to high routing densities, effectively resulting in fewer layers.
HDI flex circuits present the only practical way for designers to mount multiple large BGA packages with less than 0.8 mm pitch. They also offer the lowest cost for high-density boards with high control over power and signal integrity with appropriate stackup definitions.
Processes requiring Restriction of Hazardous Substances (RoHS) do well to use HDI flex circuits, as newer materials are available that offer higher performance with lower costs. This is an advantage over conventional boards, as these newer materials are not suitable for sequential or standard laminations.
HDI flex circuits are the best alternatives to expensive, high layer count sequential or standard laminated boards. Smaller HDI features are the only way to effectively breakout and route multiple instances of high pin-count and finer pin-pitch component devices on a single board. With all the above features and advantages, handheld consumer electronics is currently committed to using HDI flex circuits.