Designing with Ultra-Thin Flexible Printed Circuit Boards

Estimated reading time: 6 minutes

When reviewing the entire installation for the total cost of ownership, use of a rigid-flex circuit offers the maximum benefits, as it replaces the maximum number of components. Not only can surface mount components be mounted on both sides of the board, rigid-flex circuits allow the integration of the best capabilities of the resilient flex areas with the resistant rigid areas, thereby offering the highest capability and maximum vibration resistance. As it offers the smoothest transition between rigid and flex areas while preserving the benefits of each, this combination is the best option when mounting a component with a high mass.

One of the leading PCB manufacturers has more than 30 years of experience in designing, manufacturing, and assembling high-quality flex and rigid-flex printed circuit boards. Their team provides world-class expertise in manufacturing different types of flex circuits across a broad base of technologies. Their products range from the simplest, single-sided flex circuits with plated through holes, to multilayer rigid-flex types, using dual access and floating lead, buried and blind vias, with sequential build, and with controlled and differential impedance.

HDI Stackup

One of the most important aspects of initial design of a multilayer PCB is defining its appropriate stackup. This is essential for large dense PCBs with multiple pin-count BGAs, especially when standard laminate stackups are inadequate in terms of cost and performance goals. HDI stackups are a viable alternative to large number of layers providing lower cost with higher performance if properly designed.

For boards with high pin-count BGAs, there can be three types of stackups—standard lamination with through vias, sequential lamination with blind and buried vias, and buildups with micro-vias. Of these, HDI boards primarily use a buildup with micro-vias, as this technology offers several advantages:

• The vias and traces are of smaller feature sizes, leading to higher routing density and fewer layers
• Microvia patterns can be more effectively used, opening up more channels for routing, leading to potentially fewer layers
• This is the only practical way for designing with multiple large BGAs with pitch less than 0.8 mm
• Offers the lowest cost for high density boards
• Appropriate stackup definition leads to improved signal and power integrity
• Appropriate materials for processes requiring RoHS
• Newer materials are available for higher performance at lower costs, but these new materials may not be suitable for other types of laminations.

Eminent PCB manufacturers have defined HDI PCB stackups with 16 layers, where the overall board thickness is only 66±7 mils. This requires sequential build-up (SUB) and has laser-drilled micro-vias.

The Impact of Cost

Although flexible circuits are more expensive than rigid PCBs are, the costs generally rise with the layer count. Therefore, judicious options may have to be considered for minimizing the cost. For instance, two double-layer circuits could turn out to be less expensive compared to using one four-layer circuit. Other factors may lower the overall cost in favor of flex circuits. For instance, the ability to fold a flexible circuit could save space and layers. Depending on the situation, time invested in project assessment could result in significant overall savings.

Akber Roy is the CEO of RUSH PCB Inc.

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