Technical Briefing: Advanced PCB Solutions for Demanding Environments　by OKI EMS

1. Introduction

In environments like aerospace, where space, weight, and thermal loads are unforgiving, conventional hardware solutions are often inadequate. This briefing analyzes two advanced board-level technologies—Flexible Printed Circuits (FPCs) and Copper Coin Printed Wiring Boards (PWBs)—engineered to provide robust performance, three-dimensional wiring, and superior thermal management under these extreme operational constraints.

2. Flexible Printed Circuit Boards (FPCs) for High-Reliability Applications

2.1. Overview and Core Characteristics

A Flexible Printed Circuit Board (FPC) is a circuit board made by applying a copper wire pattern to a thin, flexible insulating material. Unlike traditional rigid boards, FPCs offer distinct advantages in form factor and application.

The three primary characteristics of FPCs are:

• Flexibility and 3D Wiring: FPCs can be bent and folded, which allows for free arrangement within confined areas and enables complex three-dimensional wiring configurations.
• Thin and Lightweight: FPCs are extremely thin, typically 0.1 to 0.2 mm, and lightweight, weighing approximately one-fifth as much as traditional cables. This contributes directly to the miniaturization and weight reduction of electronic devices.
• Dynamic Movement: FPCs can be formed into specific shapes, such as a bellows, to accommodate dynamic movement, including repeated bending and extension. For applications involving simple, repeated flexion, they can be engineered to withstand over 100 million cycles.

2.2. Suitability for Harsh Environments

FPCs are highly suitable for use in harsh environments, such as space and aviation, where high reliability is a critical requirement. Their material composition is key to this durability. FPCs use a thin insulating material like polyimide, which provides excellent heat resistance (above 120°C), radiation resistance, and superior electrical insulation properties, making them robust enough for the rigors of space.

2.3. Case Study: The Ikaros Solar Sail

The history of FPCs dates back to the late 1960s, following the mass production of polyimide film, which led to their adoption by NASA for aerospace applications. The same fundamental material properties that made FPCs valuable for early space missions—namely polyimide's resilience—are precisely what enabled the success of ambitious modern missions decades later, demonstrating the technology's enduring relevance.

A significant application of this technology was the use of OKI's FPCs in the JAXA "Ikaros" small solar power sail demonstrator, launched in 2010. The FPC was chosen for this mission after being evaluated for its key attributes: being lightweight, heat-resistant, and radiation-resistant. In the Ikaros craft, a continuous 14-meter FPC was used as the wiring for thin-film solar cells and various sensors attached to the sail's surface. The mission was a success, marking the world's first successful demonstration of a solar power sail.

3. Copper Coin Printed Wiring Boards (PWBs) for Enhanced Thermal Management

3.1. Functional Principle

A Copper Coin Printed Wiring Board (PWB) is a product where a solid copper coin is embedded directly into the board to conduct heat away from electronic components. The primary function of the coin is to transfer thermal energy from a component on one side of the PWB to the backside, where it can be dissipated through direct contact with a chassis or heat sink. This method is particularly advantageous for components where heat cannot be dissipated from the top surface.

3.2. Innovation: The Convex Copper Coin

To improve thermal performance, a convex (凸型, totsugata) copper coin was developed. The purpose of this design is to increase the surface area on the heat dissipation side of the board, thereby improving the efficiency of heat transfer away from the component and into the chassis.

3.3. Manufacturing Challenge and Solution

The Challenge: The introduction of the convex coin presented a manufacturing challenge. The traditional method—using pressure to deform a cylindrical coin into a through-hole—is unsuitable for convex coins. The varying diameters of the convex shape result in uneven deformation under pressure. This non-uniform deformation prevents the formation of a secure mechanical interlock, compromising the coin's stability and thermal interface within the board.

The Developed Method: A new manufacturing process was developed to reliably secure the convex coins. This method involves the following steps:

1. Create two separate multi-layer board sections (e.g., layers L1-L4 and L5-L8).
2. Drill holes in each board section that correspond to the different diameters of the convex coin.
3. Stack the board sections with a layer of prepreg material placed between them.
4. Insert the convex copper coin into the aligned holes.
5. Laminate the entire assembly. During this process, the prepreg material's resin melts and flows into the gaps, encapsulating the coin. As the thermosetting resin cures during lamination, it forms a robust, void-free bond that mechanically locks the coin in place and ensures a stable thermal pathway.

This new method has been validated through reliability evaluations, including heat cycle tests, confirming sufficient durability for demanding applications. Its true ingenuity lies in its use of existing, space-qualified materials and processes—namely multi-layer lamination and prepreg resin—to solve a novel mechanical problem. This approach minimizes qualification risk by leveraging proven manufacturing infrastructure.