Everything You Should Know about Flex-Rigid PCB Manufacturing Technology

Due to development trend of aerospace products including light weight, miniaturization, multifunction and assembly densification, higher requirement has been set up for Printed Circuit Board (PCB) technology and manufacturing process. Flexible PCBs are a type of circuit boards that are made of flexible substrate material and feature more advantages than ordinarily rigid PCBs:

• Lower thickness

• Lighter weight

• Dynamically bendable

• Accessible for 3D interconnection assembly

• Higher freedom on electronic design and mechanical design

• More space saving

Furthermore, electromagnetic signals can run quickly and smoothly in flex PCBs owing to excellent electrical performance and thermal performance of flexible substrate material so that flexible PCBs are widely applied in many industries like instrument, automotives, medical care, military and aerospace.

Based on the further development and optimization of flexible PCBs, the next step for them should be Flex-Rigid PCBs, Embedded Flexible Circuits and HDI Flex PCBs among which Flex-Rigid PCBs attract the most attention and applications. Therefore, this article will discuss properties and application fields of Flex-rigid PCBs based on their material and manufacturing technologies.

Material of Flex-Rigid PCBs

The performance of flex-rigid PCBs depends on that of substrate material of them that primarily contains flexible dielectric film and flexible adhesive film. As a leading type of flexible substrate material, flexible dielectric film mainly includes polyester (Mylar) that is usually used in low-end products, polyimide (Kapton) that is the commonest type, and fluoropolymer (PTFE) that is usually used in military and aerospace products.

As those three types of flexible materials are compared, polyimide features the highest dielectric constant with excellent electrical and mechanical properties and high-temperature resistance but is expensive and easy to absorb moisture. Similar with polyimide in terms of performance, polyester, however, features bad high-temperature resistance. Polytetrafluoroethylene is primarily used in high-frequency products with low dielectric constant. The following table demonstrates performance comparison between the above three types of flexible dielectric film.

Performance Item	Polyester	Polyimide	Polytetrafluoroethylene
Limit Tension/N^*mm^-2	172	172	20.7
Limit Elongation/%	120	70	300
Size Change After Etching/mm^*m^-1	5.0	2.5	5.0
Relative Dielectric Constant/10³Hz	3.2	3.5	22.1
Loss Angle Tangent/10³Hz	0.005	0.0025	0.0001
Volume Resistance/MΩ^*cm^-1	1012	1012	1012
Flammability	Flammable	Self-extinguishing	Inflammable
Dielectric Strength/mV^*m	300	275	17
Moisture Absorption/%	<0.8	2.7	0.01
Heat Resistance/°C	150	400	260
Soldering Test	Pass	Pass	Pass

The leading material contributing to flexible adhesive film contains acrylic acid, epoxy and polyester. Acrylic acid and polyesterimide feature excellent adhesion, high flexibility and relatively high chemical resistance and heat resistance. However, it features a relatively large coefficient of thermal expansion so its internal thickness should not be over 0.05mm. Epoxy resin features bad adhesion and is mainly applied to stick covering layer and internal layer. Moreover, it features such a low coefficient of thermal expansion that it's beneficial to thermal shock resistance improvement of plated through holes.

Manufacturing Technology of Flex-Rigid PCBs

Manufacturing technologies of flex-rigid PCBs differ from each other in accordance with different types of PCB boards and the essential technology leading to the differences is fine circuit manufacturing technology and microvia manufacturing technology. As electronic products tend to develop towards light weight and miniaturization, multifunction and assembly densification, advanced PCB boards attracting the most attention include HDI flex-rigid PCBs and embedded flex-rigid PCBs.

• Flex-Rigid PCB Manufacturing Technology

Flex-rigid PCBs are fabricated through orderly and selective layer stackup of rigid and flexible PCBs with plated through holes responsible for connection between layers. Below figure indicates the basic structure of flex-rigid PCB.

Advent of flex-rigid PCBs can effectively reduce electronic product's volume and quality by replacing harness and connectors that were usually applied in electronic products. Furthermore, flex-rigid PCBs are capable of solving contact and intense heat problems caused by harness and connectors, drastically improving reliability of devices.

As early as 1970s, flex-rigid PCBs were manufactured by stacking up rigid board onto flexible board. Consistent progress and optimization have witnessed numerous new types of flex-rigid PCB manufacturing technologies. Up to now, the most mature and practical flex-rigid PCB manufacturing technology is that glass fiber epoxy resin (FR4) is applied as external rigid board and solder mask is applied to protect rigid circuit pattern. When it comes to flexible substrate material, polyimide (PI) double-layer board covered with copper is applied as a flexible core and polyimide/acrylic film is used to protect flexible circuit pattern. Adhesion depends on Low-flow prepreg. All those elements are laminated so that flex-rigid PCBs are manufactured. The figure below demonstrates the manufacturing process of a 6-layer flex-rigid PCB.