A majority of the printed wiring board laminates bear the popular names of their constituent materials, such as Epoxy, Polyimide, PTFE, and the like, which are more often the generalization of the chemical names of the principal resin systems. Recently, products have been evolving suitable for high-performance applications. These new materials and their various combinations make the past generalizations more difficult to sustain. For instance, Epoxy and FR-4 are broad categories that accommodate characteristics such as Low Flow, Lead-Free, Multifunctional, CAF resistant, Green, and High-Speed Digital epoxy products. The slash sheets of the industry’s current laminate and prepreg specifications, IPC-4101, also reflects this proliferation.
Conventional adhesive based laminates are still popular in the flexible circuit industry, and they have demonstrated their performance in industries demanding high-reliability such as medical, military, automotive, and aerospace. Of late, adhesiveless laminates are finding their way into several applications as these laminates have superior properties. This is because the adhesive layer is often the weakest link in the set of materials, which fails in harsh environments such as in the presence of high temperatures and harsh chemicals.
Adhesive Based Laminates for Rigid-Flex Circuits
These usually come in commercial grade and military or defense-aerospace grade. The commercial grade is commonly known as FR-1, which is a sandwich of copper, adhesive, polyimide, adhesive, and copper, making it the least expensive and easily processed material with UL approval. However, the significant amount of FR adhesive lowers its reliability.
The Low-Flow type of adhesive, LF 1.5, used for the military grade, gives the laminate a better bond strength compared to that of the FR types. However, the significant amount of LF acrylic still does not improve the reliability, and this material is considered old school in military design.
Adhesiveless Laminates for Rigid-Flex Circuits
For improving the reliability, manufacturers use the AP 2.0 type of laminate, which has copper chemically or electrochemically deposited and bonded on both sides of a Polyimide base, thereby removing any requirement of adhesive. AP 2.0 types of laminates offer easy processing, repeatable manufacturability, and good stability.
For high-speed applications where a good control over impedance is important, manufacturers prefer the TK 4.0 laminates. These are formed as a sandwich of Teflon on both sides of a Polyimide base, with copper chemically or electrochemically deposited and bonded onto the outer side of the Teflon layers. The presence of Teflon requires special equipment for processing TK 4.0, and requires significant amounts of expertise. Working with TK 4.0 also presents significant dimensional stability challenges.
To achieve high reliability as well as high speeds, manufacturers prefer to use LCP 6.0 laminates as prepreg material. This laminate typically uses Liquid Crystalline Polymer (LCP) as the base, with copper deposited and bonded on both sides, either chemically or electrochemically. However, the material is difficult to process, requiring special tools and significant amounts of specialized knowledge.
The rigid-flex circuit industry uses several other newer types of laminates as well. One of them is the adhesiveless polyimide blend JT, which behaves more like Low-Flow, but can tolerate higher operating temperatures.
Another is an adhesiveless laminate specifically for high temperatures. This is a modified polyimide blend without acrylic and behaves more like AP. Meant for high reliability applications, this material has the maximum operating temperature.
For more information on the various laminates used in rigid flex printed circuits boards, or to find out if a certain type of laminate is better suited to your product capabilities and needs, please don’t hesitate to contact the team at email@example.com