Liquid crystal polymers
03/01/2002
A flex circuit substrate option
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BY RUI YANG
A new process for a liquid crystal polymer (LCP) material provides an alternative to traditional polyimide film for use as a substrate in flexible circuit construction. This development offers improvements in manufacturing and processing of LCP flexible circuits, which will eliminate some inherent limitations of polyimide circuits. It also offers new advantages to enhanced electronic device design with good electrical performance and processing capabilities. For example, a lower dielectric constant allows faster electric signal transfer, and lower moisture absorption leads to higher frequency signal and data processing.
Flex Circuit Options
Polyimide films are common substrates used to make flexible circuits that fulfill the requirements for complex electronic assemblies. Polyimide film has many excellent properties (such as thermal stability and mechanical strength), but other properties limit the speed or frequency at which electronic components may operate. Polyimide's tendency to absorb moisture interferes with high-frequency device performance.
LCP is a thermally stable thermoplastic, with an upper use temperature of more than 250°C and good inherent flame retardant properties. LCP films have a lower dielectric constant and loss factor over the functional frequency range of 1 kHz to 45 GHz with negligible moisture effects compared to polyimide films. Circuits built using LCP as the base substrate can have metal trace signal lines placed closer together without crosstalk, resulting in more densely packed circuits.
LCP circuits have been made using polymer/foil laminates and subtractive processing methods for some time, but they have not found widespread use in electronic assemblies because of limited circuit design and a lack of functional features. The key technical challenges for LCP flexible circuit fabrication have been metallizing the LCP film and patterning of the LCP substrate.
 Figure 1. Comparison of dynamic mechanical modulus of polyimide and LCP films. |
This article outlines some of the characteristics of LCP film that make it an interesting alternative to polyimide for some flexible circuit applications. LCP flexible circuits made using a sputtered substrate with chemically etched features are presented.
LCP Varieties and Properties
A base LCP film is made by extrusion of thermoplastic LCP resins. Typically, the thermoplastic resins can be classified into three types (I, II, and III) according to their heat resistance. Type III LCP can be hydrolyzed when exposed to water at elevated temperature because of the presence of aliphatic groups in its backbone. Many circuit processes and packaging assemblies undergo wet processes at high temperature. Therefore, this type of LCP film is not suitable as a flexible circuit substrate.
On the other hand, Types I and II LCP resins are aromatic polyesters with rigid-rod molecular structures. They have a higher heat resistance than the Type III resin. Type I LCP has the highest heat resistance of the three, with a melting temperature range of 300-350°C. The melting temperature range for the Type II resin is 200-250°C. It is possible to enhance the melting temperatures of these LCP films by applying a special heat treatment to change the LCP's crystalline nature and by imparting a high degree of molecular orientation. LCP films from Type I and II are suitable for electronic packaging applications.
 Table 1. Typical property values of 50-micron film in machine direction. |
LCP films offer a unique combination of properties, including chemical resistance, barrier properties, electrical insulation, temperature resistance, mechanical strength and stiffness. These beneficial properties have been known for some time, but poor film quality caused by processing difficulties has prevented the use of these materials in flexible circuits. For example, the films showed anisotropic properties with large differences in the machine and transverse directions. The new LCP films developed recently have made significant improvement in film quality and isotropic properties.
Typical Type I LCP film properties in the machine direction are shown in Table 1. Data for typical polyimide films are presented for comparison. The information in this table shows some of the important similarities and differences between the polyimide and LCP materials. For example, dynamic mechanical analysis (DMA) was used to measure the glass transition temperature and dynamic mechanical modulus behaviors. The modulus-temperature plots of the two types of LCPs along the typical polyimide film are shown in Figure 1. The data shows that both polyimide and LCP Type I maintain their mechanical strength beyond 300°C, while Type II begins losing some mechanical strength at lower temperatures.
A high moisture content in a substrate film can result in significant damage to a flexible circuit at high temperature. The evaporation of the water can cause blistering, delamination and metal corrosion. LCP films have very low moisture absorption and water vapor transmission rates, which are over an order of magnitude lower than polyimide film, as shown in Figure 2. The low moisture content in LCP films will eliminate many moisture-induced problems encountered when using polyimide film as the circuit substrate.
 Figure 2. Moisture absorption and water vapor transmission are shown for 50 µm thick films of polyimide (PI) and LCP. |
Flatness and dimensional stability are other concerns with polyimide circuits. A polyimide base film expands when it absorbs water, resulting in curling of the flexible circuit. An LCP film has low moisture absorption and a low coefficient of hydroscopic expansion (CHE). The test results show that LCP circuits are more stable and stay flatter when moisture levels change.
Circuit films can also curl as a result of thermal processing. This is because of stresses built up at interfaces as a result of a mismatch in the coefficient of thermal expansion (CTE) between polyimide and copper. Unlike polyimide, the CTE of LCP film is adjustable through thermal treatment processes. The in-plane CTEs (machine and transverse directions) can be balanced and matched to copper by applying the appropriate thermal treatment. Studies have also shown that LCP film undergoes a rapid ester interchange between chains at the interface at high temperature.1 The interchain transesterification relieves the interfacial stress. Therefore, LCP circuits show significantly diminished thermal expansion effects, resulting in a flatter circuit.
However, an LCP circuit can not be operated continuously at elevated temperatures beyond 300°C, although that is not essential for most applications.
Substrate Materials
LCP flexible circuits can be made using three types of substrates: adhesive laminates, adhesiveless laminates or metallized LCP film substrates. Adhesive laminates are constructed by laminating a dielectric film to a copper foil with an adhesive layer in between. They were developed to provide designers and fabricators with interconnects in the early history of electronics industry. However, as electronic packaging became more sophisticated, performance requirements have begun the elimination of the adhesive layer.
Adhesiveless substrates offer a variety of advantages, including thinness, light weight, flexibility, greater thermal stability, laser and plasma patternability, and use of very thin copper. LCP can be softened to a flowable state when heated and hardens when cooled. Taking advantage of the flow of the thermoplastic materials during melt, LCP can be directly laminated to copper foil in a continuous process without using an adhesive layer in between.
Good metal adhesion is a challenge for these LCP laminates. To improve adhesion, the copper surface must be roughened before lamination. Peel strength for this type of laminate structure can be significantly increased. However, when the surface is roughened, it often creates problems with circuit processing and product reliability. For example, metal remaining after copper etching is a critical issue. The remaining metal is a result of copper nodules being broken from the foil surface and buried in the LCP. Another disadvantage associated with the LCP laminate is that copper thickness is limited to the availability of the copper foil used to make the laminate. Thinner copper foils are expensive and are difficult to handle without a supporting film.
 Figure 3. A comparison of subtractive and additive process flows for the production of flexible circuits. |
In addition to the LCP laminates, a LCP film substrate can be made by a direct vacuum metallization process. The process includes deposition of a thin conductor layer onto LCP film, followed by electrolytic plating of copper to the desired thickness. Vacuum metallized polyimide films have been available for many years and are used widely in flexible circuitry applications. Some of the historically key aspects associated with the process are base film preparation, adhesion-promoting tie-layer coating and initial copper layer deposition. Direct metallization of LCP films has been more challenging than metallizing polyimide films. Some of the new issues are metal adhesion, dimensional stability and web handling at relatively high temperatures during the metallization process.
Circuit Process
LCP flexible circuit fabrication can be achieved by either subtractive or additive processes. The main difference between these processes is shown in Figure 3. Both the laminated and metallized substrates can be used to build flexible circuits. However, in high-temperature and high-humidity circuit testing, the results show that circuits made from laminates do not perform well and suffer circuit trace delamination. Examination of the failed circuits showed that the adhesion problem was a result of the roughened interface between the LCP and the copper. The porous features left on LCP surface after the metal was etched off led to circuit trace undercut. As a result, fine pitch circuits can not be built using these laminated substrates.
 Figure 4. Scanning electron micrographs of metal traces of LCP circuits made on a) a laminate substrate and b) a vapor-metallized LCP film. |
In contrast, circuits made using the direct metallized substrate performed well, and the process did not create a rough interface. Figure 4 shows scanning electron micrographs of metal traces of LCP circuits made on a laminate substrate and a vapor-metallized LCP film. The micrograph of a sputtered LCP substrate shows that the LCP film surface is relatively smooth between the metal traces. The micrograph taken of laminated substrate shows that the LCP base film surface is quite rough and porous.
Metallization of LCP film was achieved using a high-energy, vacuum sputtering process on a continuous roll-to-roll basis. Copper can be sputtered directly onto LCP film, but it is hard to achieve good adhesion. A thin tie layer coating such as chromium or nickel may be used to enhance the bond strength of the copper to the LCP and to prevent copper migration at the interface. The test results show that peel strengths from the sputtered substrates are 7-9 N/cm compared to 5-7 N/cm for the laminated substrate. The detailed LCP circuit performance at high temperature and humidity, including dimensional stability and reliability, will be presented at a conference early this year.2
 Figure 5. Complex circuit structures can be produced in LCP films using the chemical etching methods. |
A key circuit process step is substrate patterning. Several methods, such as mechanically drilling, punching, plasma etching and laser ablation, for via and other feature formation in LCP have been used. However, these physical drilling- and punching-related processes require expensive equipment set apart from the main production line. LCP circuits made using these methods can become very costly.
Chemical etching is a cost-effective method for patterning flex substrates. It is well-known for the feature formation in polyimide circuit production. To date, however, no chemical etching methods for producing via and through-holes in LCP have been reported because LCP is a chemically stable material. A recent discovery provides a breakthrough in flexible circuit development based upon a process that includes chemical etching of LCP.
The etchant solutions allow the manufacture of LCP flexible circuits in a continuous process and with fine features. Complex circuit structures, such as unsupported cantilevered leads, through-holes or other shaped voids in films having angled sidewalls, can be produced using the chemical etching methods (Figure 5). The typical chemically etched vias have a sidewall angle taper of about a 45°.
Conclusion
LCP has been demonstrated to be an advanced alternative flexible circuit substrate. It offers certain advantages for electronic applications:
(1) Excellent barrier properties and low moisture absorption may eliminate process difficulties and product reliability problems seen in polyimide circuits due to its high moisture content.
(2) The low dielectric constant and loss factor remain constant over the functional frequency range of 1 kHz to 45 GHz, with a negligible moisture effect.
(3) The thermoplastic properties of LCP films addresses the problems related to thermal processing and product reliability of polyimide circuits at elevated temperature.
LCP film has been directly metallized using a vacuum sputtering process. Unlike laminated substrates, the sputtered LCP substrate yields a good metal-to-LCP adhesion, avoids trace line undercut problems, eliminates remaining copper issues and provides effective circuit patterning resolution. Either subtractive or additive processing can be employed. AP
Acknowledgment
The author thanks 3M Microinterconnect System Division management and the LCP team for their contributions in LCP circuit process development, measurement and analytical evaluation work.
References
- J. Economy and K. Goranov, "Thermotropic liquid crystalline polymers for high performance applications," Advances in Polymer Science, Vol. 117, p 221, 1994.
- T. Hayden, "New liquid crystal polymer flex circuits to meet demanding reliability and end-use applications," International Conference on Advanced Packaging and Systems, March 2002.
Rui (Ray) Yang, senior research specialist, Microinterconnect Systems Division, can be contacted at 3M, Building 003-01-N-01, 11705 Research Blvd., Austin, TX 78759-2419; 512-984-2530; Fax: 512-984-5940; E-mail: [email protected].
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