New material, the liquid crystal polymer (LCP), has advantages over traditional PCB materials in its electrical properties (such as dielectric constant and dissipation.)
Project Name: Development of the liquid Crystal Polymer printed circuit board (LCP PCB) Manufacturing Technology for High-speed applications (ITS/092/07).
Project Summary: Driven by the prevalence of wireless Local Area Networks, high-speed telecommunication network equipment such as 40Gbps high-end routers and servers, and electrical mobile devices such as microprocessors above 5GHz, cellular phones and Personal Digital Assistants (PDAs), the demand for higher frequency and faster data transmission rates is increasing rapidly. Traditional printed circuit board (PCB) laminate materials have their limitations to support these high-speed applications. New material, the liquid crystal polymer (LCP), has advantages over traditional PCB materials in its electrical properties (such as dielectric constant and dissipation factors) in high frequency applications (over 5 GHz), dimensional stability, and resistance to moisture absorption. However, research study to investigate the manufacturability of LCP (as a thermoplastic) is required to help the local industry to master this technology.
R&D methodology: LCP offers advantages in a number of properties over traditional PCB materials. However, there are still a few manufacturability problems that will be encountered in the fabrication of LCP PCBs. This project will try to solve two major processes issues in LCP-PCB fabrication, namely I) the drilling, desmearing and hole metallization processes; and II) the comparatively high out-of-plane (z-direction) CTE of the LCP.
I) Drilling, desmearing and hole metallization processes
As a thermoplastic, when LCP is subjected to serious mechanical deformation (mechanical drilling process), it responds in different way in comparison with traditional PCB materials. In this regards, drilling should be re-engineered to avoid over-heating in order to retain a good integrity of holes, and minimize the formation of smear, especially for small-hole drilling. The
same situation applies to the laser drilling of microvias. After drilling, the hole should be desmeared and conditioned for the subsequent metallization process. However, as the LCP exhibits a strong chemical resistance, the conventional permanganate desmearing method is no longer applicable. Plasma desmearing is one of the potential candidates but modification of the parameters is required. This project will investigate different ways to remove the smear and find out the most effective means and tailor-made it for the LCP. Following desmearing, the metallization process should be processed to produce reliable electroplated holes that are capable of withstanding various thermomechanical stresses during production and operation. The method of the metallization will be selected from the conventional electroless copper plating, direct metallization, or solid-vias with plugging techniques.
II) Out-of-plane (z-direction) CTE
There has been concern that the LCP may impose serious stress on the barrel copper of vias of PCBs as it possesses a relatively high out-of-plane CTE (~150ppm/oC from 30° C to 150° C) in comparison with other conventional PCB materials. There are studies suggested that properly designed LCP PCBs in terms of the board thickness, via diameter, and copper plating thickness can improve their reliability. Unfortunately, details of which are not disclosed. This part of the project will therefore focus on studying and enhancing the thermal reliability of LCP PCBs by using various techniques such as reengineering the lay-up of LCP PCBs, adopting the solid-via technologies, improving the electroplating quality and devising a new circuit design rule.
Objectives and Benefits
The objectives of this project are:
1.To acquire technical know-how in drilling, desmear and metallization process in LCP-PCB
2.To develop PCB design parameters to improve the reliability of LCP-PCBs.
1.Develop a distinctive processing technology in the drilling, desmearing and metallization process in high-speed LCP PCB manufacturing;
2.Develop various design parameters of the high-speed PCB using LCP materials to improve their quality and reliability;
3.Promote and transfer the know-how and technology for high-speed LCP PCB fabrication through a website; and
4.Setup a database and information link to support Hong Kong PCB manufacturers with the latest information on high-speed LCP PCB fabrication.
Summary of Findings
The manufacturing techniques for fabricating LCP PCB, including lamination, drilling, desmearing and metallization, have been developed and the problems associated with the manufacturing have been addressed. The design parameters concerning the PTH reliability of LCP PCB have also been studied.
As high temperature lamination is required for LCP, traditional stacking materials such as kraft paper and release sheet, which can not withstand temperatures above 250° C, are no longer applicable; instead, ceramic-filled fibre paper can be used as a presspad and skived Teflon or the shiny side of copper foil can be used to act as a release film. On the other hand, excessively high resin flow was observed on laminating LCP, particularly in high-layer count LCP test panels. To cope with this problem, sealing of the edges of the lay-ups were used to limit the flow of LCP. A thinner LCP laminate might also be an option to help to ease the resin flow, and can be a future study topic.
There was no problem in laser drilling LCP. However, problems were encountered during mechanical drilling. Due to the difference in the properties of the polymers, it is more difficult to evacuate LCP drilling chips compared with conventional PCB materials. Cutting speed and chip load, in particular the latter, were found to play an important role in the drilling quality. A lower cutting speed and chip load were required to drill a hole with good quality. Further lowering of the cutting speed and chip load was also required for drilling a high layer count LCP and for smaller holes. Peck drilling might be required for high-layer count drilling in order to improve the hole quality.
Desmearing and metallization
Conventional permanganate may not be suitable for LCP desmearing due to its low desmearing rate resulting from the chemical inertness of LCP, and could not provide a robust peel strength between the deposited copper and LCP. CF4 + O2 plasma desmearing, which is commonly used in polyimide desmearing, is also not applicable as it tends to smooth the LCP surface rather than roughening it, which results in metallization problems after subsequent electroless copper plating. N2+ H2 plasma was found to be effective in desmearing LCP in terms of the desmearing rate and the effect on surface conditioning.
Effect of PCB design parameters on the reliability of LCP PCBs
LCP has a comparatively high out-of-plane CTE that may impose a higher stress on the PTH when it is subject to thermal cycling conditions. However, as LCP is capable of being fabricated into a thin film laminate, this helps to release the stress, to a certain extent. Therefore there is no special concern for the design parameters when designing LCP PCB. In other words, LCP is suitable for PCB fabrication.
All in all, LCP has produced some problems which have not been encountered in using conventional PCB materials, however, by adopting suitable techniques and adjusting existing process accordingly, industry is capable of mastering this new material to manufacture highly advanced LCP PCB to meet the demanding requirements of new high speed applications.
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