Development of Rigid Flex Printed Circuits (RFPC) Manufacturing through Adoption of Advanced Critical Equipment.(GHS/063/04)

Portable electronic products such as cellular phones and cameras prevail in consumable market. The products features are in light weight and compact with high density interconnection and 3D packaging configurations. This drives PCB designers to adopt new features in printed circuit board (PCB) in an effort to keep pace with the miniaturization trend. Rigid Flex Printed Circuit (RFPC) is the “new” emerging product that is widely adopted. It helps reducing weight and overall system cost; providing better electrical repeatability as well as shortening assembly time.

The manufacturing know-how, however, are limited to a few flex companies in US and Japan. Local and southern China PCB manufacturers of RFPC have to face the challenges about the reliability problems associated with both rigid and flex materials in a single PCB. Demand for developing these technologies from local industry is overwhelming. This proposal is therefore submitted in order to help them to acquire the technologies and therefore to compete with foreign RFPC companies.

R&D methodology

RFPC is comprised of at least two kinds of materials with different properties. It is estimated that there are three times as many process steps involved in building rigid flex circuitry than are needed to manufacture multilayer PCB. This makes RFPCs difficult to fabricate. Problems encountered during the fabrication of RFPCs can be categorized into three main areas: drilling and desmearing, lamination, and reliability.

(I) Drilling and desmearing
Conventional desmearing with permanganate chemical used in the production of rigid boards is no longer applicable in RFPC, as the acrylic adhesive, which is commonly used in RFPC, is not compatible with strong alkaline chemicals. To solve this shortcoming, desmearing is a critical step and plasma technology should be adopted. Although there are many benefits to using plasma, plasma etching has not yet become a common technique in local industry, since most engineers lack the experience and know-how to control and optimize the process.It is a technology with very low throughputs, and the problem of uneven etching within chambers is still unresolved. The research study will focus on how to achieve an even higher etch rate for holes within different panels in a vacuum chamber and within a panel. To achieve a cost-effective plasma desmear with a good-quality yield, and in order to achieve a higher etch rate and even etching, the design of the vacuum plasma chamber has to be improved.

(II) Lamination
Flexible dielectrics, in general, show poor dimensional stability. With decreasing land diameter, lamination should be carefully carried out in order to achieve good registration. Meanwhile, attention has paid to the positional shift of flex against rigid, which is the one of main causes of the mis-registration of the final product. To tackle this problem, abrasive blasting will be studied to improve the registration as well as the production yield of RFPCs. This technology, in conjunction with plasma technology, alters the topography and wettability of polyimide. Typically, the polyimide flex materials have smooth surfaces that are difficult to laminate. A physical blasting and plasma that micro-roughens the surface of polyimide helps to improve its adhesion with bond-ply after lamination and so enhances its thermal reliability. It may also help to increase the friction between polyimide and bond-ply while laying up, which prevents or minimizes the positional shift of layers against the others and therefore helps improve registration.

(III) Reliability
Reliability is another crucial research area that will be focused upon. Both acrylic and modified epoxy adhesive, which are used to enhance the adhesion between copper and polyimide; coverlay; copper; and so forth, have a very low glass transition temperature but a relatively high thermal expansion rate. When RFPC is undertaken to an elevated temperature during assembly or a thermal stress test, the flexible polyimide and adhesives expand much more quickly than the barrel copper in the Z-direction; thus fracturing the copper and leading to lifted pads and barrel copper cracks.

The proposed project will help local industry to accomplish the following:

  1. Acquire the technical know-how required to produce RFPCs
  2. Develop innovative technologies to overcome technological challenges.
  3. Launch an RFPC production line unique to the local industry.
  4. Gain competitive edge over the RFPC manufacturers from foreign countries such as Taiwan, Korea, and Japan.

Project impact
In summary, this project will benefit a number of target user groups in the following aspects:

  1. RFPC manufacturers: The RFPC technologies will be developed and customized for a few RFPC manufacturers who are not only showing interest, but who also want to enter into the RFPC business.
  2. Medium-sized PCB manufacturers: Through a series of technical seminars, RFPC fabrication technology and its reliability will be made known to this group.
  3. Large-sized PCB manufacturers: Through consultancy services, seminars, and workshops, this group will not only acquaint themselves with the gaps in the technology for manufacturing RFPCs, but also with the know-how to bridge the gaps.
  4. PCB material fabricators: Through prototype and consultancy services, this group will be able to customize the advanced RFPC fabrication technologies and findings in this project to formulate and fabricate an RFPC prototype.
  5. PCB and Electronic industries at large: Through publications and exhibitions, these user groups will be exposed to some of the advanced RFPC designs and manufacturing technologies as well as the functional reliability of their future end-products that adopt RFPC.


  1. Develop distinctive technologies (e.g., desmearing, lamination, and reliability testing technologies) for RFPC fabrication.
  2. Assist in setting up an advanced RFPC production line in the Guangdong region
  3. Promote and transfer the know-how on RFPC fabrication for PCB industries through exhibition and publications (e.g. newsletter and technical report)
  4. Conduct one seminar and one training workshop in advanced RFPC fabrication technologies
  5. Set up a database for potential RFPC entrepreneurs
  6. Set up a technical library for manufacturers, and quality and reliability standards for advanced RFPCs

Within the scope of this study, several critical aspects of the technologies associated with the manufacture of RFPCs have been studied, and the following are some major conclusions:

  1. The traditional chemical desmear method was found to be ineffective for removing debris generated in the process of hole drilling. On the other hand, plasma desmear was shown to be an effective means to remove such debris.
  2. It is possible to optimize the productivity of plasma etching and the uniformity of the etched holes by the proper selection of operating parameters for the plasma etching process.
  3. Quality problems such as blistering, squeeze-out and conformity of coverlay that are associated with the lamination of RFPCs could be resolved using improved lamination techniques.
  4. In cases where the traditional oxide treatment cannot be used, pumice scrubbing is considered to be a good alternative among the other surface treatment methods to give a high adhesion strength to the coverlay lamination.
  5. All the three types of lay-up constructions that were studied passed the solder stress test and the insulation resistance test.
  6. The lay-up, where there is limited extension of coverlay to the rigid section of RFPCs, was found to have the highest resistance to thermal stresses.However, it is worthy to note that materials from different suppliers have slightly varying physical properties that might affect the performance of these materials in the production process. The study illustrated above serves as a reference. Manufacturers therefore need to fine-tune their process for individual material suppliers.

As one project cannot fully cover the entire scope of RFPCs, some technologies such as hole metallization were not studied. The industry will be required to acquire and fine-tune them according to their needs. Material advancement is another aspect that was beyond the scope of this project, however, it is one of the key factors determining the prevalence of RFPCs in the coming years. The existing flexible materials still have many shortcomings, such as dimensional instability and high production costs, which remain to be solved. Currently, a few suppliers are offering Liquid Crystalline Polymer (LCP) as an alternative to replace existing flexible materials. However, modifications of traditional PCB production facilities such as lamination, drilling and hole metallization are required in order to use LCP. Nevertheless, this kind of material advancement is expected to continue and is the driving force towards success.

Project Commencement Date:
May 1, 2005

Project Completion Date:
October 31, 2006

Principal Investigator:
Dr. Winco K.C. Yung
Tel (852) 2766-6599

Project Team Member:
1.Dr. Winco K.C. Yung
2.Prof T.M. Yue
3.Mr. James Tam
4.Mr. C.P. Lee
5.Mr. Jack Wang
6. Mr. Steve Wang
7.Mr. Patrick Wong
8.Ms. Carrie Cheung
9.Ms. Dawn Law

Sponsoring Company:
1.Onpress Printed Circuits Ltd.
2.Yantat (Hong Kong) Industrial Ltd.
3.Techwise Circuit Co. Ltd.
4.Topsearch Printed Circuits (HK) Ltd.
5.Rayben PCB Ltd.