Flexible circuit market drivers: flexible circuitry manufacture has grown at four times the rate of rigid boards in 2004. But what technologies and applications are driving this expansion?

Flex circuitry finds applications in very many situations in many sectors throughout our industry. Flex and, in extreme situations, flex-rigid technology offers a combination of dense electronic interconnection and the ability to resolve 3D structural issues that readily out-performs other solutions such as rigid boards cables and connectors. The rapidly developing markets of mobile phones and digital cameras present some of the most exciting markets in electronics. Here the need for density is driven both by the system intelligence and high-resolution display and for flexibility and 3D resolution to enable portability and novelty in design. Flex circuitry is proving to be the ideal solution and the mobile phone and digital camera market sector offers an exciting opportunity for flex circuits and the materials involved.

Between 2005 and 2010 the demand for flex circuitry is expected to double in size. Within the overall FPC market, digital cameras and mobile phones account for 33% of value now and this share is expected to increase to 40% by 2010. In fact, by 2010 the camera/phone market will account for 45% of flex circuit activity (volume of production) world wide.

The next sector down in the value scale is military and aerospace that accounted for another 33% of value in 2004. BPA expects this sector to retain a 33% value share to 2010. In terms of numbers made and materials used, military and aerospace accounts for some 20% of volumes of activity and materials used (indicating the additional prices in this market characterised by many low volume/high price applications).

In Japan alone, we expect the flex-rigid circuit board market for digital cameras and high-end mobile phones to increase from $107 million in 2004 to $224 million by 2010. These applications account for more than half of multilayer FPC production in Japan.

Miniaturization and increased functionality will continue to drive demand for smaller, finer features. The move to the additional capability of the flex-rigid solution is curtailed by cost. The cost of flex-rigid currently outweighs the benefits offered for present designs and interconnect products, discrete flex and connectors will continue to be used in most mobile phones for the next five years.

Currently, although most of the major mobile phone handset manufacturers see a requirement for flex-rigid substrates in their handsets, the major barrier currently being cited by these companies is the high cost of flex-rigid in comparison to alternative technologies (i.e. rigid boards with conventional flex). However, we at BPA believe that the driving forces of increasing functionality, changes in mechanical design and fine pitch connector challenges will create a demand for flex-rigid in this sector from 2007 onwards.

Mobile Phones

Over the past five years mobile phones have shrunk in volume size from 240cc to less than 30cc, and in weight from 300g to less than 80g. At the same time functionality has advanced from a limited memory capacity and a small black and white display to an impressive item of integrated data equipment, almost a small pocket PC, with internet access, gaming, graphics, and high-resolution colour display. At the same time the mobile has coped with the shift from simple voice-only, circuit-switched, access to high-speed, digital data packet switching access.

With each additional capability, the mobile phone has gained weight and volume and then shrunk as further technological advances are made. A downward trend in weight and volume, observed as 2G technology matured, was temporarily set back by the addition of camera modules, and again with the introduction of 3G. Larger, higher resolution displays essential for clearer viewing of photographs and video clips, together with the extra silicon for memory and processing, initially indicated larger, heavier models. It is estimated that the area of silicon required for 3G mobile phones is triple that for 2G. Size reduction for this is achieved with thinned silicon, stacked packages and system-in-packages. A thinner FPC interconnect can contribute towards slimming the display module and so thinner materials and finer lines and spaces are part of the FPC technology trend.

The overall trend is also toward increased use of flex interconnect for additional functions such as camera modules, backlighting, power boosters, speakers, microphone and other electronics. Most designers, however, are still focused on using (low cost) discrete connectors as the means of attaching the flex component to the main substrate and the display module for example, is purchased with flex component attached.

In 2004, the market for mobile phones worldwide was more than 660 million units, with 684 million units actually shipped by the manufacturers. The market had increased by well over 20% on the 2003 figure. Of the key manufacturers, Nokia, Motorola and Siemens together accounted for 358 million units. Nokia maintained its lead position with 207 million units (a 12% increase on 2003). Motorola stood in number two position with 104 million units shipped. Samsung increased shipments by 57% (87 million units) to overtake Siemens (49 million units) and become the number three supplier. LG also enjoyed a huge increase in shipments to become the fifth largest supplier (44 million units) by the end of 2004. Sony Ericsson ranked sixth with just 6.2% of the market and Alcatel can only boast less than 4% market share.

In China in 2003, Bird became the largest domestic producer of mobile phone handsets. While Bird is not yet in the top five worldwide, the company has shown remarkable growth, with shipments increasing from two million units in 2001, seven million in 2002, 11 million units in 2003 and 13 million in 2004. Motorola and Nokia occupy second (14%) and fourth (9.7%) place respectively in the Chinese marketplace.

China produced two hundred and thirty million phones in 2004 of which approximately fifty percent were exported. In contrast, Taiwan produced approximately fifty four million handsets. However, price competition has forced a reduction in R & D spending by many of the Chinese companies while Taiwanese companies have identified the potential of the market and are investing in developing feature-rich, high colour resolution camera phones.

The markets for mobile phones in Japan, North America and Europe are near saturation in terms of just 'phones per person.' The future market in these regions for mobile phones will therefore arise from upgrade or replacement of existing handsets. In the newer, developing regions, new subscribers will be looking for the lower cost entry-level models. BPA's forecast of world-wide mobile phone demand demonstrates the impact of this division between the markets for a basic functional model and the latest sophisticated model.

Form Factor

Approximately 37% of mobile phones sold worldwide in 2004 were of clamshell design, while in Europe, clamshells accounted for only 14% and most of these were Japanese or Korean models. This was up from 25% worldwide in 2003 and just 8% in Europe. More than 50% of the models displayed in showrooms at the beginning of 2005 are clamshell, and clamshell is expected to be the dominant form factor from 2007 onwards. For this style of phone, the flex interconnect may be either a (polyimide-polyimide or polyimide-glass/epoxy) flex-rigid construction for the board-to-board; or rigid board and flex interconnect attached with a discrete connector.

Display

The pixel resolution has a direct impact on the FPC interconnect. To drive a display, the interconnect from driver IC to display array must provide connection to each of the vertical and horizontal arrays and in triplicate to one or the other for RGB colour. Typically therefore, a 132 X 176 display would require (3X132)+176 or 132+(3X176) tracks i.e. 572 or 660 tracks.

To minimise the overall thickness of the display module and the flexible interconnect, the designers usually aim to keep the metal layer count to minimum. As display resolution increases the options are to reduce the track width or double the metal layer count. Typically, track widths for single-sided interconnects are 30[micro]m while those for double are 60[micro]m.

BPA has established that for display flexes, the trend is towards thinner materials, and finer lines and spaces, to avoid use of multilayer flex where possible. This will drive lines and spaces for single-sided flex down towards 10[micro]m as standard rather than exceptional by 2010 and 12[micro]m for double-sided.

Camera Phones

By 2009, BPA expects 85% of all mobile phones to have at least one camera sensor. Accompanying this trend will be an increase in the pixel resolution of the camera sensor and this, together with video streaming, will affect the layer count and feature sizes on the flexible interconnect between mainboard and sensor module.

As with displays, this increase in resolution of the camera sensor will have an impact on the flexible circuit interconnect. The quantity of data that will be fed back from the CMOS sensor will increase with every increase in sensor resolution. Most of the OEMs interviewed believed that the sensor could be accommodated on double-sided flex with only one anticipating a move to 4 or even 6 layers.

For the most part, the sensor interconnects are a double-sided flex with track widths not less than 70[micro]m. However, the declared layer count depends on the type of EMI shielding employed: either a low-cost silver paint outer layer, or a more expensive hatch-patterned outer layer fabricated as the outer layer of a multilayer construction. These flex interconnects will see a reduction in standard line widths from 75[micro]m to 50[micro]m by 2009, while high-end sensors up to 8 megapixel will require finer line widths below 20[micro]m to achieve the routing.

The figure below shows a 0.1 megapixel sensor module from Agilent's Sensor division (now part of Flextronics). The flex interconnect which can be clearly seen supplies 18 circuit tracks to the 8 mm sq. module. For these early modules, the track widths are at least 150[micro]m and are pitched at the termination to suit a standard connector.

[FIGURE 3 OMITTED]

Flex-rigid is not on the immediate horizon. Although a move to flex-rigid is seriously considered by many designers, many see flex-rigid as an extremely costly option even with the cost advantages of Asian production. We can't see flex-rigid become a mainstream design option for at least three years. What BPA does see is a trend towards novel frontages and opening mechanisms which will differentiate between brands and therefore there will be increased usage in flexible interconnects to enable the new mechanisms.

At BPA we confidently expect that the minimum line/space requirements for hinge interconnects for advanced phones will move below 25[micro]m by 2009. A key issue with the hinge interconnect is the dynamic flexing requirement. The loosest specification was for a minimum of 70,000 cycles. In Japan, some manufacturers demand >200,000 cycles. Typically, most OEMs demand >100,000 cycles. The dynamic flexing performance is being improved by reducing the thickness of both the polyimide substrate and the polyimide component of the adhesive backed coverlay, i.e. 25[micro]m with a downward trend to 12.5[micro]m, and also by the adoption of air-gapped constructions.

Francesca Stern is a lead consultant for BPA Consulting Ltd. Email f.stern@bpaconsulting.com or call +44 1306 875500

 Mobile Phones and Digital Cameras  183 Automotive                          18 Industrial and Instrumentation      41 Medical                             32 Computer/Business Retail            31 Consumer                            13 Military and Aerospace             188 Telecom/Datacon Infrastructure      40  Total = $546 Million  Figure 1. Worldwide Demand for Flex-Rigid PCBs--2004.  Note: Table made from pie chart.            Electronic      Application by Substrate Technology Sector    System          Flex                    Flex-Rigid  Telecom/  Rack-based      Rack to rack            Limited applications Datacom   equipment       interconnects           Mobile phones   Interconnects for       Flex-rigid in high-end                           displays, backlight,    clamshell mobile phone                           camera sensor, power    handsets and in camera                           module Multilayer       modules                           mainboard in high-end                           & clamshell Japanese                           mobile phone handsets           Transmission                            4-6 layers, very cost-           control                                 competitive            Electronic      Application by Substrate Technology Sector    System          Rigid  Telecom/  Rack-based      Multilayer backplane/ Datacom   equipment       motherboard           Mobile phones   Main substrate is multilayer &                           microvia build-up           Transmission    Mainboard with discrete           control         connector  KY610204  Table 1. Telecomms Applications Requiring Flex and Flex-Rigid Circuits.         Replacement Handset  New Handset  2004          382              302 2005          405              314 2006          441              330 2007          485              357 2008          529              374 2009          592              404  Figure 2. Worldwide Market for Mobile Phones 2004-2009.  Note: Table made from bar graph.