IEEC - Ericsson DH338 Digital Portable Cellular Phone

Cellular communications technology was first conceived by AT&T in the late 1940s, and working prototypes were completed as early as the 1970s. In 1983, the first commercial cellular system came on-line, in Chicago. At the time, AT&T dared to project a subscriber base of 1 million by the year 2000.

AT&T was wrong! In 1994, the worldwide cellular phone subscriber base topped 48 million, about half of which were in the US. Western Europe as well as Asia Pacific have also become major cellular phone markets. These numbers are even more staggering if expressed as cellular penetration, i.e. the percentage of the total population subscribing to cellular phone service.

Cellular Phone Penetration - 1994
Country	Penetration Rate
Scandinavia (Sweden, Norway, Finland, etc.)	~12%
United States	~9%
Singapore	~8%
United Kingdom	~5%
Germany	~4%
Japan	~3%
Thailand	~2%

Cellular phone sales are forecast to grow at more than 20% compound annual average growth rate until the end of the decade. Cellular phones are not just popular with wealthy people in developed countries. Rapidly industrializing countries in Asia, but also in Eastern Europe and elsewhere, find that it is considerably faster and less costly to construct cellular networks rather than old-fashioned wired phone lines.

A cellular phone is essentially a two-way radio transceiver that patches into the telephone system via a cellular network of base stations. Rather than communicating directly with another phone handset, it is in contact with the nearest base station, itself a radio transceiver that is directly wired into the plain old telephone system (POTS). The base stations are arranged in a cellular network, with each cell defined by its base station. As the cellular phone user moves spatially, he crosses into a different cell and gets handed over to the new base station. This is done in apparent real time, so that the user can continue his call while moving through different cells. For a more detailed discussion of cellular telephony and personal communications services, please refer to our September 1995 bulletin Ericsson DH338 Digital Cellular Phone.

Ericsson DH338 Digital Cellular Phone

Worldwide, Motorola, Ericsson, and Nokia are the dominant cellular phone suppliers, and together they dominate about 70% of the worldwide market. Ericsson first introduced its line of handheld cellular phones in 1987. The Ericsson DH338 is the world's smallest digital cellular phone. It is in fact a dual analog/cellular phone, reflecting the fact that a broad digital cellular network is not yet in place in the US. The phone uses TDMA technology for the D-AMPS cellular network. Ericsson has sold over one million digital cellular phones worldwide. This DH338 cellular phone was assembled in Sweden with components from various sources.

Overall Construction

The Ericsson measures 5" x 1.8" x 1" (LxWxH) and weighs 224 grams, including 103 grams for the nickel metal hydride (NiMH) 6V battery. This makes the battery the single largest and heaviest component, typical in today's cellular phones.

The phone consists of front and back covers which sandwich two printed circuit boards that run along the entire length of the phone. The top board (or LCD/keypad board) is mostly responsible for input/output functions and contains the keypad, the LCD module, and the microphone (the loudspeaker is attached to the inside of the front cover). The lower or main board contains most of the RF and digital processing components. This clean separation of functionalities is typical of most of the cellular phones manufactured today. Usually two rigid boards are used, although some -- like Nokia phones -- will replace the LCD/keypad board with a flexible substrate.

LCD/Keypad PCB Assembly

The LCD/keypad board (shown earlier) is a four-layer FR4 substrate about 30 mil thick in total, with a dielectric thickness of 8 mil. Minimum linewidth measure 7 mil, minimum line spacing 12 mil. Plated through holes of various diameters (minimum 22 mil) exist. The board is gold plated.

Overall, the LCD/keypad board is of moderate complexity. Sixty-one surface mount passive components are mounted onto the board, but only a single active device: the LCD driver which is mounted directly onto the LCD module.

Main PCB Assembly

The main board, on the other hand, is of much greater complexity. While it is of the same size as the LCD/keypad board, it carries 24 integrated circuits, which are surface mounted on the front and back of the board. In addition, 298 surface mount passive components are located on the board. As the photos below show, all RF components are located on the upper half of both sides of the board, whereas all logic components are located on the lower half. A gasket made of silicone elastomer embedded with cabosil separates the two halves along the plated line and aides in EMI shielding.

Front (Left) and Back (Right) of the Main Board

The main board is an eight-layer FR4 construction with a thickness of about 48 mil. Minimum linewidth was measured at 5 mil, minimum line spacing at 6.5 mil. The hole density is 68 holes per square inch, with areas of considerably higher local densities. Hole sizes vary, and are as low as 12 mil with 24 mil pads. The board is gold-plated.

LCD Module

The Ericsson phone has a 1.3" x 0.5" LCD screen that can display three lines of twelve characters each. This screen is attached to the phone as a fully integrated LCD module with its own LCD driver. The back glass substrate of the LCD screen extends to almost twice the height of the actual LCD screen. The LCD driver is mounted on this protrusion as a flip chip device. The silicon -- manufactured by Philips -- has gold pads which mount onto Indium Tin Oxide (ITO) traces. ITO is a transparent conductive material that is used in LCDs to electrically excite the individual pixels. In this case, the ITO traces were extended beyond the actual LCD screen to serve as contact pads for the flip chip LCD driver. The ITO appears to be of greater transparency than usually found in such an application.

This chip-on-glass construction does not use conductive adhesives. Instead, either ultrasonic pressure bonding (probably), or thermosonic pressure bonding was used to mount the silicon. Furthermore, no underfill was used; encapsulant surrounds the die, but does not cover it or underfill it. The photo below shows the flip chip device, the side encapsulant and the ITO traces.

The entire LCD module is fastened mechanically and electrically to the LCD/keypad board via a row of six 'C' - shaped clips (see earlier photo of LCD/keypad board). These clips keep the LCD module suspended above the printed circuit board. Additional spacers keep the LCD from being pressed against the board. The ITO traces on the LCD module extend to the clips for the electrical connection. The chamfered glass substrate of the LCD module is snapped into the clips during assembly. In addition, an adhesive material is located between the clips and the glass, but likely serves only mechanical purposes.

Please contact Prismark Partners LLC or the Integrated Electronics Engineering Center (IEEC) at Binghamton University to receive a full copy of this report, and to discuss the opportunities and challenges cellular communications technologies present to your business.

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