VGA Imagers in Camera PhonesDespite a drive for higher resolution, the old standard is sticking around.
GILES HUMPSTON, TESSERA
Early cell phones had very low resolution imagers, typically 0.1 megapixels. This resulted in barely acceptable picture quality. Consequently, the imager resolution moved quickly from the common intermediate format to video graphics array (VGA), which provides three times the number of pixels and a noticeable improvement in picture quality compared with common intermediate format (Table 1).
Table 1. The standard format and resolution of solid-state imagers run from QCIF, with just 25,344 pixels, up to UXGA, with 2 megapixels. Courtesy of the Video Electronics Standards Association.
The increased pixel number confirmed a perception that more pixels mean better-quality photos. This perception was bolstered by the fact that megapixels are a readily quantifiable metric that can be used for sales purposes, despite the tenuous relationship between megapixels and picture quality.
Given the competition between cell phone manufacturers for models with headline pixel numbers, industry analysts were predicting that the cameras on mid- and low-end phones would be progressively increasing in resolution. However, reports of actual manufactured units show that these forecasts were wrong.
When looking at the past production and future volume forecast for VGA camera modules in cell phones, it’s clear that VGA was the resolution of choice in a higher proportion of phones for much longer than originally expected, and the prediction of substitution by higher resolutions has had to be revised continually (Figure 1). The persistence of VGA is the result of a combination of factors.
Figure 1. Recent surveys indicate less of a drop in VGA use in camera phones than surveys conducted in 2002 and 2004 revealed. Source: Techno Systems Research.
One reason why consumers prefer VGA camera modules over higher resolutions is the way the camera phone is used. The pictures captured by the camera are usually viewed only on the tiny cell phone screen. Even on high-end phones, the screen is typically only 350 × 450 pixels, which falls somewhere between common intermediate format and VGA resolution. An image with greater detail simply cannot be displayed, so there is no benefit to having a camera with a resolution higher than VGA.
Allied with the above is the changing use of camera phones. With the introduction of third-generation (3G) networks, bandwidth exists to provide real-time video and audio communication at the same time. To do so, the camera phone must be pointing toward the user while it displays the image of the other party on the cell phone screen. To economize on network bandwidth and cell phone battery drain, low-resolution camera modules are used extensively for this application. Consequently, many high-end phones now have a high-resolution (typically a 2-, 3- or 5-megapixel) camera for taking pictures and a VGA camera for video conferencing.Thin is fashionable
Moreover, thin is the latest fashion trend for portable electronic products. Higher-resolution imagers tend to require taller optics, especially when adding features such as autofocus and zoom to improve picture quality. These optical functions are usually achieved with micromechanical systems that are impressive feats of mechanical miniaturization but that add to the overall camera module height and cost. Because fashion drives sales, the slimmest phones are the most desirable. This constraint has slowed the transition to high-resolution cameras to the rate at which optical innovation can deliver compact optics for high-resolution cameras.
It is inevitable that the VGA camera module eventually will be rendered obsolete as higher-resolution cameras become smaller and less expensive, and their power consumption declines. However, in the short term, the predicted virtual elimination of VGA from camera phones is unlikely; several technical and economic developments will ensure its persistence for a number of years. These include solder-reflow compatibility, wafer-level packaging and camera phones designed for emerging markets where cost is the principal driver of component selection.
Traditionally, camera modules are manufactured using chip-on-board assembly. In this approach, the image sensor die is attached to a substrate and interconnected to it by wire bonds. A turret housing the camera lenses and other optical elements is glued in place over the imager. Camera modules manufactured using this method are temperature-sensitive, so the camera module is built separately from the rest of the phone and eventually is interfaced to the phone by a flying lead and connector. This arrangement is not mechanically robust, and interconnect failures are one of the leading causes of camera phone returns.
A great improvement is the manufacturing technology that uses solder reflow to assemble the camera module with the other components. This interconnect technology is best suited to small die with low numbers of interconnects, giving VGA an advantage over higher-resolution cameras.
Tessera’s Shellcase RT wafer-level packaging technology uses a glass-silicon structure to provide environmental protection to the active face of the semiconductor and a low-cost redistribution layer to connect the die bond pads to a ball grid array interface. The wraparound wiring trace can be seen under a protective dielectric layer as well as the cover glass that protects the front image sensor.
For a camera module built using chip-on-board assembly, the costs of assembly are incremental for each module fabricated. Wafer-level packaging is an alternative where the components are fabricated and assembled while in wafer form. The assembly costs are shared among the good components on the wafer, greatly reducing manufacturing costs per part. The technology to package image sensor die at the wafer level with solder-reflow interconnects has been available for a number of years and is one of the reasons for the low cost of VGA camera modules.
The same technology now has become available for camera lenses, which means that with both the lens and the sensor packaged with wafer-level technology, the camera module is smaller and more robust (Figure 2). It also costs 30 percent less, making it affordable for incorporation into budget mobile phones that previously did not have cameras and for use as the second video conferencing camera in midrange phones.
Figure 2. A conventional camera module is shown alongside a much smaller next-generation product, where the imager packaging and lens stack assembly are done at the wafer level. Courtesy of Tessera.
Because the Western world is the dominant market for camera phones, it’s easy to overlook the rest of the market. In developing countries that may lack land-line phone systems, a low-cost wireless phone is an alternative. These markets tend to prefer camera phones, and the VGA camera module has the dual advantages of manufacturing volume and lowest price.VGA longevity
The VGA camera module is well matched to the capability of camera phones for most applications consumers actually use. Just at the point where a shift to higher-resolution formats was gaining traction in terms of market share, the market for low-end camera phones with VGA cameras has blossomed. Meanwhile, new technologies for VGA cameras provide improved reliability, reduced size and decreased cost. Combined, these influences will extend the life of the VGA-resolution camera module, which is projected to stabilize at between 25 and 30 percent of the camera phone market for the foreseeable future (Figure 3).
Figure 3. The actual and forecast penetration of VGA cameras in cell phones is shown with data gathered in a 2008 market survey. Owing to new technologies, lower cost and other markets, VGA use is forecast to stabilize at around 25 percent. Courtesy of Techno Systems Research.
Meet the author
Giles Humpston is director of research and development at Tessera in Buckinghamshire, UK; e-mail: firstname.lastname@example.org