Camera Module Assembly and Test Challenges

Image sensor device production has skyrocketed in recent years, fueled primarily by the growth of cell phones with cameras. The assembly and test capability and capacity of these CMOS sensor packages, known as camera modules (Fig.1 ), have also rapidly evolved during the past five years — and the technology is proving to be quite challenging compared with standard assembly technology. An immediate challenge is the fusion of semiconductor assembly with optics technology. Another camera module assembly challenge is optimal particle control beyond what is required for standard assembly processes. Assembly companies must address the particle control requirement through special attention to facilities control, equipment and materials selection, process flow and strict discipline among line personnel. The rapid emergence of this product has not provided the industry the time to set standards for test and reliability requirements. Assembly companies must work together with sensor manufacturers, optics suppliers and the original equipment manufacturers (OEMs) to set these standards.

Table 1 shows different image sensor display formats. In the late 1990s, camera phones were predominantly using the CIF format, which is of a fairly low resolution. The VGA became popular by 2000, and in 2004, we saw the introduction of the SXGA-type or 1.3 megapixel (MP) camera phones. More than 65% of cell phones are expected to be sporting cameras (Fig. 2 ) by 2008 — amounting to more than 600 million cell phones with cameras.

A typical fixed-focus camera module design cross-section (Fig. 3 ) shows a stacked-die version in which the image sensor is on top of a signal-processing die. The typical fixed-focus lens system for a 1.3 MP module is comprised of a mount, lens barrel, IR filter and three or more lens elements. The number of lens elements varies with optical design requirements. The IR filter eliminates longer-wavelength radiation, which would create noise in the sensor. A flexible circuit with passive components is attached to the bottom of the laminate substrate. Modules are designed so that manufacturers have options, based on factors such as cost, size, available technology and personal preference.

Particles are the primary cause of yield loss in camera module assembly — more than 90% of defects are related to particles. Figure 4 shows a pareto of an assembly-related defect of a 1.3 MP module.

1. Examples of camera modules.

To keep device sizes relatively small, pixel sizes are smaller for MP sensors. Most MP CMOS image sensors currently have pixel sizes of 2.8-3.3 µm. Sensors that are 3.2 MPs will shrink to the range of 2.2 µm this year. Pixels on CCD sensors are even smaller. The maximum allowable particle size on a die typically is equal to 1 pixel. While particle control has been a challenge for CIF and VGA camera module assembly, particle control strategies for MP sensors need even further improvement. Examples of particles on image sensor die are shown in Figure 5 . Continuous particle reduction is a necessary practice for assembly engineers.

Assembly and test

A basic camera module assembly process flow is shown in Figure 6 . This process is broken down into five separate flows for wafer, substrate, mount, flexible circuit and module. Many of these processes are common for the assembly of standard packages. The flow, however, clearly shows that special processes are required for camera module assembly and test. Furthermore, even standard processes need careful control for enhanced particle control.

2. Forecast of cell phones with camera modules.

In the wafer flow, the saw process is critical because it generates silicon particles. Saw process optimization techniques are required to reduce silicon particle contamination on the sensor area. For the substrate flow, various cleaning and inspection steps are used to eliminate particles. Vacuum cleaning is commonly used to reduce particles. As pixel sizes shrink, however, wet cleaning is being adopted for effective cleaning. Various wet cleaners for image sensor products are now commercially available. Inspections, though costly, must be carried out at various steps to ensure a particle-free process. Mount flow is unique for camera modules. This is where the IR glass is sawn to required size and attached to the mount. Completed mount assembly also goes through cleaning and inspection steps. The mount is then attached to the substrate that contains the wire-bonded device. Another special process is barrel insertion, where the lens barrels are inserted into the mount.

3. Cross-section of a typical megapixel camera module.

Module flow is also unique to camera modules. This is where the lens barrel is focused and locked into position in the mount, and the module is tested electrically and optically and also for particle-related failure. These unique steps in the flow posed challenges that assembly houses had to quickly come up to speed on in the past few years.

4. Defect pareto of a 1.3 megapixel camera module.

Wafer, substrate, mount and flex circuit flow involve commonly automated process steps. One exception is barrel insertion, which is often done manually but can be automated. The module process steps are less commonly automated in current state-of-the-art processing, although some manufacturers are making progress at fully automating this flow as well.

5. Examples of particle defects on image sensor die.

The flex-attach process is typically achieved with a hot bar system. Flex attach is commonly done in assembly using standard surface-mount technology that uses common alloys, such as lead-tin eutectic. These common solder alloys melt at higher temperatures than the lens system can handle. Most of the camera modules today either use a plastics lens system or a combination of glass and plastic lens elements. Because the plastics lens system is already focused and locked into place, it is critical to perform the flex-attach process while maintaining the lens temperature below the maximum service temperature. If the lenses are overheated during flex attach, the module will go out of focus and must be rejected or reworked.

Alternatively, the flex can be attached using standard surface-mount materials and processes before inserting the lens barrel. In this case, the barrel insertion and focus must be done after attaching the flex. The module flow must be done with a dangling flex circuit. This makes automation of focus, barrel lock, image test and functional test more difficult, which is a disadvantage. An advantage of this flow is that the separate hot bar flex-attach process is eliminated. The manufacturer's decision in choosing between the two flex-attach processes depends on cost and yield considerations, as well as on their ability to automate.

Table 2 shows cleanroom requirements by assembly process steps. The portion of the assembly process where the die is exposed is most sensitive to particles. Until the module is encased in the lens system, assembly is typically carried out in a Class 100 environment, which is not required for standard or advanced package assembly, and adds to the cost.

Among the five separate flows shown in Figure 6 , flows for wafer, substrate and mount are most critical from a particle-related defect standpoint. This is because the sensor die is exposed during these process steps. A Class 100 cleanroom environment is recommended up to the barrel insertion process, while the remainder of the assembly process should be done in a Class 10,000 cleanroom. The actual cleanroom particle count must be checked at regular intervals and at strategic places within the room. For example, a particle detector should be placed near the processing areas, where the die is exposed to the environment. This provides data on the actual particle control level at critical process locations.

6. Typical process flow.

The environment inside the equipment should also be monitored regularly for particles. It is possible that, while the cleanroom particle count is well under control, the particles inside some of this equipment is outside the specifications of a Class 100 cleanroom. Proper airflow inside the equipment is the key to reducing particles. Keeping equipment covers open or taking them off completely are examples of ways to reduce particle levels inside the machines.

Assembly line layout is another area of special concern. Smooth, laminar airflow inside the cleanroom is important to reduce particle count. Adequate space between equipment, workbenches and shelves must be allowed. A good camera module assembly floor equipment layout will look sparse compared with that of a standard packaging assembly floor.

Careful consideration must be given to cleaning and inspection steps within the process flow. All jigs, fixtures, carrier frames and trays must be cleaned prior to use inside a Class 100 cleanroom environment. Use of an ultrasonic wet cleaning process is common.

Particles can be generated during the backgrinding process from the tape that is attached on the die's top surface. Standard backgrinding tape can be used if the wafer goes through a wet cleaning process before die attach. Silicon particles can be a major source of particle contamination during the wafer saw process. The standard deionized water cleaning during the saw process may not be adequate for removing silicon dust. Two steps can be taken to reduce silicon particle contamination from the saw process. First, the saw process must be optimized to reduce the silicon dust accumulation on the wafer. For example, it has been observed that a two-pass saw process shows less silicon dust-related contamination vs. a standard one-pass saw process. Secondly, a separate wet cleaning process after wafer saw can further reduce any silicon particle contamination. Sawn wafers should be inspected for cleanliness before the die-attach process.

7. Semi-automatic dry cleaning equipment using deionized airblow and vacuum.

On the mount flow, the IR glass saw process should use a cleaning process similar to that of the wafer saw. After the IR glass is attached to the mount, the mount subassembly is attached to the substrate subassembly that has the sensor die attached and wire bonded. Before the mount attach, the mount must also be cleaned and inspected. Wet cleaning before mount attach can significantly reduce particles and improve yield.

The remainder of the module flow and flex-attach flow is done in a Class 10,000 cleanroom. By this point, the camera module is less susceptible to particle defects because the sensor die and IR filter are already encased.

Cleaning frequency and methodology of equipment in the cleanroom must be optimized. For example, if adhesive cure ovens are not regularly cleaned, they can be a large source of contamination. As process conditions are improved, proven by higher assembly yield, some inspection steps can be eliminated or reduced to a sampling basis, thus reducing process cost.

Equipment material and design can inherently generate particles during operation. All handling areas and moving parts of equipment should be made so that particle generation from friction is reduced from both a design and materials selection standpoint.

MP module assembly may require equipment with tighter process control specifications. For example, MP modules need a tighter optical center for sensor center alignment. The current requirement is ±50 to ±75 µm for VGA designs, and ±25 to ±50 µm for MP design.

Standard packaging materials used in camera modules include silicon die, gold wire, laminate substrates, ceramic substrates, flex circuits, passive components, and connectors. There are, however, several materials that are somewhat unique to camera modules.

In the case of adhesives, outgassing properties are important in terms of potential contamination. Adhesives with higher outgassing may contaminate the sensor surface or IR glass during the cure process.

For the mount and lens barrel, plastic materials used include liquid crystal polymers (LCPs), polyphenelyene sulfides (PPSs), polyphenelyene oxides (PPOs) and polycarbonate/acrylonitrile butadiene styrenes (PC/ABSs). One key criterium is the materials' ability to withstand the adhesive cure temperature. LCPs and PPSs have a higher temperature of deflection compared with that of PPOs and PC/ABSs. For this reason, LCPs and PPSs are widely used for mount and lens barrel. For fixed-focus modules, the lens barrel is either screwed or unscrewed during the focus test, depending on test methodology. Choosing the right material combination between the mount and lens barrel reduces particle generation during the barrel insertion and focus test process. A test was done on mount and barrel, each made of LCP and PPS materials. Simulated focus operations were performed on each of these mounts and barrels, and then the IR glass was examined for particles. The LCP material produced no particles on the IR glass, while PPS produced a small amount of particles. Table 3 summarizes the results.

In the case of the IR filter, particles can be generated from the IR coating material. Depending on the coating material and process, it may produce flakes during process conditions and also during reliability test conditions. IR filters from each supplier should be tested under the conditions corresponding to actual process and reliability test conditions to ensure the integrity of the coating material.

Focus and test, an integral part of the camera module process flow, are unique because of the optics involved. There are no industry standards for focus and test methodologies. Every device manufacturer uses its own internally developed methods. This poses a challenge for assembly companies, because millions of dollars may be spent on developing processes and equipment for focus and test for a certain integrated device manufacturer — but these may not be transferable to other devices or may require expensive capital for software and hardware modifications.

The image sensor industry lacks standards for reliability test requirements for camera modules. Table 4 shows the results of a survey of 13 companies' qualification requirements for camera modules. High-temperature storage, damp heat and temperature cycle were selected because these tests are common to all companies. In addition to these tests, each company has additional tests, such as cold storage, mechanical shock, vibration, etc. The survey results show that there is very little commonality among the requirements of the various companies, despite all the modules being used in cell phones. Going forward, standards must be established to streamline the process flow, material selection and reduce costs.

Summary

Camera modules in cell phone applications are relatively immature in terms of assembly and test technology. They have ramped from zero to very high volume in a matter of three or four years. It poses several challenges compared with standard assembly technology, such as optics and stringent particle control. Also, as camera modules move into the realm of 3.2 MP and beyond, the development and use of the autofocus and zoom functions will be required — adding challenges not within the scope of a semiconductor assembly.

Another challenge is the price erosion of camera modules, making it difficult for the supply chain to make a profit. Assembly companies must learn to make modules more efficiently and in low-cost areas. Even then, this price-reduction requirement may very well force many assembly companies out of the business. Alternatively, we may see a stronger relationship among sensor manufacturers, assembly companies and even cell phone makers, or the emergence of the so-called "mega-integrators" providing the total camera module solution to the cell phone market.