By Kevin Brennan, Delta Design, Littleton, Mass.
As packages continue to shrink, they are becoming increasingly more difficult to handle. Combine their smaller sizes with growing volume and a wider variation in form factors due to chip-scale packaging variants, and the situation is ripe for a new handling method. Test hardware is also forcing a change in handling methods. Increasing tester pin counts and the implementation of DFT (design-for-test) and BIST (built-in self-test) techniques are enabling higher levels of parallelism for a wide variety of devices. Handling non-singulated devices offers an efficient way to position many devices for test while maintaining short index times. One technique that is gaining interest is test-on-strip (TOS) handling. Imple-menting TOS involves a process change in the test cell. Several factors must be considered to determine if implementing TOS will benefit your application. While it is unlikely that TOS will replace either existing gravity or pick-and-place technologies, it will deliver benefits that are likely to position it as one of the primary future handling technologies. No Device Kit Needed A primary advantage of a TOS handler is the limited form factors it must deal with. Current handlers require a device kit for each package type handled. With proper planning, a single-strip outline can be employed for many package types, eliminating multiple device kits. Only the contactor needs to be changed when testing different package types. The result is a significant reduction in handler setup time. This is especially beneficial for CSP packages where form factors can vary by die size. Escalating tester pin counts, and the reduced pin counts required for testing by DFT and BIST, are increasing the number of devices that can be tested in parallel. At the same time, these factors are reducing test time. This, however, poses a challenge for non-singulated handling, where index and sort times typically increase with parallelism. Typically, high parallelism is only pursued if test times are long enough to mask the index time and sorting time for tested devices. Using TOS typically takes the same amount of time to position part of the strip for testing as it does to position the entire strip. When testing is complete, the TOS handler maps the test results to the location on the strip. Sorting is accomplished offline so that throughput and utilization of the tester are not impacted. Planning Needed Taking advantage of the common form factor and higher parallelism benefits of TOS requires some planning. The size and layout of the strip must account for all varieties of packages produced today and in the future. Additionally, the parallelism you test in today as well as in the future must also be considered. TOS handlers can be configured in stand-alone test cells, comparable to current non-singulated handlers. The handlers may also be configured with other process equipment in-line. In-Line Configuration The in-line configuration works well if all processing steps can be balanced to minimize bottlenecks. When this is achieved, the result is an optimal use of equipment and a shorter cycle time to process devices through the backend. In most applications, however, TOS handlers, like non-singulated handlers, need to be configured in stand-alone cells due to the mismatch of processing times involving different process steps. The software systems required to support TOS, however, offer the added benefit of improving cycle time.
One of the primary differences between today's singulated handling and TOS handling is the need for a software system to manage data and hardware components to transfer data to and from equipment that processes the strips. This need exists because tested devices are not sorted until after the entire strip is tested and singulated. During testing, a strip map is created that logs test results to a part's location in the strip. The map is then downloaded, as needed, by follow-on process steps. Each step adds its own information, as required, and ships the strip information back to the central database. Software Tracking To implement the software tracking of strips, users must incorporate a unique ID onto each strip. Every piece of process equipment, in fact, requires a barcode reader able to interpret these strips' IDs. Additionally, a network connection to the central database to send and receive information is needed. Thus, TOS handling requires a certain amount of infrastructure support. Once TOS is in place, the opportunity exists to reduce the amount of paper at the backend. The strip IDs are all that is needed to ensure correct processing of the strips. When the strip arrives at each piece of equipment, the ID is read, and the equipment setup is verified to match the processing required by the strip. Errors due to incorrect setup are dramatically reduced. Gathering information as devices are being processed delivers personnel realtime information on activity, enabling an immediate reaction to process problems. The system also is aware of the loading on each piece of process equipment to which it is connected. Resource Allocation By employing this information, the TOS system can dynamically allocate resources for the most efficient use of equipment and shorter cycle times. TOS offers the potential to improve throughput in the test cell and in the backend. This is particularly where test times are shrinking, parallelism is increasing and the equipment set-due to package variations-is becoming a larger part of the overall processing time. Conclusion The advantages do not come without some significant changes taking place in the backend. Strip layouts must be changed to ensure that they can be efficiently processed. A data management infrastructure must be put in place, with the processes downstream of test, to process the strips correctly. Several manufacturers who have implemented TOS are evaluating the benefits of making these changes, and the industry is watching these early adopters to gauge their own plans for TOS.
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