Back-end assembly: Closing the traceability gap
10/01/2001
BY JEFF WOOLSTENHULME
 |
In semiconductor manufacturing, the high value of semiconductor wafers has traditionally placed a heightened emphasis on wafer traceability - the ability to automatically track a wafer through the fabrication process. By tracking a wafer through its entire lifecycle, manufacturers can more easily understand where defects are occurring in the process and what is causing them. This has helped manufacturers reach higher levels of process control and, ultimately, improved yield.
Where is Back-end Traceability?
While traceability has been a consistent theme in front-end manufacturing for years, historically there has been very limited use of traceability in the back end. In fact, once a wafer was singulated, a device would typically not be marked with an ID code until final packaging, if at all. And, while marking an IC package may help manufacturers track the package itself, if there was a problem with the final IC, there was no absolute way to identify where defects may have occurred. Nor was there a sure way of knowing what wafer a specific die came from, so the ability to pinpoint potential defect trends relating to a specific wafer was limited, at best.
While there are a variety of reasons why the use of back-end component traceability has been limited, one barrier of entry has been the complex and often expensive nature of machine vision systems of the past. In an industry where cost sensitivity can be high and profit margins low, many vision systems have been difficult to justify for the purposes of reading ID codes. Even if a company could absorb the cost, the technology could prove to be too complex for many potential users.
More Traceability, New Standards
Today, device manufacturers are looking at adding more traceability in the back end to gain a better understanding of their process flow. To facilitate increased traceability, Semiconductor Equipment and Materials International is developing new standards for identification of components, such as interconnect substrates, ball grid array and chip scale packages, and bare die. The benefits of this can be seen in a number of situations. For example, if a CPU doesn't perform to the specified processing speed, it may be shipped back to the manufacturer for failure analysis testing. Because the manufacturer marked the die with an ID code, process engineers can easily trace the component's manufacturing history and identify where problems may have occurred in the process. This may limit the scope of necessary rework and testing.
Another area where component traceability can be a great advantage is in process tracking. With increasing customer demands for short turnarounds and just-in-time delivery, manufacturers can use part tracking to determine where a specific batch of parts is in the process at any given time.
New Vision Technologies
Low-cost vision sensors are helping to break down the old barriers of entry, thereby opening the door to the adoption of back-end traceabilty. Unlike expensive, complex systems of the past, today's vision sensors are highly cost-effective, offer the same type of performance and reliability found in front-end wafer identification systems, and are easy to integrate with a variety of equipment. Most are equipped with software tools for reading a wide range of commonly used bar codes, such as EC200 2D Data Matrix and IBM 412. Furthermore, today's vision sensors are far easier to use than many earlier systems, and they no longer require knowledge of programming languages or previous vision experience. Finally, some of the latest products feature built-in networking capabilities that authorize fab personnel to set up and modify ID applications from remote sites and monitor reading activity from any location in the facility.
As device manufacturers in the back-end continue to realize the value of tracking components through assembly all the way to the end user, the traceability gap between a singulated wafer and a final IC product will continue to close. The ability to identify any component at any point is helping manufacturers reduce rework costs and improve process tracking. New technologies, such as low-cost vision sensors, have eliminated the cost and complexity barriers of the past, and they are playing an increasingly important role in today's back-end traceability and yield improvement strategies. AP
Jeff Woolstenhulme, semiconductor industry marketing manager, can be contacted at Cognex Corp., One Vision Drive, Natick, MA 01760; 508-650-3145; Fax: 508-650-3344; E-mail: [email protected].