The key stages of semiconductor lifecycle management (SLiM) were presented in my previous blogs, so now we will delve into more specific details of a successful SLiM program. (See: Nuts & Bolts of Semiconductor Lifecycle Management, Part 1 and Nuts & Bolts of Semiconductor Lifecycle Management, Part 2.)
In this article we will take a look at device storage and wafer management issues, methodologies, and proper management of inventory levels. Here are the critical issues and steps to consider in wafer storage for extending the lifecycles of semiconductor products.
Many military contractors procure semiconductor devices in advance of project needs once end-of-life notices have been received. This technique puts the burden of cost and storage on the user but requires a crystal ball to determine the project needs far into the future. By working with a strategic SLiM supplier, contractors can avoid a last-time-buy purchase and can postpone the cost outlays until the budget year of the project build.
The first determining factor for device storage is the storage method. Devices can be stored in die/wafer form or as packaged units. There are several advantages to storing devices in die/wafer form, including flexibility, lower cost, more compact storage, and potentially higher reliability.
Flexibility is achieved as die/wafer inventory can be used to satisfy many user requirements for such things as package type, screening level, and speed grades. With packaged devices, the flexibility is lost as these product choices must be made at assembly. The only costs immediately incurred are the wafer cost and the costs of maintaining the storage environment. Other costs such as assembly, test, screening, marking, and packaging are postponed until specific orders are received. Die and wafers stored in wafer storage containers or waffle packs also take up a fraction of the space of finished devices, providing the benefit of reduced space requirements and lower cost.
Finally, and most critically, storage in die/wafer form can reduce the potential failure mechanisms that are inherent in storage of finished devices, especially when compared to devices in plastic packaging. Mishandling of die/wafers can result in corrosion or scratching of the die or bond pads, while mishandling of ceramic packaged parts can result in package cracking, corrosion, or lead solderability issues. In addition to these package issues, plastic devices are also subject to other potential problems due to moisture issues, including delamination, outgassing, and the requirement for repeated testing and repacking. Overall, the risks to product damage are reduced when stored in die/wafer form.
Now that the decision has been made that die/wafer storage is the preferred method, the second critical factor is to develop proper storage facilities that maintain the necessary environment to ensure long-term viability. Successful wafer storage over 10 to 20 years is possible without product degradation.
The proper conditions include a controlled atmosphere (dry, nitrogen-purged storage is preferred), steady-state temperature conditions, cleanliness of the facility (low levels of gases and particulates), and careful handling and proper removal and replacement as wafers are pulled for builds.
With all the proper controls in place to maintain and build from the wafer bank, the remaining issue is planning for the time when the wafer supply runs out. A prepared SLiM provider can anticipate the lowering levels of inventory and undertake product redesign to continue the availability of critical devices for as long as key military and aerospace programs need the devices.