Moisture Prevention for Packaged Semiconductors

Moisture has always been a bugaboo in the backside of electronics systems. Whether in ambient working environments, shipping through humid tropical regions, outright liquid spillage or a myriad of potential mishaps, moisture is responsible for adding significant cost to electronics. Given this reality, electronics manufacturers must weigh the cost of prevention measures against potential disasters of nightmarish proportions.

In traditional semiconductor packaging, judicious materials selection, carefully controlled assembly environments, innovative handling systems, and shipping with dessicants inside hermetically sealed bags and containers has kept moisture problems to a minimum. Yet many plastic compounds and organic materials used as package substrates or package encapsulants are still permeable and allow moisture to enter. Today, an up tick in high-reliability products for military, aerospace and sensitive industrial applications, plus a growing concern over elevated temperatures for lead-free processing, has placed renewed emphasis on moisture control among semiconductor package providers and their customers.

The basic trouble with moisture absorbed inside semiconductor packages is that it builds up pressure when rapidly heated during solder reflow attachment to PCBs. This highly accelerated expansion, coupled with differing rates for coefficient of thermal expansion (CTE) of package construction materials, may create pressure forces that the package cannot withstand. Although present during eutectic solder operations, insignificant moisture amounts were acceptable at 180-200°C. However, any moisture presence during lead-free processes in the range of 230-260°C can build vapor pressure sufficient to cause mini-explosions (popcorning) or materials delamination that destroy the package.

Some industry associations and individual companies have recognized the heightened moisture alert, and have begun to develop specific industry guidelines for controlling moisture in electronics manufacturing. In 1999, the Association Connecting Electronics Industries (IPC, Northbrook, Ill.), along with the Joint Electron Device Engineering Council (JEDEC, Arlington, Va.), released J-STD-033A — Standard for Handling, Packing, Shipping, and Use of Moisture/Reflow Sensitive SMDs. Later in 2002, that standard was revised to include sections on bake process and dry storage. The content of these documents provides guidelines for handling, dry packing and storage, baking and tracking components to control moisture absorption prior to the package arrival in a production setting.

According to this group, "The risk of failure during reflow is directly related to the concentration of moisture at the critical interface, which is near the center of the package." Since the maximum acceptable moisture content and rate of moisture diffusion vary by package, each new device must be categorized based on how long it takes to absorb a critical level of moisture in a standard production environment. Even veteran industry practitioners are often puzzled about moisture concerns, since lower process temperatures in the pre-lead-free era ensured rare encounters. However, with the advent of lead-free processing awareness, the relatively recent standards documents are sure to get close scrutiny by semiconductor packaging professionals.

The red and yellow areas indicate delamination/cracks within the BGA caused by excessive moisture expansion during the reflow process. (Source: Sonoscan Inc.)

Following the joint IPC/JEDEC collaboration on standards, the Surface Mount Technology Association formed its Moisture-Sensitive Device (MSD) Council, also to promote control of moisture in electronic components. As downstream recipients of packaged semiconductors, PCB assemblers will be on the frontline of controlling reflow processes, and must work closely with suppliers to ensure delivery of moisture-free packages. In fact, they often employ moisture tracking systems equipped with RF tags, local control units and proprietary software to monitor moisture content from package assembly, to shipping/handling to production floor assembly operations.

The degree of moisture in a package, coupled with elevated lead-free processing reflow temperatures and the dissimilar materials in semiconductor packages, poses serious issues that must be considered when mounting the packages on PCBs. Also remember, with two-sided assemblies, some packages will pass through the reflow process twice. Why risk failure with moisture-laden packages when the moisture can be controlled by following available industry guidelines? For more information on controlling moisture in electronics manufacturing, visit www.ipc.org or www.smta.org/msd/msd.cfm.

For additional information on assembly and packaging, go to www.semiconductor.net/assembly .