APPLICATIONS ENVELOPE FOR PEMs

Humidity:

Operation in humid environments relative to Al corrosion has been the main concern with PEMs over the years. PEM performance under accelerated biased humidity conditions has improved dramatically since the 1970's, as shown in reference [1] and figure 5. However, the perception of the 1970's still remains in many sectors, even though today's commercial use of PEMs registers in the billions per year, and field experience indicates that corrosion is rarely the cause of failure. Despite the significant improvement in moisture test and field performance, PEMs are still non-hermetic and are thus permeable to moisture. Therefore, they are susceptible to corrosion induced wear-out. Hermetic packages are not expected to exhibit this wear-out mechanism; i.e., if they are sealed in a dry ambient and the integrity of their seals are maintained over life. It is pointed out that a study comparing PDIPs and hermetic CERDIPs exposed to prolonged temperature cycle, followed by bias under 98%RH, showed a 4X higher failure rate for CERDIPs due to moisture induced corrosion [16]. The higher failure rate was attributed to the loss of hermeticity of the glass seals.

In the 1980's it became impractical to perform characterization studies of PEMs under standard 85/85 THB conditions, as their capability was in the thousands of hours (Table 4). Therefore, a more accelerated moisture test called HAST (Highly Accelerated Stress Test) was developed [21-23]. Typically, this test is conducted at 85% RH in the range of 110^oC to 140^oC (JEDEC 22-A110), although the humidity can be varied and higher temperatures have been evaluated. Wear-out characterization performed on PSG (phosphosilicate glass) passivated CMOS Logic ICs, using both HAST and 85/85 THB at 18 volts static bias is shown in Figure 6. This figure demonstrates the increase in median life gained through improvements in mold compound purity and lead frame construction, as well as from the reduction of Cl and other ionic contaminants incurred during the manufacturing process. The activation energy, empirically determined is in close agreement with Peck's 0.9eV [25]. Peck's model [24,25] was used to establish the acceleration factors relative to 85/85 in order to predict the set of curves shown in Figure 7. Extrapolation to 1% cumulative failure was based on an average log-normal s = 0.5 and MTF of 18,000 hours. A voltage acceleration factor for the process [1] was used to extrapolate from 18 volts to 5 volts.

It is cautioned that not all PEMs yield equivalent results on moisture tests. Lifetimes may vary depending on the wafer process, package, and materials used. For example, HAST wear-out data generated at 145^oC/85%RH/18V on silicon nitride/PSG sandwich passivation indicates a 10X longer lifetime than PSG (Figure 8). Thus, Figure 7 may provide a conservative estimate in many cases. Each supplier's data should be reviewed relative to moisture performance.

COMPARISON OF PLASMA ENHANCED NITRIDE (PEN) AND PHOSPHOSILICATE GLASS (PSG) PASSIVATION FOR Al CORROSION

FIGURE 8

The characterization and prediction of long term humidity storage is made difficult by the fact that wear-out distributions can not easily be obtained over a practical period of time without the presence of a bias voltage. Available long term 85/85 storage and THB data are shown in Table 4. The unbiased saturated autoclave test (121^oC, 100% RH, 15 psig) is routinely used to monitor PEMs. Although unbiased autoclave does not correlate well with 85/85 THB, it is a good accelerated test for monitoring lot-to-lot variability. PEMs generally do not have a problem passing the specified durations of 96 to 192 hours (Table 3). However, for optimum results when storing parts prior to system use, it makes sense to keep the ambient humidity as low as possible. Recommended conditions are 55% RH at 30^oC, which is the upper limit of the typical PEM manufacturing environment.

A special case exists with the storage of certain Surface Mount Devices (SMDs) relative to "popcorn cracking", as described previously. Shipment in dry pack containers may be necessary on large, high pin count packages. Moisture sensitive devices are first subjected to a dehydration bake and then placed in a moisture resistant bag, along with a desiccant packet and moisture indicator card. The bag is then vacuum sealed. After opening the bag, the moisture indicator card should always be checked to ensure the seal was not impaired. The parts then should be board mounted within 48 hours, depending on ambient conditions, or stored in an environment which prevents the total absorbed moisture from exceeding 0.1% of the total package weight. Recently released industry standards, such as JEDEC A112 and A113 (moisture sensitivity classification and qualification preconditioning for SMDs), and the proposed JEP113 (Symbols and Labels for Moisture-Sensitive Devices) have established six categories of moisture sensitivity. These range from nonsensitive Level 1 packages (unlimited floor life at 30^oC/90%RH) to extremely moisture sensitive Level 6 (maximum floor life of 6 hours at conditions of 30^oC/60%RH)