APPLICATIONS ENVELOPE FOR PEMs

Temperature:

Ambient operating temperatures specified for PEMs fall within the range of -55^oC to +125^oC, with some rated at these outer extremes. Individual devices may require more reduced ambient temperatures, depending on electrical characteristics, power dissipation, or specific package constraints. Other ranges typically specified are: -40^oC to +125^oC, -40^oC to +85^oC, and 0^oC to +70^oC. The maximum junction temperature for PEMs is +150^oC. High temperature operating life (HTOL) performed on PEMs is usually conducted at ambients of 125^oC to 150^oC for a minimum of 1000 hours. In some cases, PEMs can be stressed at 175^oC for shorter durations, as shown in Appendix Table A3. Operation of PEMs in dry ambients, within device ratings, generally yield results comparable to hermetic parts, as discussed previously.

The storage temperature range usually specified for PEMs is 65^oC to +150^oC. As mentioned previously, certain devices may require reduced storage temperatures, depending on temperature cycle limitations, or other package constraints. Routine 1000 hour storage data collected at +150^oC on PEMs show no issues (Table 2). Temperatures exceeding the glass transition temperature (Tg) - typically 155^oC to 165^oC - of most mold compounds may result in leaching out impurities over time, particularly from the flame retardants, which may not otherwise occur. In addition, the TCE of the mold compound increases dramatically above the Tg threshold. Therefore, it may be inappropriate to extrapolate test results obtained above 150^oC down through the Tg [18]. The primary failure mechanism associated with high temperature storage is Au-Al intermetallic degradation, which results in weakening the Au wire attachment to the Al bond pad. Reference [19] reports an activation energy of 0.8 eV for this mechanism in the presence of the mold compound, from which the Arrhenius acceleration factors can be derived. It has also been reported that this mechanism is unlikely to occur under 150^oC, the upper storage limit of PEMs, as the Au-Al reaction threshold is above 150^oC [17].

The acceleration factors for temperature cycle often used are based on the Coffin-Manson relationship [12], which equates the acceleration factor to the absolute change in temperature at stress divided by that at use, all raised to the power n. Although n can vary, a value of n=4 is generally used [20]. This relationship is mostly applicable to bond wire fatigue failure and may not hold for all temperature cycle failure mechanisms [12]. At Intersil PEMs are qualified for 1000 cycles at either -65^oC to +150^oC (automotive) or -40^oC to +125^oC (industrial). Figure 4 plots the equivalent number of cycles for both these conditions at lower delta temperatures, using the model described.