A new method for equivalent acceleration of JEDEC moisture sensitivity levels

Abstract

In order to devise an equivalent accelerated moisture sensitivity test, the JEDEC specification J-STD-020C has recommended an accelerated preconditioning time of 40 hrs exposure under 60degC / 60% RH, which is considered equivalent to the standard moisture sensitivity level 3 (MSL-3) of 216 hrs soak time under 30degC / 60% RH. However, the existing methodology for the accelerated moisture sensitivity test was developed based on the equivalency of local moisture concentration at the interest of location for leaded packages only. The failure mechanism is restricted to the potential delamination between mold compound and leadframe. In addition, such an equivalency requires the activation energy of molding compound for moisture diffusion in the range of 0.4 - 0.48 eV. This paper introduces a new method to accelerate JEDEC/IPC moisture sensitivity level testing. The methodology is developed based on the equivalency of both local moisture concentration and overall moisture distribution of packages. The local moisture concentration equivalency ensures identical adhesion strength and vapor pressure at interfaces of the interest, and the overall moisture distribution equivalency results in the same condition of applied driving forces, such as thermal and hygroscopic stresses, during reflow. In our previous study (Xie et al., 2007), this methodology was applied to a molded matrix array package, and an accelerated soak time subjected to 60degC / 60% RH was established. In this paper, the further reduction of soak time using 85degC / 60% RH is investigated. An ultra-thin stacked-die chip scale package (CSP) is used as the test vehicle. Extensive experiments have been carried out to obtain the failure rate as function of soak time under various environmental conditions. Finite element analysis was performed to obtain the equivalency conditions. According to finite element modeling results, it has been found that, at 70hrs under 60degC / 60% RH and 45 hrs under 85degC / 60% RH, respectively, both the local moisture concentration at critical interface and overall moisture distribution of package become identical to that under the standard MSL-3. Such an equivalency of the new accelerated test conditions has been proven by the test results. Failure site and failure mode indicates that the proposed accelerated tests are well correlated with the standard MSL-3. The new methodology can be extended to other packages.