William D. Bjorndahl

Benefits of a Bayesian approach to anomaly and failure investigations

It is often the case in failure and anomaly investigations that data is either limited or so wide ranging that it is difficult to bring focus to a key root cause. For this reason, a disciplined approach incorporating root cause trees (Ishikawa Diagrams) is usually taken to develop and track root cause hypotheses and analyses. During the investigation, statistical tools can be used to evaluate various hypotheses of failure. However, in many cases, there is limited failure data and it is often necessary to set up accelerated life tests involving many samples in order to induce failures under controlled conditions so that a statistically significant population of failures can be obtained. Root cause is sometimes achieved only after extensive and expensive efforts to reduce the number of root cause hypotheses. Other times, root cause investigations are truncated to “most probable cause” based on the evidence available and expert opinion.

Package size and epoxy mass effects on package hermeticity requirements

Hermetic packages are required to protect sensitive electronic components from atmospheric constituents. The primary constituent of concern is water. Hermeticity requirements for large sealed assemblies are somewhat difficult to address because for normal leak rates, long bomb times are required in order to get enough helium into the package so that during leak rate measurement, a detectable concentration of helium comes out. Additionally, because many packaging houses seal in an atmosphere containing a significant partial pressure of helium, application of the current specification inadvertently requires larger packages to be more hermetic than smaller ones. Logically, small packages should have more stringent leak rate requirements than large packages, since for an equivalent leak rate the time to reach a given concentration of moisture in a larger package is longer. The trend in hermetically sealed assemblies has been to larger sizes. Examples include multi-chip modules (MCMs) and chip-on-board assemblies. This paper illustrates the inconsistencies which can occur when performing leak measurements on large packages. Also, the effect of gettering by moisture absorbing substances is evaluated both analytically and by examining residual gas analysis test results. It is found that moisture gettering by epoxies delays considerably the time for moisture build-up.

Robustness of advanced surface mount technology for space applications

Advanced surface mount technology, which includes ball grid array packaging, is quickly becoming a preferred technology for ground based applications. Compared to leaded quad flat packs, ball grid array packaging offers increased manufacturing yields, reduced parasitics and reworkability. Reliability for space applications is an issue which is being aggressively worked. A methodology for examination of various part type/size/board combinations in terms of thermal cycle reliability has been previously developed (1997). Important factors in determining this reliability are: thermal expansion differences between component packages and the printed wiring board (PWB), processes and materials used in creating the interconnect, and the overall package size dimensions. This paper reviews literature data on the reliability of the advanced surface mountable package/board level interconnect. Experimental data are presented to indicate the importance of board level design and package selection in order to effect optimal reliability. The experimental results were fit to a two parameter Weibull model. The boards containing a constraining core provided a factor of 2 increase in thermal cycle robustness over the conventional glass-epoxy/copper board. The results indicate that the material stack-up within a PWB can have a significant effect on surface mount reliability.