| Created: 5/21/95 |
Updated: 4/18/97 |
Use of a Stress-margin Approach to Study Failures in PWBs
A. Dasgupta, G. Ganguly, B. Mathieu, S. Bandarkar, R. Agarwal
Point of Contact: dasgupta@calce.umd.edu
Objectives
Establish a systematic methodology to
precipitate failures in printed wiring boards with
simultaneous combinations of highly accelerated
temperature cycles and 6-axis random
vibrational loads. The purpose is to validate
design models, uncover design flaws, and
ruggedize the prototypes through design and/or
process changes. The entire process of using
stress margins to improve the intrinsic reliability
of the product will be systematically developed
through experimentation and simulation.
Background
There is significant controversy in the literature
regarding the validity of accelerated testing and
the methodology to extrapolate failure
information to predict field failures. In this
study, the aim is to establish scientific limits for
failure precipitation by highly accelerated
stresses, without altering the failure
mechanisms. The role of such testing in
successive product ruggedization needs to be
demonstrated. Finally, a methodology needs to
be documented for combining physics-of-failure
simulations with the accelerated test data to
obtain a comparative estimate of product
ruggedness, relative to other successful products
of the manufacturer. The goal is thus a relative
estimate of the product's reliability, rather than
an absolute reliability prediction through some
empirical acceleration transform. The crux of
this method is to devise a test that can be
performed cost-effectively within a few hours to
audit the quality and reliability of a product on
a relative scale.
Approach
An experimental setup has been established at
CALCE EPRC to apply simultaneous
combinations of highly accelerated thermo-
mechanical and vibrational loads to PWB test
articles. Failure stimulation facilities include a
QUALMARK OVS-1 combined 6-axis random
vibration and high-rate temperature test chamber
and an MTS high-strain-rate servohydraulic test
frame. Failure detection equipment includes a
128-channel ANATECH event detector. Failure
analysis equipment includes optical and
scanning electron microscopes, environmental
scanning electron microscope, infrared
microscope and a C-scanning acoustic
microscope.
The initial test program includes a hybrid serial
interface module from two vendors and a
current-mode coupler from two vendors. These
test articles were supplied under funding from
Boeing. The goal is to apply accelerated
stresses to establish the destruct limits (if any)
of these test articles. These destruct limits will
be compared later to those of existing packages
known to have acceptable reliability, and to
failure mechanisms in similar articles being
tested.
Work Accomplished
- The experimental facilities have been
established.
- The Qualmark test chamber has been
instrumented and characterized in terms of
its vibrational characteristics.
- The specific load history to be applied to
these test articles has been developed and
approved by Boeing and the respective
product manufacturers. Preliminary
exchanges have occurred with the vendors to
examine the designs of the test articles and
to consider fixture designs. However, none
of the test samples have yet been received at
CALCE. At Boeing's request, the test
sequence has been postponed to the last
quarter of 1993. The funding received in
the '92-'93 year has been allocated to
complete this test program.
