| Created: 11/2/96 |
Updated: 4/18/97 |
Project Number: C97-04
Effect of Delamination and Cracking on PEM Reliability
Point of Contact: Dr. Pat McCluskey
email: mcclupa@calce.umd.edu
Phone: (301) 405-0047
Fax: (301) 314-9269
Objectives
To determine if there is any correlation between pre-existing delamination
and time-to-failure by moisture-related failure mechanisms.
Background
Delamination in plastic encapsulated microcircuits (PEMs) can occur during
manufacturing, assembly, or use. Perhaps the most common source of delamination
is ôpopcorningö during the solder reflow phase of assembly.
In this instance, delamination occurs when the stresses resulting from
the vaporization of absorbed moisture together with those resulting from
mismatches in the coefficients of thermal expansion of the various package
materials exceed the adhesion strength at one or more package interfaces.
Previous studies at the CALCE EPRC (C95-23) have shown that it is possible
to control the amount of delamination by controlling the reflow ramp rate.
The question of whether delamination is a serious reliability concern
is critically important in electronic systems which have been fielded for
several years and are now being certified for continued use. Often delamination
is observed in systems which was not seen when they were originally fielded.
In order to recertify these systems it is necessary to know if this delamination
will have a detrimental effect on the reliability of the system, in order
to determine if delaminated parts which show no other signs of degradation
need to be replaced.
Previous studies have shown that delamination at the top surface of
the die can degrade the reliability of PEMs by increasing the shear stress
on the wirebonds. This can lead to lifted ball bonds, broken wedge bonds,
and sheared bond wires either immediately upon delamination or with repeated
temperature cycling.
There is still controversy, however, concerning the effect of delamination
on the susceptibility of PEMs to moisture-related failure. Some tests have
shown a correlation between delamination and time-to-failure by metallization
corrosion, while others have indicated no correlation. In addition, studies
conducted at the CALCE EPRC involving the degradation analysis of stored
parts (C96-31) have not identified a correlation between delamination and
corrosion in parts fielded for ten years or more. In this project we will
definitively determine whether there is a greater susceptibility for moisture-related
failure in PEMs with pre-existing delamination.
Approach
-
Introduce controlled amounts of delamination into PEMs by preconditioning
them at various IR reflow ramp rates.
-
Quantify the amount of delamination and cracking with C-SAM
-
Gravimetrically determine the relative moisture absorption of packages
with varying levels of delamination.
-
Electrically test samples for functionality
-
Expose the samples to a highly-accelerated temperature-humidity environment
(HAST) of 140 C, 85% RH for 1000 hours.
-
Electrically test samples for functionality every 100 hours.
-
Conduct failure analysis
-
Plot the failure distributions and determine the delamination-related acceleration
factor.
-
Conduct degradation analysis on selected unfailed samples including bond
shear testing to determine wirebond strength.
Work Accomplished
Correlation between delamination and moisture absorption is detailed in
"The effect of delamination on moisture-related failure of PEMs"
Correlation between delamination and time-to-failure in unbiased HAST
is detailed in "The effect of delamination on moisture-related failure
of PEMs"
