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Reliability testing and data analysis of lead-free

solder joints

By John H. Lau [Unimicron Technology Corporation]
T hirty years ago, the author the procedure starts with a design Reliability testing and data analysis

asked key people in solder
This section discusses various aspects
materials, solder mechanics, of the interconnects of a particular of reliability testing and data analysis.
semiconductor integrated circuit (IC)
and solder manufacturing to write a book package with, e.g., the given chip size, the Examples of thermal cycling and drop
chapter in their areas of expertise. The solder alloys, the molding compound, and tests of a fan-out lead-free package are
result was a book called Solder Joint the corresponding printed circuit board also given.
Reliability: Theory and Applications (PCB) and demonstrates that the design is Definition of reliability. In this study,
[1], published in 1991. Today, students, electrically, thermally, mechanically, and reliability of an interconnect (e.g., solder
engineers, and researchers all over the chemically sound. The next step in the bump, solder joint, or microbump) of
world still use the book as their reference. process is for a certain number of samples a particular semiconductor package
In the past 30 years, however, of the sound or reliable design to be built in an electronic product is defined as
soldering technology has changed and tested under certain conditions for the probability that the interconnect
dramatically. In order to comply with the a certain period of time. The test data will perform its intended function
restriction of the use of certain hazardous (failures) are then analyzed and fitted for a specified period under a given
substances (RoHS) regulations, the into a life-distribution designation for operating condition without failure [2-
most challenging one is from tin-lead the interconnects. Next, failure analysis 5]. Numerically speaking, reliability
solders to lead-free solders. Therefore, should be done on the failed samples to is the percent of survivors; that is,
the reliability of lead-free solder joints is find out the root cause and understand R(x) = 1 – F(x), where R(x) is the
under scrutiny. the reason for their failure. In this study, reliability (survival) function and F(x)
Conducting reliability engineering of the focus is on reliability testing and is the cumulative distribution function
lead-free solder joints consists of three data analysis. Emphasis is placed on (CDF). Life distribution is a theoretical
major tasks: design for reliability (DFR), acceleration models and factors. Some population model used to describe
reliability testing and data analysis, and recommendations in these areas will also the lifetime of an interconnect, and is
failure analysis (see Figure 1). Usually, be proposed. defined as the CDF, that is, F(x) for the
interconnect population. Therefore,
the one and only way to determine the
interconnect reliability is by reliability
testing to determine the F(x).
Objective of reliability testing. The
objective of reliability tests is to obtain
failures (the more, the better) and to
best fit the failure data to determine
the parameters of the CDF of a chosen
probability distribution (e.g., Weibull).
The number of items (i.e., sample size)
to be tested should be such that the final
data are statistically significant. The
reliability test time is unknown, but
usually takes a while (e.g., a few months
for thermal cycling tests). It should be
noted and emphasized that as soon as the
life distribution F(x) of the interconnects
is estimated by reliability testing, the
reliability R(x), failure rate, cumulative
failure rate, average failure rate, mean
time to failure, etc., of the interconnects
are readily determined [2-5].
Most reliability tests are accelerated
tests, with increased intensity of exposure
Figure 1: Reliability engineering: design for reliability, reliability testing and data analysis, and failure analysis.

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