Accelerated testing is a complex subject. Much confusion exists in the
industry, because many tests, that have quite different purposes, are called
'accelerated tests'. The main differentiation in purpose is between product
screens and testing of prototypes/test vehicles. In a previous TechNet
discourse I wrote:
Product Screens:
Burn-In: Is a functional test for some time (typically 8 to 24 hours)
involving perhaps worst case but still realistic operational environments.
Environmental Stress Screening (ESS): A screening procedure involving
environmental stresses of limited duration, such as thermal cycling, thermal
shock, vibration, mechanical shock, etc., designed to precipitate specific
suspected 'latent defects' or product weaknesses to failure (so these
failures can be detected) without causing significant damage to good product.
Highly Accelerated Stress Screening (HASS): A more aggressive screening
procedure involving environmental stresses, such as thermal cycling, thermal
shock, vibration, mechanical shock, etc., designed to precipitate specific
suspected 'latent defects' or product weaknesses to failure more quickly than
ESS. Higher acceleration means higher stress levels which can translate into
causing significant damage to good product.
Accelerated Tests for Test Vehicles:
Accelerated Reliability Test: This is a test on test vehicles designed to
produce failures with the same damage mechanism product would experience in
the field on an accelerated basis. The purpose is to create a data base from
which product reliability in the field can be estimated.
HALT (Highly Accelerated Life Testing): This is a test on product prototypes
to cause failures with very high stress regimens, but somehow still related
to the operational environment of the product. The damage mechanisms employed
in HALT, and thus the failures, may or may not have any connection of the
experiences of product in the field. This is an attempt to do an accelerated
reliability test quickly, which is can lead in many cases to misleading
information, because the high acceleration has brought about damage
mechanisms and/or material behavior that are not pertinent for product
reliability.
HAST (Highly Accelerated Stress Testing): This is a test on product
prototypes to cause failures with very high stress regimens which show the
least robust portions of a design. By beefing up the observed failure sites
(often successively) a more robust product is arrived at. The damage
mechanisms employed in HAST, and thus the failures, may or may not have any
connection of the experiences of product in the field.
>From your question, I assume that you are primarily interested in Accelerated
Reliability Testing, where the results can be related via some acceleration
transform yielding an acceIeration factor to some equivalent life at actual
operating conditions. For this purpose, accelerated testing requires:
(1) that, in fact, the same damage mechanism active during product operation
is dominant during accelerated testing,
(2) that the acceleration of this damage mechanism by some appropriate change
in the operating conditions to the test conditions can be expressed in an
acceleration transform, which is known.
Both of these requirements are frequently not present.
However, IPC-SM-785, 'Guidelines for Accelerated Reliability Testing of
Surface Mount Solder Attachments' does provide in Table 2 equivalent test
cycles for a variety of service lives. The equations to generate equivalent
service lives and test durations for other conditions are stated.
Temperature cycling -55 to 125oC is appropriate for bare boards to accelerate
the failures of PTHs, but not for assemblies, because these test conditions
generate dominant failure mechanisms absent in most field operations.
For the temperature/humidity test conditions that you stated, you need to
differentiate between SIR and CAF failures. There are some acceleration
transforms available for SIR, but experimental activation energies observed
vary significantly. For CAF, difficulties in finding a correlation from
failures at high humidity conditions to typical failure conditions, has led
to the postulation of some threshold condition. Thus, there can not really be
a satisfactory answer to your question. I would, however, suggest a look at
Engelmaier, W., and L. Turbini, "Design for Reliability in Advanced
Electronic Packaging," Proc. Surface Mount International Conf., San Jose, CA,
August 1995, pp. 844-879, which deals with the underlying issues.
Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting
23 Gunther Street
Mendham, NJ 07945 USA
Phone & Fax: 201-543-2747
E-mail: [log in to unmask]
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