Hi Jim
In a message dated 10/08/97 23:17:27, you wrote:
>I enjoy reading your comments on the IPC Technet and I've read quite a number
>of your articles and documents about solder joint reliability.
>I have just done an extensive study of HALT/HASS testing originated by Mr.
Hobbs
>and it seems that he is telling me things that contrast with your research
and
>suggestions.. For instance..IF I understand, he says it's OK to HALT a
product at >the highest ramp rate possible, maybe 60 degC/min. BUT IF I under
stand your >document on accelerated reli test..you suggest a test ramp in line
with the end >use environment and actually 30 degC/min.+ is classed as a
thermal shock (havoc >with thermal mismatches!). I find what I think is quite
a contrast of opinions on >test stimulus.
>Question: How do you feel/opinion about HALT/HASS testing?
>Should it be applied? How?
>I appreciate any info you give and highly value your opinions!!!

Apparently Mr. Hobbs is spreading his gospel vigorously; I have heard a number
of comments/questions regarding his message recently. Unfortunately, Mr. Hobbs
(or his students) does apparently not discriminate between applications for
which his philosophy is valid and those for which it is not.
For solder joints, HALT is likely giving you results that do not correlate
with the loading conditions your product sees in the field. Further, even if
you were to redesign your product to increse the survivability in a HALT test,
you may not increase (and perhaps decrease) the reliability of your product in
the field. Take as an example the famous -65<->+125C cycling misapplied to
electronic assemblies; to survive these testing extremes, it is necessary to
virtually match the CTE's of components and substrates. Real product has power
dissipation with components operating at (sometimes significantly) higher
temperatures than the PCBs; conditions for which a larger PCB CTE will give
you improved reliability. This, of course, does not even touch on the fact,
that in a  -65<->+125C shock-type HALT you are confounding a number of damage
mechansms not significantly active (or even present) in real product
operation.

Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting
23 Gunther Street
Mendham, NJ  07945  USA
Phone & Fax: 973-543-2747
E-mail: [log in to unmask]

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