Hi George,
Wished it were that simple.
First, IPC/J-Std-001 does not have a set reliability goal, not even a
"fictionalized targeted reliability goal". In fact, it does not address
reliability at all; for that you need to go to IPC-D-279, Design Guidelines
for Reliable Surface Mount Technology Printed Board Assemblies. The different
IPC Classes categorize different perceived quality levels, but depending on
the damage mechanism we are talking about, the measureable reliability
difference between the Classes may be small or even non-existant. For solder
joints the quality stressed in IPC/J-Std-001 is quite important for loading
conditions like high-level vibration and/or mechanical shock, but for thermal
cyclic loaded solder joints only proper wetting is really important, excepting
grossly defectve solder joints, of course.
Second, there are in fact results from studies that show for at least chip
components one can actually get a reliability improvement by having
displacements according to Class 2 over the tighter requirements of class 3.
Third, given an assembly with "a slightly off-center 20 mil pitch component,
ie 1/3 of the lead width," my recommendation would be to leave this
compomemt(s) strictly alone. With the very compliant 20-mil-pitch leads even
severe vibrations or mechanical shocks will not affect the solder joints (the
leads may break off at the shoulder, however), and the risk of damaging the
PCB or the solder joints of adjacent components is not worth it, given only a
violation of a paper requirement without technical foundation; Class 3 is a
hang-over from the old now (and previously) obsolete MIL-specs.
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask]
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