Ultrasonic cleaning is just fine for most of the parts in use today. The
original "ban" on U/S cleaning grew out of problems with metal TO-5 cans and
similar packages used in the military, combined with 25 kHz sonics. With the
change to plastic encapsulated parts, which can absorb most of the U/S, and
milder sonics (either 40 kHz or sweep frequency  sonics) the problem is
rarely encountered today.

IPC has two test methods for assessing ability to stand up to the enhancement
of milder cleaning agents offered by sonics.
-method 1 is used to assess the durability of parts loose in a basket.
-method 2 is used to assess the durability of parts soldered (restricted) to
a pwb.
Check you IPC TM 650 for these methods. The U/S task group, chaired by Erin
Yeager (now at Loctite), did this work.

For background, there were two studies undertaken to take a fresh look at
sonics:
The first was started by the US Navy at China Lake/EMPF, under Kathi
Johnson's direction. The second was was a series of papers that were
published by GEC Hirst on families of components. As I recall, there were
about five papers in that series. The rule of thumb that emerged from the GEC
work was that just about all the components would take at least 10x the
typical exposure time seen in a typical cleaning cycle without any problem.
Brian Richards, now at NPL, was the driving force for all these studies,
which were presented at China Lake. IPC should have copies of  all the GEC
reports (unclassified) prepared by Dr Richards.

The use of sonics as an incoming inspection tool was documented by S. Stach
(founder of AAT) Jonas Alexonnis in a paper at NEPCON East. To conserve space
in state of the art pacemakers, ICs were piggybacked on the PWB, then
soldered in place. The PWAs then were subjected to an ultrasonic cleaning
regimen prior to shipment to the customer. Any infant mortality in the ICs
was weeded out in the post-cleaning testing--- which is preferable to the
alternative method of determining relability after implanting the pacemaker
in the customer!

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