Maybe it's due to "Stress" on the leads of components?
(on the line of: if you bend a lead too often it will break)
Maybe the ultrasonics weaken the leads.

Joe M.

Funny NYPD wrote:
>> but I do know that untrasonic cleaning can cause microscopic 
> pitting on surfaces. 
>  
> My understanding is: it only affects and cleans the outside surface (dirts, flux residual, etc.) on circuit board cleaning process, will the wire-bone inside the chip be pill-off too without liquid bubble contact? That's what really confused me.
> 
> Funny N.
> Au Group Electronics, http://www.AuElectronics.com
> http://www.AuElectronics.com/products
> http://augroups.blogspot.com/
> 
> 
> 
> 
> ________________________________
> From: sergio masci <smplx@allotrope.net>
> To: Microcontroller discussion list - Public. <piclist@mit.edu>
> Sent: Sun, February 7, 2010 8:51:51 AM
> Subject: Re: [TECH]Ultrasonic Cleaning for circuit boards.
> 
> 
> 
> On Sat, 6 Feb 2010, Jonathan Hallameyer wrote:
> 
>> I took a nasa soldering class, same thing  63/37 only, and no ultrasonic
>> cleaning for stuffed boards.   As far as 63/37 only,  the other stuff may be
>> alright, but none of it has been used for 100+ years like lead solder. There
>> may be finally be coming good alternatives, but for a while, lead free meant
>> higher temperatures for soldering.
> 
> It's not so much about temperature as the formation of tin wiskers. It's 
> easy to make low temp solders without using lead. True they are much more 
> expensive than standard tin / lead solder. One of the actions of lead in 
> standard Sn/Pb solder is to inhibit the growth of tin wiskers. These are 
> tiny little fillaments that grow from the surface of tin. They can cause 
> trouble in circuits as they are conductive. Look up tin wiskers.
> 
>>  I'd say lead free was pushed before good
>> alternatives were available, but if it werent pushed, who says they would
>> have been developed... So catch .22 there
>>
>> And you nailed it on the reasoning for the no ultrasonic cleaning.  Our
>> instructor kept it simple.  Some dies are wirebonded using ultrasounds, so
>> no ultrasonic cleaning to be extra sure...   Though I guess you could say
>> some were bonded with just heat, so no heating the leads ;p
>>
> 
> I don't know why ultrasonic cleaning is banned from military or NASA 
> circuits but I do know that untrasonic cleaning can cause microscopic 
> pitting on surfaces. Maybe there is the risk of pitting of the surface of 
> the chip buried inside the plastic package.
> 
> Regards
> Sergio
> 
> 
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