>From the Electronic Manufacturing Productivity Facility in Indianapolis,
prepared by Tim Crawford:
Phil,
To settle the argument and make everyone happy, they're all right. The
thing about ultrasonic cleaning, as we all know, is that transducers
outside the tank "vibrate" the walls of the tank causing positive and
negative pressures in the solvent. These positive and negative pressures
in turn cause microscopic air bubbles to expand and contract giving the
cleaning process that extra "scrubbing" action. Air bubbles eventually
reach their resonant frequency and implode, causing an additional cleaning
power. The best analogy of resonant frequency is that of a tire on your car
when it is out of balance. Everything is fine until you get to 46 MPH then
it feels like your whole car is ready to implode.
Now let's back up to the electronic component. The same resonant frequency
that causes air bubbles to implode CAN also cause delicate wire bonds to
vibrate, fatigue and break. I stress "CAN" because there are many
variables that influence the resonant frequency of a wire bond. Wire
length, wire diameter, wire material, wire geometry (Z height of wire
connection), package material, package type and of course, the integrity of
the bond itself. The assembly that the component is being mounted onto can
also influence the resonant frequency. Board material, board thickness,
population density, even where the component is located on the assembly.
The problem is that there is no way of knowing what the resonant frequency
of every wire bond, in every component, of every assembly. To say that
this one component should not be cleaned using ultrasonic energy is hard
because it is very possible that it CAN be cleaned in a different system or
a different solvent or on a different assembly.
The military banned the use of ultrasonic energy back in the 1950's when it
noted that ultrasonic energy seemed to be contributing to component
failure. Some important notes to that are the fact that wire bonding
techniques were not as good then as they are today and, based on the
technology then, cleaning without ultrasonic energy was acceptable. It was
kind of an "If it's damaging the components and we don't need to do it,
don't do it." kind of thing. But as technology progressed, standoffs got
smaller, leads closer, populations denser, the need was increased. In
addition, wire bonding techniques improved and our understanding of
ultrasonic energy has increased.
The bottom line is that ultrasonic energy can safely be used in most cases,
but you cannot blanketly state that it is safe. Nor can you blanketly
state that it is unsafe on this component. In-house testing must be done
on actual product to determine impact on reliability. Clean some
assemblies with ultrasonic energy and clean another group of assemblies
(same solvent, time, temperatures) with ultrasonic energy off. Test them
functionally, then subject them to ESS tests to see if the components have
been fatigued. Delid the components, if possible, and do pull tests on the
wire bonds themselves. Good luck.
>----------
>>>Resent-Date: Fri, 6 Jun 1997 13:27:00 -0500
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>>>From: [log in to unmask]
>>>Date: Fri, 6 Jun 1997 14:23:14 -0400 (EDT)
>>>To: [log in to unmask]
>>>Subject: Ultrasonic Cleaning
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>>>
>>>I have a argument going on ultrasonic cleaning of PWA's with different types
>>>of component.
>>>
>>>Several opinions have come up:
>>>(1) Never ultrasonic clean if you have hybrids with wire bond connections.
>>>(2) Hybrids are OK to clean in an ultrasonic if an encapsulant is used.
>>>(3) Hybrids are OK to clean if they supplier follows standard wire bond
>>>requirements.
>>>(4) Do any of the above rules apply to multichip modules.
>>>(5) Do Ball Grid Array fall in any of the above categorties.
>>>
>>>Phil Hinton
>>>[log in to unmask]
***ALL NEW EMPF PHONE NUMBERS***
Jack Crawford, HelpLine Coordinator
Electronics Mfg. Productivity Facility
****NEW--317.655.3688
****FAX 317-655-3699
714 N Senate Ave, Suite 100
Indianapolis IN 46202-3112
VISIT OUR HOME PAGE AT: http://www.empf.org
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