Actually no, the thing about ultrasound is that at the sort of impedance
mismatch you are looking at here the realitty is that at a package to air
interface you have a virtual 100% reflection. Not sure that those
ultrasonics units are set up to use through transmission mode though,
although a couple of hours with a soldering iron and hmmmmmmmm.
Don't forget that in transmission mode all you are going to see are a number
of weaker or stronger signals delayed by the speed of ultrasound
transmission through those various materials so all you are looking for
basically is a signal.
We used a similar technique back in the '70's on the space shuttle where we
transmitted ultrasonic energy into a liquid filled bearing which was holding
a silicone rubber based wheel( imagine a skateboard wheel with the center of
the bearing filled with liquid and a probe attached to the outer sleeve of
the bearing transmitting into the liquid). We had a pair of these set up
with one set up to transmit and one to receive. The units were set up about
2 inches apart (Imagine half a roller skate with VERY soft tires) and then
ran them over the tiles of the shuttle. Once the signal disappeared you had
an air gap QED.
So in this case the transmission was from the probe, through the liquid,
into the Silicone rubber, through the tile into the skin of the shuttle
where it reflected off of the back of the outer skin in the direction of the
second probe, which when it hit it picked up the signal which was
transmitted from the skin, through the tile into the silicon rubber into the
liquid and onto the receiver.
This technique is actually very common in non destructive testing for
checking the integrity of sections of aircraft wings for delamination of the
honeycombe structure and for checking for water ingress in wing sections due
to pinholes in the outer skin.
John Burke
(408) 515 4992
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bev Christian
Sent: Monday, September 21, 2009 3:17 PM
To: [log in to unmask]
Subject: Re: [TN] Acoustic Microscopy for Under fill Coverage
John and others,
But wouldn't it be even worse trying to use transmission mode because of the
nature if the circuit board underneath the component (copper barrier(s),
glass fiber bundles and epoxy?
Nobody from Sonotek or Sonoscan on TechNet?
Bev
----- Original Message -----
From: John Burke <[log in to unmask]>
To: 'TechNet E-Mail Forum' <[log in to unmask]>; Bev Christian
Sent: Mon Sep 21 18:00:16 2009
Subject: RE: [TN] Acoustic Microscopy for Under fill Coverage
Not a discontinuity as such just an interface, and you are right, the
signal reflections will be from every interface, so for a BGA (without a
lid) the interfaces will be as follows:
1 Liquid to top of package molding compound
2 Molding compound to silicon
3 Silicon to die attach
4 Die attach to substrate
5 4 or so interfaces in the substrate itself (copper to substrate
pre-preg)
6 Substrate to under fill (or NOT!)
Depending of the acoustic impedance between the Material travelling
from/Material travelling to, more or less ultrasound energy will be
reflected at each of the interfaces.
Of the layers above I believe that the strongest reflection will be from
level 6 above.
It is 30 years since I last played with non destruct testing using
ultrasonics, but I would bet from my experiences then that you would do far
better using through transmission for this scenario rather than reflected
signals due to Bev's comment, particularly due to the substrate interface
reflections masking the ones between the package and the under fill.
John Burke
(408) 515 4992
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bev Christian
Sent: Monday, September 21, 2009 2:07 PM
To: [log in to unmask]
Subject: Re: [TN] Acoustic Microscopy for Under fill Coverage
Since when can an acoustic signal punch through a BGA? I know you can do if
for flip chips, but aren't there too many density discontinuities for CSPs
abd BGAs?
Bev
----- Original Message -----
From: TechNet <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Sent: Mon Sep 21 14:40:48 2009
Subject: [TN] Acoustic Microscopy for Underfill Coverage
Anybody know a good lab in the North East that does acoustic microscopy? I
need to verify under fill coverage under BGAs.
Thanks,
Amol
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