O.K. all you nay-sayers, I'll back up Paul's statement. Based on
internal data, a single digit DPM failure rate on BGA's was finally
traced back to our 40kHz "Sweep Frequency" 500W transducer stencil wash
used to clean misprinted boards. It took several years, numerous DOE's,
and millions of assemblies to finally trace the root cause to the
ultrasonic cleaner. One of the reasons the cleaner was overlooked for
so long, is that we still don't have a working theory of the physics
behind the failure. PARTICULARLY because "bare boards", without any
components, that are washed in the ultrasonic cleaner will end up with
broken solder joints on BGA's. The failure mode is separation of the
BGA land pad from the laminate and it is extremely dependent on the land
pad size, as well as other dependencies such as package design and PCB
layout. We don't see the effect when the land pad is larger than
0.29mm. And yes, we are very aware of all the other confounding effects
that also cause pad separation when the pad size is smaller than 0.29mm.
We suspect there is dependency on proximity to the transducer, but we
ultimately decided it would be easier to just increase pad size and not
wash boards where we couldn't increase pad size, than pursue another
large sample study. (We can turn the defect on and off, at a DPM level,
on specific PCB layouts, by washing with ultrasonic and not washing with
ultrasonic). Because of the pad size dependency, the low DPM
occurrence, and design dependencies, I suspect all the "other" published
studies would never find this failure mode.
-Ryan
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Wednesday, August 03, 2005 6:39 AM
To: [log in to unmask]
Subject: Re: [TN] Question on cleaning boards with BGA.
Hi Paul! What is the physics behind the ultrasonic cleaning system
causing
cracked BGA solder joints? Is your comment based on published data or
internal data?
Dave Hillman
Rockwell Collins
[log in to unmask]
Paul Edwards
<paul@SURFACE-ART
.COM>
To
Sent by: TechNet [log in to unmask]
<[log in to unmask]>
cc
Subject
08/02/2005 09:41 Re: [TN] Question on cleaning
PM boards with BGA.
Please respond to
TechNet E-Mail
Forum
<[log in to unmask]>
; Please respond
to
Paul Edwards
<paul@SURFACE-ART
.COM>
Never use an ultrasonic cleaning system with BGAs unless you want to
generate cracked solder joints...
Assuming your ECN is at the former label location which is why you need
complete adhesive removal... find out from your label supplier what type
of adhesive they are using and have them suggest something...
We use an alcohol/glycol/water mix to remove most label adhesives...
Paul Edwards
Surface Art Engineering
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David Harman
Sent: Tuesday, August 02, 2005 4:58 PM
To: [log in to unmask]
Subject: [TN] Question on cleaning boards with BGA.
We are remanufacturing (reworking some of product) due to an ECR
release. We apply a label with non-conductive adhesive that once
removed leaves adhesive on the PCBA. This adhesive prevents the board
from passing due to not making contact. The current process requires an
operator to use IPA and a soft scrub to remove it. This process is time
consuming. My CM has suggested that we ultrasonic the boards using S-10
solvent.
My boards have BGAs, interconnect pins, and switches along with the
normal SMT parts. Will using this s-10 solvent in an ultrasonic cleaner
work, and will it not cause any reliability issues with BGAs or other
parts on the boards?
Anyone have any other suggestions for auto cleaning my boards to remove
the adhesive. We use the no clean process.
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