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From:
"Stadem, Richard" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard
Date:
Thu, 4 Aug 2005 09:03:35 -0500
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Yes. That is a good idea. Certainly something to think about. If a pwb is going to go through a process, then it should be qualified using the process. 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dehoyos, Ramon
Sent: Thursday, August 04, 2005 8:20 AM
To: [log in to unmask]
Subject: Re: [TN] Question on cleaning boards with BGA.

 



        This leads me to believe that perhaps a USC process not as a cleaner but as a test of how good the board was fabricated can be a part of a standard test.   Just a thought.
        Ramon




	

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard
Sent: Thursday, August 04, 2005 8:36 AM
To: [log in to unmask]
Subject: Re: [TN] Question on cleaning boards with BGA.

Hi, Ryan and Paul
Your situation described below is somewhat confusing. Was the problem a single digit DPU seen on every assembly that had been washed in the US board cleaner? And was it the same pad or set of pads? This indicates something that I would classify as a design or fabrication issue, perhaps exacerbated by the US cleaning. If US cleaning of the bare PWB was enough to weaken the pad attachment, then the question comes to mind about whether the 40 Mhz frequency is appropriate for PWB cleaning, as opposed to components soldered on to a PWB. But I also question whether the frequency or the fabrication method is what led to the failure, which in this case I assume is catastrophic delamination of the pad from the PWB and its trace. Single pads are not usually electrically functional.
Also, I doubt that the "loose" pad is going to cause a broken solder joint. Do you mean a lifted pad, separated from its trace? If anything, when stresses are applied to the BGA, the ball soldered to the loose pad is the one that is not going to crack, as the pad is not attached to anything. Your observation that it did not happen on pad sizes larger than .012" (.29mm) was a good one. 
When working all day with CSPs whose pad sizes are .015" diameter or less, I quickly learned that a different foil lamination process was needed, as those pads not attached to a trace were found to be very weak. They are so small that the amount of "glue" holding them to the board is proportionally small also, so it sure does not take much of a difference in the Cte to cause them to lift. Usually this is not a problem, because as I already stated; the pads without traces are not typically functional from an electrical standpoint anyway. I worked with the board supplier, and they changed to a different laminating process or foil "glue" that was much stronger, and it fixed the problem.
But that leads me back to my confusion about why you had an occasional electrical failure. Can you clarify the failure mode?

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian Ellis
Sent: Thursday, August 04, 2005 1:19 AM
To: [log in to unmask]
Subject: Re: [TN] Question on cleaning boards with BGA.

Thanks for this info. It raises a number of questions in my mind. Would US affect the peel strength of very fine tracks (<100 µm)? If so, energy intensity at the board, frequency(ies)? etc.? I can easily imagine that an imploding cavity close to the track could shake up a tenuous bond, where the copper treatment is dimensioned so that the pitch between asperities is > the track width. I think this is a subject that could be looked at by someone???? Perhaps by Gould or one of the other copper "makers"??? Maybe it needs a new treatment??? Then, what about HDIS additive copper???

Brian

Ryan Grant wrote:
> 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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