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November 2012

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From:
"Goodyear, Patrick" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Goodyear, Patrick
Date:
Wed, 14 Nov 2012 20:26:18 +0000
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Thanks all, these and earlier posts give me the confidence to try it.   I don't have a reflow oven but plan on using a small convection toaster oven to do the deed, it will go to 600deg, nice thing about the toaster is it heats both sides. 

The board is already toast so I might as well butter it up and see what happens.  This is for a non-work related job, I had a computer board go intermittent after some temperature cycles, suspect a cracked solder joint on one of the chips.   I already have a replacement this is a learning experience, hopefully I will be smarter in the end.    Learning is doing.   I will heat it slowly then bake it for a few minutes, then allow natural cooling.   

Thanks again!  
Pat 

-----Original Message-----
From: Amol Kane [mailto:[log in to unmask]] 
Sent: Wednesday, November 14, 2012 10:13 AM
To: TechNet E-Mail Forum; Goodyear, Patrick
Subject: RE: [TN] Reflux and reflow BGA rework

Patrick,
 if you have any heavy components directly underneath the part you are trying to rework, then they may potentially come off if the component weight overcomes the surface tension forces. The components on the rest of the board on the bottom side should be OK as they will not be heated to reflow temperatures with a well-designed rework profile. You can always add a tiny chip bonder/epoxy bead (Loctite 3609 comes to mind) to keep such components on the board during rework. The curing time/temp of chip bonders is such that they cure first before the solder hits reflow temperatures.

The following is a thumb rule I picked up from technet by EMPF you can use to gauge if you need the epoxy application:
Here is the rule of thumb from the EMPF Boot Camp: Four grams per square centimeter (25 gm/inch^2) of solderable surface area on the device or board, which ever is less.

    
Regards,
Amol

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Goodyear, Patrick
Sent: Wednesday, November 14, 2012 9:08 AM
To: [log in to unmask]
Subject: Re: [TN] Reflux and reflow BGA rework

Ok since I have less than zero experience on this subject so my knowledge level is nil at best I have a question.   No laughing since I really don't have a clue. 
I have a double sided board, that needs a BGA reflow, has high density chips on both sides, how is it positioned so that the chips on the down side remain on the board?    Can Kapton tape be used to hold the chips in place?  Does the weight of the board hold them in place?  What about the chips that have a lower profile?  As you can surmise I have a potential project but I'm completely clueless.   
 
Pat 
Clueless in Los Osos


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Wednesday, November 14, 2012 8:06 AM
To: [log in to unmask]
Subject: Re: [TN] Reflux and reflow BGA rework

Absolutely agree with that. You need to understand if you have a plating issue or some other issue that will prevent good soldering. But if you have a BGA that is slightly skewed, for example, or was the subject of an improper profile (insufficient heat) during initial reflow, or some other process issue, then you can use the method I described. 
But absolutely, you need to understand the root cause of the defect before you blindly start reflowing BGAs.


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Wednesday, November 14, 2012 9:28 AM
To: [log in to unmask]
Subject: Re: [TN] Reflux and reflow BGA rework

Hi George -  good discussion and I completely agree. I should have been more detailed in my response. We do not use the "reflux and reflow" 
process as a routine procedure. As you detailed, we do a root cause investigation and do not allow the process to be used without some due diligence. And as Richard detailed in his response, our "reflux and reflow" process is a detailed process with specific procedure steps that must be used. Use of the "reflux and reflow" process without conscious thought can get you into trouble as you suggested. A good example was one case where we found that a lot of boards had crack microvias in the BGA pads so when the BGA was reflowed, the solder electrically  "reconnected" 
the microvia giving the impression that there was a bad solder joint when in fact there was a bad microvia fabrication process.  I agree with you - blindly using a "reflux and reflow" process can be dangerous practice.

Dave



From:   "Wenger, George M." <[log in to unmask]>
To:     <[log in to unmask]>
Date:   11/13/2012 09:30 PM
Subject:        Re: [TN] Reflux and reflow BGA rework
Sent by:        TechNet <[log in to unmask]>



Julie / Dave,

I'm going to stick my neck out and say that injecting flux and re-reflowing is a risky repair process.  You may be able to make a part that wasn't working work again but just because it works again doesn't mean you have reliable solder interconnections.  I'm aware of a case were injecting flux and re-reflowing did fix PCBA product well enough to pass functional test and those "fixed" boards failed after being deployed.  The problem with "fixing" non-working BGAs is that the non-working BGAs may have been "fixed" but you don't know what was "fixed" and you don't know how good the "fix" is.

Before fixing any non-working BGAs I think the important thing to do first is identify what you are trying to fix.  Is the BGA non-working because of Head-n-Pillow, Double Reflow, Insufficient solder paste dispensing, Plugged stencil aperture and no dispensed solder paste, Poor wetting, De-wetting, etc.

Regards,
George
George M. Wenger
Senior Principal Reliability / FMA Engineer Andrew Corporation - Wireless Network Solutions
40 Technology Drive, Warren, NJ 07059
(908) 546-4531 Office (732) 309-8964 Mobile
E-mail: [log in to unmask]

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Tuesday, November 13, 2012 9:22 AM
To: [log in to unmask]
Subject: Re: [TN] Reflux and reflow BGA rework

Hi Julie - I don't know of any standards that cover this topic unless there is something in the IPC-7711 specification. When we use our BGA repair procedure, we use specific fluxes that are easily removed and we are very conscious about the volume of flux used. "Bigger the glob, better the job" is not a applicable process philosophy when utilizing fluxes for BGA repair. Our focus for the process is to insure that we are using flux in such a way that we don't cause a reliability issue that was not there prior to our BGA repair actions. The example we discussed of the flux packed under the BGA in the previous Technet thread is a classic case of not understanding how much flux was necessary for successful BGA repair.

Dave Hillman
Rockwell Collins
[log in to unmask]



From:   Julie Silk <[log in to unmask]>
To:     <[log in to unmask]>
Date:   11/13/2012 07:51 AM
Subject:        [TN] Reflux and reflow BGA rework
Sent by:        TechNet <[log in to unmask]>



A discussion of the practice of re-flux and reflow of BGAs to rework them has emerged within the "limits of flux residue" discussion.  I'm taking it out into a separate thread.  This rework process injects flux under the BGA, then reflows the BGA.  The part is not removed.  It will frequently make a part that wasn't working work again.  The heat damage to the board is less (fewer cycles) than a replacement process.  The question Joyce asked about whether there are standards and reliability studies has not been answered.  What is the proper procedure for this rework process?  Are there official standards?  Studies of effectiveness / reliability?




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