"yes I said hundreds of BGAs by simply adding flux"   What type of flux and how much flux, control or uncontrolled?

Victor,

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Wednesday, November 14, 2012 8:49 AM
To: [log in to unmask]
Subject: Re: [TN] Reflux and reflow BGA rework

All of those things are easily identified with a good X-ray machine and an X-ray tech that knows what he/she is doing.

I myself have reworked hundreds, yes I said hundreds of BGAs by simply adding flux and reflowing on commercial and high-rel product in previous lives. As I stated before, you have to have a qualified process, but it is easily done and is a very viable process. And believe it or not, it can be done with a good tacky flux. Alpha WS 1208 is my favorite but there are others that may be even better. Liquid benchtop flux does not work very well, as it does not stick around long enough during reflow to do a good job.

Here is the process for a leaded water-soluble process. I have done this with leaded and lead-free solder but never with no-clean or RMA flux, but I believe it can be done:

1. Pre-bake the CCA at 105 deg. C for a minimum of four hours to remove moisture from the board and the part.

2. Mask off the area around the BGA using a low-outgassing, ESD-safe polyimide (Kapton) tape. Do not use any old Kapton tape, and do not use high-temp crepe masking tape. Seal the tape down to the PWB as well as possible if there is room immediately around the BGA. You can also use a peelable maskant.

3. Apply a small bead or line of tacky flux evenly around the perimeter of the BGA with an ESD-safe syringe (from EFD). The bead should be as high as the edge of the BGA and about the same distance away.
 
4. Start the rework machine. The tacky flux turns liquid as soon as the CCA is heated from underneath at about 100 deg. Fahrenheit (not C) during the pre-heat stage of a machine rework process, and then can be blown under the BGA with a grounded air nozzle set at 15 PSI to evenly distribute it over all of the solder balls, but not blast it away. It will leave only a very fine sheen of flux on the solder balls, no droplets or blobs. Excess flux will be blown out onto the tape. All you have to do is blow the flux under the BGA from a complete circle around the BGA, so the air blows the flux under the BGA from every angle while the machine is in the pre-heat stage. This very effectively distributes a thin sheen of flux over all of the solder balls and the PWB pads. At about 15 PSI, you will not blow all of the flux away, but there will be no droplets or blobs of flux under the part.
 
5. With an Airvac DRS 25 or equivalent rework machine, you can then lower the nozzle over the BGA and continue to reflow the BGA with a proper profile.
 
6. When finished, carefully remove the tape. Manually wash by directing hot (145 deg. F) de-ionized water at an angle from all four sides under the BGA in an ESD-approved sink. Follow that with a good in-line machine wash process and bake dry for 1 hour at 105 deg. C.

7. Re-inspect under the X-ray, as well as visibly under the microscope at an angle or using inspection mirrors or an Ersascope. The condition of the solder balls around the perimeter will indicate the condition of those further under the BGA. The X-ray images should show round, uniform solder joints.

The advantage of this rework method, as has been stated in this string, is that you do not have to remove the BGA, possibly losing an otherwise good BGA, and you do not have to re-ball it, and with a single heat cycle you can bring the product back to full conformance to the IPC-610 or J-STD-001 solder joint specs. You need to evaluate whether you want to do this to a $10 CCA and a $5 part. Maybe not. But if your CCA is $15,000 and the BGA is $800 it is certainly a process you want to have fully qualified and in your back pocket, just in case.

If you choose to remove and replace the BGA, you will subject the CCA to 1 temp excursion to remove, another to dress the site and clean the solder from the pads, another to replace the part. Counting the original reflow of the entire CCA (or two reflows if double sided) you will have subjected the CCA to a minimum of 5 temperature excursions. So there is a significant advantage to being able to perform this rework in a single excursion from a reliability perspective.

And as I stated many times before, there is no need to print additional solder paste during a BGA rework, except for reworking CBGAs or CCGAs with Pb90 solder balls or columns. That is a completely different scenario. Sufficient unto today are our sins thereof.


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wenger, George M.
Sent: Tuesday, November 13, 2012 9:28 PM
To: [log in to unmask]
Subject: Re: [TN] Reflux and reflow BGA rework

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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