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Remove,Re-ball then Re-use BGA safely !!!

Hany A. Salam

#20417

Remove,Re-ball then Re-use BGA safely !!! | 19 June, 2002

Dear all:

I wonder how to avoid cracking the BGA if I want to remove it from the PCB assembly, re-ball it and solder it again. I need an answer considering:

1- The assembly could have been exposed to the ambient humidity for a long time.

2- The assembly has many PTH components that cannot resist baking like, plastic sockets & electrolytic capacitors.

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#20418

Remove,Re-ball then Re-use BGA safely !!! | 19 June, 2002

Try this: * Remove PTH components that cannot resist baking like, plastic sockets & electrolytic capacitors. [NOTE: It may be econonomic to sacrifice some of the PTH components that cannot resist baking.] * Bake the board. * Remove, reball, and replace the BGA. * Replace PTH components that cannot resist baking like, plastic sockets & electrolytic capacitors.

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Hany A. Salam

#20435

Remove,Re-ball then Re-use BGA safely !!! | 20 June, 2002

Thanks Dave but unluckily my sweet BGA�s are surrounded by plastic connectors with hundreds of pins. Also my PCB�s are 4-layers so just thinking of doing what you suggest means ruining them for me.

To be honest I tried bypassing the baking issue before the BGA removal and it was OK most of the time. But I was asked to justify this process according to the JEDEC standard J-STD-033 which obligates me to do the baking ,so what do you think I have to do ?

Regards Hany

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#20438

Remove,Re-ball then Re-use BGA safely !!! | 20 June, 2002

Tell them that you use the "Abreviated Bake Method".

Seriously, how do know if you've ruined the BGA {or someother component] or not? How do KNOW it worked fine?

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Dason C

#20447

Remove,Re-ball then Re-use BGA safely !!! | 20 June, 2002

Dave mention b/4 to me, we don't have any industrial standard for the BGA rework. One of my customer request us to bake at 125C for 6 hours b/4 rework. I am lucky and we don't have the component which you are using.

I did a evaluation and we found component weight will reduce more than 50% of the moisture weight after place in the cabinet with 55C and less than 5 % for 48 hours. We are currently in second stages and the component will reflow and C-SAM. The components are under soaking and I should have the prelim result in next two weeks. I had no idea will it work at the present moment. Just for your idea.

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ianchan

#20556

Remove,Re-ball then Re-use BGA safely !!! | 1 July, 2002

Dason: Hi mate,

Could you help clarify this abit? ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ we found component weight will reduce more than 50% of the moisture weight after place in the cabinet with 55C and less than 5 % for 48 hours ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

was the component moisture weight reduced by 50% at 55C with 48 hours duration inside the cabinet? where does the 5% figure factor in?

Kindly advise as we are interested to follow certain advise as part of our continual improvement plans. Thanks, Dason.

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Dason C

#20567

Remove,Re-ball then Re-use BGA safely !!! | 2 July, 2002

Ian, First, I baked the parts at 125C for 48 hours and check the weight Second, I soaked the parts above 30C and 60% for 96 hours and check the weight Calculate the weight gain, suppose only moisture add to the parts during soaking, say a Third, dry the parts at 55C below 5% for 48 hours, existing oven when the temperature at 55C, the humidity will be around 5 to 8%. The test cabinet able to control less than 5%, average around 3% at 55C. Check the weight and calculate the moisture remain on the parts, say b Moisture reduction = (a-b)/a X 100%.

I am under stage 2 evalaution by using 0.0001g scale and I should have my result this week and prelim report in next week. The evalaution will include the C-SAM. I can share with you for yr. reference when available.

Dason

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#20570

Remove,Re-ball then Re-use BGA safely !!! | 2 July, 2002

Dason,

Just a word of caution about your experiment. Evaluating moisture content by weight gain is a very simplistic model, based on the assumption that the moisture content is uniform across the package.

In fact, recent technical papers have clearly demonstrated that the moisture content is never uniform across the thickness of the package. A moisture gradient always exists and moisture is either moving in or our of the package, based on the history of exposure and subsequent dry storage or baking. In addition, the actual location of the moisture gradient is a lot more critical than the overall moisture content to define the risk of defects during reflow.

When moisture is initially absorbed, a large weight content can be located in the perimeter of the package without causing any functional problem. On the other hand, when a package is being dried, moisture will be removed from the outside first and the the more critical gradient located near the center will be removed last. In other words, even a very small content by weight could cause component failure.

The above physical phenomenon has been well documented, moisture diffusion models have been developed and proven, and this is what forms the basis of the industry standard J-STD-033. This explains why the default bake durations have been significantly compared to earlier standard that were based on weight content models. (i.e. from 24 to 48 hours at 125C).

If you want more information I can e-mail you an excellent technical paper called "Handling of Highly-Moisture Sensitive Components - An Analysis of Low-Humidity Containment and Baking Schedule", written by Dr Shook from Lucent, one of the world leading experts on this subject.

Regards,

Francois Monette Cogiscan Inc. Tel : 450-534-2644 Fax : 450-534-0092 fmonette@cogiscan.com www.cogiscan.com

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dason_c

#20577

Remove,Re-ball then Re-use BGA safely !!! | 2 July, 2002

Francois, thank you very much for your recommendation twice. Is it any harmful for my evaluation? The component will go thro the reflow after drying process and not just only check the weight. The evaluation will go thro the C-SAM as Dr. Shook document. The only different I will using the lower temperature, ie 55C. Our industrial doesn�t have any standard for the BGA rework and my company insist me to try on the evaluation. I can�t tell you is 48 hours work or 96 hours will work or longer at the present moment but when I have more data then I can share with you if you like. One of the customer request us to bake the board at 125C for 6 hours before rework. Should I request to bake at 125C for 48 hours or 40C for 68 days or any recommendation?

I always see the recommendation on baking but I don�t see any paper or article the dry method is not working. I am very appreciately if you can provide me some of the data and the dry is not working otherwise I don�t see any reason that I should stop at the present moment.

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#20587

Remove,Re-ball then Re-use BGA safely !!! | 3 July, 2002

Dason,

I apologize if some of the information sounds redundant but I don't think I explained my point very well (and the standard can get really confusing at times). I definitely agree with using C-SAM to look for internal defects. My only advise is to be careful not to draw general conclusions based on a small sample of components. It is possible that you will not see any defects in your experiment but that does not mean that all components will always be safe using this same process.

The actual drying effect of storage at any temperature and RH level varies significantly based on the physical properties of each component, as well as the actual duration and conditions of exposure and subsequent dry storage. There are years of research and thousands of samples that went into the creation and ongoing revisions to J-STD-033. In order to provide safe guidelines, all simplifications must be based on a worse case condition. Otherwise you will jeopardize the reliability of some of your components.

The standard specifies that you must insure that your moisture-sensitive components are still within their specified floor life, or to completely bake them prior to any reflow cycle (such as localized hot air rework). Of course this is only important if you plan to re-use the components. The standard provides a complete bake table that takes into account all important parameters described above. The upcoming revision includes bake cycles at three different temperatures, namely 40C, 90C and 125C. The 90C was added specifically for drying populated boards that cannot withstand 125C. The actual duration of the bake cycle varies based on the specific MS level and body thickness of the component that must be baked. If you want to simplify your internal procedure you should use the maximum default values. (i.e. 48 hours at 125C, or 10 days at 90C, or 68 days at 40C).

In order to determine the minimum drying duration at 55C, and to be consistent with J-STD-033, you should use the moisture diffusion models identified in Dr.Shook's paper and calculate the worse case scenario. If you do this, you will end up with a duration somewhere between 10 days and 68 days, consistent with the bake duration at 90C and 40C. My suggestion would be to increase your temperature at 90C if possible, to reduce your cycle time and to be in line with the industry standard. Ideally you would also track the exposure time of your components on the boards up to the rework process. This would avoid baking parts that are still within their specified floor life.

I have no clue where the 6 hours at 125C comes from but it is much too short to be safe and certainly not in line with the present industry standard. You might want to ask your customer where they got that number from.

I am not sure if I understand exactly what kind of additional data you are looking for but if you want to contact me offline I might be able to provide some relevant information.

Rgds, Francois Tel : 450-534-2644 fmonette@cogiscan.com

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#20711

Remove,Re-ball then Re-use BGA safely !!! | 16 July, 2002

Our BGA Rework process has resulted in 100% yield. We always bake BGA chips at 125 degrees C for at least 8 hours before installation. We always bake loaded boards at 90 degrees C for at least 12 hours before BGA removal. Any parts that went through the wave solder machine or reflow oven will easily withstand 90 degrees C. If you are using a reballing process that requires you soak the chip in water to remove the paper, "such as with Winslow Automation Solder Ball Preforms" then you need to bake your chip again after it is exposed to water before installation. BGA Components are extremely moisture sensitive and will warp if not baked before installation.

See: http://www.pcb-repair.com/bgarework.htm

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