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Solder reflow temperatures too high

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SMTRework

#46594

Solder reflow temperatures too high | 11 January, 2007

I know this is not the correct forum to post chip "reworking" questions but you guys have been very helpful in the past.

My question is this, when we rework BGA components most of the time the solder reflows at a normal temp and all is well. On occasion, we'll get BGAs that simply will not reflow at any normal temp level. I believe these chips may have been generating high operating temperatures and thus possibly raising the reflow threshold of the solder below it? Does this sound right?

This wouldn't be a problem, but what were after is some of these BGAs are hard to find and would like to re-use the component if possible. But due to the high reflow temps we're damaging the BGA during the removal process.

I have notice that introducing new solder to old solder that will not reflow at normal temps cures the problem just enough to clean the old solder off or remove the component in question. But with BGAs it's nearly impossible to introduce new solder due to it's design.

Is there some king of special flux or liquid that will aid in the reflow of solder that's been exposed to high temps?

Thanks again. -Joe

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RDR

#46600

Solder reflow temperatures too high | 11 January, 2007

You using lead free BGAS now? What is your rework equipment and process?

I am sure we can help you out here with some more detail, you should get the package data sheets for your problem BGAs and determine ball alloy or they may even be CCGA packages?

Russ

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#46608

Solder reflow temperatures too high | 11 January, 2007

Joe: I know this is not the correct forum to post chip "reworking" questions. Reply: Rework questions are as appropriate as the next topic here on SMTnet

J: BGA will not reflow at any normal temperature level. I believe these chips may have been generating high operating temperatures and thus possibly raising the reflow threshold of the solder below it? Does this sound right? R: No, the reflow temperature is governed by: * Alloy of the solder * Thermal mass of area being soldered * Oxidation levels of components being soldered. It�s more likely that these BGA balls are leadfree or a non-collapsing alloy, as mentioned in an earlier posting by Russ. The alloy just has a higher melting temperature than you�re accustomed to seeing. Alternately, it's possible these problem BGA have an internal slug of metal as a heat sink that dissipates some of the heat that you supply to melt the solder.

J: Adding new solder to old solder that will not reflow at normal temps cures the problem just enough to clean the old solder off or remove the component in question. R: This makes sense. Mixing an alloy with a lower liquidus temperature to a high melting temperature will lower the liquidus temperature of the higher melt alloy, because the new melting temperature is combination of the two liquidus temperatures of the solder alloys.

J: Is there some king of special flux or liquid that will aid in the reflow of high melting point solder? R: Not that we know of.

J: We would like to re-use the component some of these hard to find BGA, but the high reflow temperatures are damaging the BGA during the removal process. R: Try doing both of the following: * Preheating the board at 125*C prior to considering removing the BGA. * Using a syringe to dispense lots of a low melting point solder under the BGA. [Use an alloy with a MUCH lower than the near eutectic solder that you commonly use. Ask your paste supplier for suggestions. Try to avoid using alloys containing bismuth, because of potential reliability issues with high operating temperature equipment.] Then doing BGA removal. OR Remove the BGA by cutting the board, the dunking the BGA in a solder pot of near eutectic solder.

Look here: http://circuitsassembly.com/cms/content/view/2578/95/

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SMTRework

#46649

Solder reflow temperatures too high | 14 January, 2007

Hi Russ and Dave,

Thanks for the replies..

These are PBGA's usually 31 x 31mm. They were manufactured about 3 years ago. I don't believe they are lead free. Data sheets are not available for these BGAs due to the competitive market.

With that said....

J: BGA will not reflow at any normal temperature level. I believe these chips may have been generating high operating temperatures and thus possibly raising the reflow threshold of the solder below it? Does this sound right? R: No, the reflow temperature is governed by: * Alloy of the solder * Thermal mass of area being soldered * Oxidation levels of components being soldered. It�s more likely that these BGA balls are lead-free or a non-collapsing alloy, as mentioned in an earlier posting by Russ. The alloy just has a higher melting temperature than you�re accustomed to seeing. Alternately, it's possible these problem BGA have an internal slug of metal as a heat sink that dissipates some of the heat that you supply to melt the solder.

----> I think the main cause of this condition is #3 because both #1 and #2 never change from board to board, these are identical boards with identical BGA revisions. These BGA's (as stated before) are about 3 years old.. I don't believe they are lead free.. The board manufacture date is 2004. With the boards in question the BGA's are fine it's just they are lifting from the PCB tearing the pads, thus a need for the removal, repair and re-use of the chip.

J: Adding new solder to old solder that will not reflow at normal temps cures the problem just enough to clean the old solder off or remove the component in question. R: This makes sense. Mixing an alloy with a lower liquidus temperature to a high melting temperature will lower the liquidus temperature of the higher melt alloy, because the new melting temperature is combination of the two liquidus temperatures of the solder alloys.

----> Yes, this is great, I just need to figure out how to get (for example sake) a "liquid form of solder" to add to the BGA's solder spheres before the removal attempt. This should make removal of the BGA's happen at a normal temp.

J: Is there some king of special flux or liquid that will aid in the reflow of high melting point solder? R: Not that we know of.

J: We would like to re-use the component some of these hard to find BGA, but the high reflow temperatures are damaging the BGA during the removal process. R: Try doing both of the following: * Preheating the board at 125*C prior to considering removing the BGA. * Using a syringe to dispense lots of a low melting point solder under the BGA. [Use an alloy with a MUCH lower than the near eutectic solder that you commonly use. Ask your paste supplier for suggestions. Try to avoid using alloys containing bismuth, because of potential reliability issues with high operating temperature equipment.] Then doing BGA removal. OR Remove the BGA by cutting the board, the dunking the BGA in a solder pot of near eutectic solder.

----> Pre-heating the board is now at about that temp already. I did try to apply some solder paste but it seemed to be too think to use in a syringe applicator. Is there a way to thin out the paste? The boards need to be preserved for re-use. I developed a fantastic way of "Micro Welding" the torn traces for nearly impossible separation happening again after the repair.

Thanks again for all the help..

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SMTRework

#46650

Solder reflow temperatures too high | 14 January, 2007

Also, I was beginning to think that the humidity level in the facility was a factor.. but I don't believe that's the case.. as this condition seems to occur regardless of the humidity level..

Equipment being used is as follows..

Metcal BGA 3500 Kester flux 951 being applied under BGA before removal

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