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warped boarf after reflowing

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#67276

warped boarf after reflowing | 17 September, 2012

Hi I did a reflow on a laptop board and after the reflow the board is warped in the area I did the reflow in, and now the laptop screen does not turn on at all when I put it back into the laptop. Original problem was the wifi (I did the reflow more or less as a test to see if i could fix it or not).

So I was wondering where I went wrong- could it be too much heat I used to cause warping? I had a pre heater on the bottom set to the max (250 C) which brought the top of the board to a temp of arround 80C. Then I used hot air nozzle to further heat the southbridge chipset on top. I had to set the temp to arround 450 C in order to get one of my thermal meters to show a temp of 270 C on the top of the board on the southbridge chipset. I let the chipset roast for about 2 minutes then I cut the heat down. I also used a no clean flux as I heated the board.

If that did not cause the warping maybe it was my holding jig? it has a spring loaded arm on it which puts pressure on the board. Maybe that is what caused it? I must admit when I loaded the part I did have it set very tight.

Also I noticed lots of residue from the no clean flux left behind, I cleaned it off and testes it again just incase that was causing a short out but even after cleaning it does not work (the laptop board). any ideas where I went wrong?

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#67277

warped boarf after reflowing | 17 September, 2012

Hi,

Do you want to replace the SB ou just reflow it again?

Do you suspect there is some "colder solder joint" in one of the SB balls ?

In my opinion you are heating the board a lot... Did you put some thermocouple on the ball to check the temperature? As the temperature of solder paste become liquidus above +/-220°C, you dont need to reach 270°C on the top of component... You maybe can consider 250°C on the balls, where the solder paste is placed.

When I need to repair a bga NB or SB I used to set the bottom heater in 150°C.. it's enough to take it off and solder it again on the board...

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#67280

warped boarf after reflowing | 17 September, 2012

thank you for your reply. I felt like I was using too much heat but I thought it would not work unless I did so. I did not suspect a cold solder joint, however I did another test on a scrap board. I heated the board without my clamps attached (I put the board on top of the U bracket instead of inside the groves) and heated it up real hot again and it did not warp even with the extra heat. I did the test again but this time put the board in the U bracket clamp and used the same heat and it warped big time. I think it was causing too much pressure, and with all the heat it caused lots of warping.

It warped so much that I believe it may be causing the SB chip to not line up properly because in that area is where the warp starts.

One question I have is where to place the thermocouple? when I measured the temp I put it right next to the bottom of the SB chip, I could not fit the probe under the chip in the ball area. Is this area (next to the bottom) good enough?

I am going to heat the laptop board up and try to unbend the warp but I think the board may be scrap now but it cant hurt to test it :-)

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#67294

warped boarf after reflowing | 18 September, 2012

Hi,

Regarding to thermocouple placement, you should use a precision thin drill. Drill through the bottom of the board to the top as follows: One hole in the center of the part, preferably into one of the center balls, if available. One hole into the corner of the part, preferably into one of the balls in the outside row. See the picture attached. This same procedure I use when I need to check the temperature on LGAs 775.. both reflow or rework of bgas.

Attachments:

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reflow oven profiler

Reflow Oven