I have been trying to rework a lead-free BGA. For some reason, when I remove the BGA the corner pads on the board come off and the board must be scrapped. If I do manage to remove the BGA successfully, when I solder the BGA, the corners of the BGA warp up and doesn't allow me to get a good connection on the pads. I have tried using a larger nozzle, increased temperature, and nothing seems to work. the board is ENIG and it does contain a large ground plane.
There are a lot of variables in BGA rework in general, and the switch to lead free has only magnified some of the sore spots.
Perhaps with a little bit more information someone here can be of help.
What is the size and thickness of the PCB? What is the size and style of the BGA compoent in question? What type of rework equipment do you have at your disposal?
You should never have an issue with pads being removed during the removal process. When you are prepping the site is typically when pad damage occurs. If your machine is lifting the component too soon and tearing pads off, then you have a machine problem. If you are not using a machine, and are using some kind of hand held heating device then you have other problems not addressable here.
As far as warping of the BGA, consider that your heating process is too quick. If you ramp to temperature slowly enough the package will have ample time to heat evenly, and help to avoid warp. It is not neccessarily a matter of a larger nozzle or a higher temp. Bottom side heating is CRITICAL to pb Free rework. Generally speaking, you cannot drive enough heat through the top of a BGA package to create good solder joints at the board level. It is so very important that the PCB substrate is up to temp as well. Otherwise you get collapsed spheres and poor connections.
I can see this going towards a discussion of IR vs convective rework machines. IR = Good for Bottom heating, not so good for Component heating. Forced Air convection = Good for bottom heating, Good for component reflow. (Thats why they build PCBs that way)
If at all possible try to have as much of the PCB heated evenly for the process. Try to keep the PCB at around 140 to 160C while your topside temps (depending upon the rework machine) may go from 270 to 290C
Good Luck. Lots of knowledge in this group, I'm sure between all of us we can get you on a better path.
The board thickness is 50 thousandths. The style of the BGA is a 324 ball BGA that has an isolated center region. The machine that I am using is the Air-Vac DRS25. I'm not too sure why the pads are being removed when I am desoldering. I know when I ran the profile, two corners underneath the BGA were at an average of 234C.
While your peak temperature measurement is probably correct, you probably ripped the pads from the board long before reaching that temperature. By using a steep temperature ramp, you are causing the corners of the BGA to curl away from the board and the center of the BGA. This curling movement of the corners of the BGA could be so strong that it is pulling the solder ball and the pad on the board with it before the solder of the ball becomes molten.
For more on this, search the fine SMTnet Archives on BGA warp*
Agreed, try slowing the ramp rate down and possibly increasing bottom side temp to 170 if you have that ability. If youre using a hairdryer, youre screwed. Sometimes the 4 degrees C/sec is too fast especially with the FR-4 boardered BGAs.
What would a good ramp rate be. I am running from 205-217C in 17 seconds. My reflow is 217-235 for 68 seconds. The bottom preheaters are set at 200C and the nozzle is 2075 & 295 for each process respectively.
*Maximum slope is a time/temperature relationship that measures how fast the temperature on the printed circuit board changes. The ramp�up rate is usually somewhere between 1.0 �C and 3.0 �C per second, often falling between 2.0 �C and 3.0 �C (4 �F to 5 �F) per second. If the rate exceeds the maximum slope, potential damage to components from thermal shock or cracking can occur. Solder paste can also have a spattering effect. The preheat section is where the solvent in the paste begins to evaporate, and if the rise rate (or temperature level) is too low, evaporation of flux volatiles is incomplete.
Did you use the auto profile feature on your DRS25 machine to arrive at your profile?
Our DRS25 has been doing quite well at larger BGA components. Most critical is a good preheat until the PCB reaches at least 140c, followed by a 2degC/sec ramp to soak at 170 for 30 to 45 seconds to get those volatiles out of the solder paste, then another ramp to the reflow temperature, and holding peak temperature setting of 270C for about 60 seconds.
With a good preheat and logical ramp rates you can minimize warpage of the BGA device (or the PCB)
I agree it is likely that the pads are being pulled off by the corners of the BGA itself warping upwards before reflow, during the removal.
Remember that good rework is not about speed, it is about patience. You can do it right, or you can do it "right now".
The autoprofile feature on our DRS25 was used to create all the basic profiles we use, but we always tweak them a bit to get exactly what we are looking for. So far we find it easy to use one of the "canned" profiles, and edit them as we choose.
I have never had a problem with any other BGA's that I reworked. I recently found out that the BGA that we are trying to rework is made poorly and warps automatically from the heat. There is nothing that we could do to prevent this. Also the board has a huge ground plane and it changes the way the profile usual works. The maximum temperature that the boards gets to is 190C and I know it shouldnt get this high. I do use the auto profiler for creating profiles, but this one is the one that gives the most trouble.