Author Topic: BGA reball, why do you need a preheater?  (Read 5016 times)

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Offline u666sa

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BGA reball, why do you need a preheater?
« on: March 07, 2014, 07:33:17 pm »
Help a complete newb please  :palm:

Let's say we want to reball a BGA, video chip on laptop motherboard, and we have this Aoyue 2703A+ hot air station with proper sized nozzle and an automate feature with 5 segment profile for the hot air gun.  What will happen if I just use that station to take off chip, clean it up, reball using a proper kit, and then solder it back?  Will it work just fine?

Why would I need a preheater?

Can I do it without a preheater?
 

Offline ert829

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Re: BGA reball, why do you need a preheater?
« Reply #1 on: March 07, 2014, 08:04:40 pm »
For what it is worth, here is my experience....I put a circuit board from a computer monitor (whose BGA re-scaler chip lost good electrical connection) in my kitchen oven at 375F for 8 minutes. After re-install, the original screen blinking problem went away. (for 2+ months so far!)  No preheat of board/chip, I just turned on the oven and when it hit 375F, I put my board in and waited 8 minutes.  I removed the board and let it cool down before re-installing.  No reballing.  Pretty crude, but it worked.

Anectdote...The problem was caused by the large amount of heat generated by the monitor's built in power supply.  It is a 30 inch monitor, so it generated a lot of heat.  I left the back cover of the monitor off, so the heat now easily escapes.  Others who reflowed their scaler chips in the kitchen oven, but put the monitor back cover back on had their re-scaler chip separate again.  I also put a heat sink in the rescaler chip (with double sided thermal tape) and attached a usb powered fan.  The heat sink doesn't even get warm, so I think it is probably unnecessary.  Just leaving the cover off (faces the wall) seems to have solved the problem.
 

Offline Rasz

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Re: BGA reball, why do you need a preheater?
« Reply #2 on: March 07, 2014, 08:15:52 pm »
-it will minimize chance of you lifting tracks while picking up bga chip
-will let you use less heat on the chip, that means less stress to whole pcb and surrounding components
-make sure new balls properly solder to the pads = no cold joints
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Offline pickle9000

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Re: BGA reball, why do you need a preheater?
« Reply #3 on: March 07, 2014, 08:23:18 pm »
Rasz is correct on all counts.

I would suggest playing around a bit, find an old laptop that works and try removing and reinstalling chips. Test after each round. In reality you should have a proper setup but I'm always amazed at what people can do after some practice.

 

Offline u666sa

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Re: BGA reball, why do you need a preheater?
« Reply #4 on: March 07, 2014, 08:28:53 pm »
I see, so it is better to have a preheater.

Here is a new question tho.  Let's say I have a smaller preheater -- budget problems -- it is a bit smaller than a motherboard, so a motherboard will not fit and hold in place on preheater's holders.  Is it ok to rest the board over, maybe rig something up to hold in place, couple of bricks or books?  Will that work?
 

Offline pickle9000

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Re: BGA reball, why do you need a preheater?
« Reply #5 on: March 07, 2014, 08:35:40 pm »
You only need to preheat the area you work on. Mine is 5x5 inches and that will handle most anything. Normally the board will need some air space between itself and the preheater. It's needs to be stable while removing or installing a chip. 
 

Offline marshallh

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Re: BGA reball, why do you need a preheater?
« Reply #6 on: March 07, 2014, 11:29:46 pm »
I've had best results with whole-pcb preheating.

Basically, when you shoot a bunch of heat at only part of the PCB, it must expand. Since it's bound in place by the rest of the cold PCB, it must deform up or down to expand. This is negligible on small boards but it is very easy to see on big boards.

The amount the PCB must deform depends on the relative temperature difference. A room temp PCB at 27 C with a small portion heated to 400 C will not make equal contact across a large BGA.
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Offline pickle9000

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Re: BGA reball, why do you need a preheater?
« Reply #7 on: March 08, 2014, 12:35:26 am »
I can't say for sure that deformation has ever been a problem for me (at home or work). I have seen it but reorienting the pcb seems to solve the issue (for me). I can say that I have not done a perfect job every time. Don't feel bad pulling a chip and starting over it's part of the game.
 


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