Desolder BGA chip from PCB, suggestion for newbie?

[tske]

Hi, I have never used a hot air gun, now I want to desolder a BGA chip from a PCB, which I don't care about the chip, but the rest of PCB must be preserved, and the original site cannot have short circuit.
Do I need flux or other fluid? What's the recommend temperature, air speed, gun-to-chip distance, heat time? Thanks.

[Buriedcode]

There are many factors that determine the "success" - be it to preserve the original board and pads, or to preserve the chip itself.  Generally (not always but usually) boards with BGA devices are multilayer and will most likely have a few power/ground planes - that will sink heat away from the site meaning it takes longer to heat up that area - and the longer you heat the board, the further the heat spreads and the greater the potential for board damage.

I have successfully removed BGA devices with a simple temperature controlled hot air station (cheap 898D) but it needed the board to be preheated to reduce the temperature difference.  But that is just the first part.  Even if you don't plan on soldering a new chip to the board - if you don't want shorts between the remaining pads they will need at least some clean up.  Doesn't have to be perfect, but, again, due to the ground planes, it can be hard to get any remaining solder to melt.  For that I would advise a reasonably powerful iron, > 65W with a large tip + solder wick and plenty of flux.  You would to gently lay the solder wick over the pads and let the iron heat it up. Do not press down with any force or drag the iron/wick from side to side with any force.  Let it melt the solder and "ride" on top of the pads/flux - otherwise you could just swipe pads right off (if you don't plan on using the pads again, this may not be a problem, but still looks awful).

So yes, you need flux, plenty of it.  And given its a BGA you want liquid or tacky - putting a line of it along each edge of the chip means that once you start heating, it'll liquify, and flood under the chip.

As for temperature, I never go above 360C, but... it all depends on the nozzle width, width of chip, how far away from the chip you're at etc..  there isn't a particular "this distance, at this temp, for this time".
Sure, higher temperature, means it heats up the board quicker - but it also raises the maximum temp the board can get to, and it should never get to 400C...

Best advice I can give if you value the board you're working on - find some scrap boards, and practice.  And keep going until you can lift a chip fairly quickly without any signs of bubbling/warping or heaven forbid, scorching on the board.  Smaller PCB's are easier as they have less thermal mass, but something like a modern graphics card will most certainly require preheating, or it will take ages and permenantly damage the board.

How big of a chip are we talking? a 5x5mm? or something beastly like BGA289? And a photo of the board will help

[tske]

Thank you very much for your detailed reply. The size is something like 1cm square. Will direct heating on the chip by solder iron work? Or a hot air station is a must?

[tooki]

Thank you very much for your detailed reply. The size is something like 1cm square. Will direct heating on the chip by solder iron work? Or a hot air station is a must?

First of all, I’ll just comment that Buriedcode’s entire reply is excellent advice. Do not underestimate the importance of preheating the board, it makes a HUGE difference.

I would recommend getting a cheap hot air unit regardless. They’re useful not only for rework, but also for doing heatshrink tubing. I have a cheap one that cost something like $50 and it’s great. Money well spent.

As for whether you can remove a chip with a soldering iron: eh… maybe? If it’s one of the modern “wafer scale package” BGAs that doesn’t have any plastic at all — that is, you’re actually handling the silicon chip itself — it’s probably quite easy (never done that myself, but I’ll keep an eye out for a scrap board where I can try that!). In a normal epoxy-encapsulated chip, the thermal conductivity isn’t great, so a normal size soldering iron tip is not going to be able to pump heat in fast enough to spread to the whole chip. If you have a very large tip, one that covers most of the chip, it might work. (I have done that with very small chips before, but nothing as big as you describe.) Hot air has the advantage of being able to apply heat to the entire package.

So if it were me, I’d preheat the board to 150C or so and use hot air. Make sure to shield nearby components with tape (Kapton, aluminum, or copper) to keep them from being shifted or blown away with the hot air.

I’d use a gel or tacky flux.

[amyk]

I don't care about the chip, but the rest of PCB must be preserved

You can also consider milling off the chip in that case.