BGA Reballing, maybe a dumb question... but...

[made2hack]

Ok,

so, why is BGA reballing done with little solder balls? Why isn't a solder stencil made and then solder paste added, and then re-flowed to attach the chip to the board?

I mean, why the use of the balls? Seems it would be easier with solder paste?

What am I missing?

[rolycat]

IANAEE, but:

When BGA devices are initially soldered, they usually do use solder paste (as well as the balls). The balls self-centre the chips and ensure that there is enough solder to make reliable connections despite the absence of pins.

When reworking a board, it's often difficult to apply a stencil due to the already populated components,  so once the chip has been reballed it is typically reattached by fluxing either the pads or the balls prior to reflow.

[Skippy]

I don't remember, and the video's 50 minutes long so I'm not going through it again to find it, but I think in this video:



the soldering expert does exactly what you suggest to a BGA - he gets a solder stencil with a grid of holes in it, readds solder paste to the bottom of the chip then resolders the chip onto the board.

[wraper]

No, it's not easier and you won't get the balls of the right size, getting all of them equal size is not so easy too. When you are heating the stencil, it usually tends to bend because of the thermal expansion and solder paste tends to bleed. Reflowing the paste without a stencil is impossible as it will flow all over the chip. Removing the stencil after the reballing might be not so easy as flux solidifies.

[made2hack]

@Skippy, good video, I liked it, and yeah, he uses some kind of paste on the stencil (I think).

Granted in the commentary they said he used balls, however, you can notice what looked like paste and not balls.

[DanielS]

Granted in the commentary they said he used balls, however, you can notice what looked like paste and not balls.

You wouldn't spread balls with a squeegee and the excess would roll all over the shop. Attaching balls is a completely different process where the ball-thick stencil usually gets loaded by hopper/tumbler and balls are initially attached to the chip by flux.

He is clearly applying paste, reflowing it and then has to punch reflowed pads out of the stencil if they get stuck. Using solder paste and a paste stencil is much more accessible than dealing with balls: all you need is the same stuff you already have to deal with surface-mount PCBs.

[Hardcorefs]

Ok,

so, why is BGA reballing done with little solder balls? Why isn't a solder stencil made and then solder paste added, and then re-flowed to attach the chip to the board?

I mean, why the use of the balls? Seems it would be easier with solder paste?

What am I missing?

Solder paste IS little balls…….. with added flux

[Teledog]

Reballed chips several times (not professionally, mind you).
Clean the chip pads very well with lots of rosin flux & braid, then clean well with IPA & Kimwipe.
Inspect the pads with a microscope, if dirty -redo, if damaged, pitch!
Slather on thick rosin flux (yes, a paste, but no solder balls).
Line up the chip pads to the appropriate stencil & tape it on (from behind) to the stencil with Kapton tape.
Mask off excess stencil holes with kapton.
Pour/wiggle on the appropriate sized balls over the stencil to fill in the pad holes (over a bowl- to recover the excess balls).
The balls will stick to the flux paste.
Place the chip/stencil on a hotplate, chip backside down, balls/stencil up & monitor the temp ( ~210-230C with a laser pointer thermometer) then use a heat gun on low until the balls heat, melt  & settle on the pads ..you'll know it when you see it! The stencil tends to warp (hence,  use lots of kapton).
Then turn off all the heat , but don't pull it off the heat plate  too quickly.
Let cool & remove the stencil.
if all is well, be proud of yourself! If not, try, try again! :-)
****be careful with the lead balls as they are toxic & they stick to EVERYTHING! Wash hands very well after playing with that stuff!!***

[made2hack]

@Teledog, interesting.

Do you know if the balls are already alloys? As in 60/40 ? Or, do they split 60% of the balls tin, and 40% lead. I would imagine the allow is built in to the little balls.

[ruffy91]

The balls are already alloy. If they weren't they wouldn't melt until the temperature reaches about 800°C