You've just been given a board with a shorted BGA site. If management decides the board is worth salvaging, there's only one thing to do; remove the BGA component, clean the site, paste the site, and replace the BGA.
You've done this many times before, but still, as any one who has attempted BGA removal and replacement knows, BGA rework at any level can be finicky and challenging.
Yes, it is complicated enough, but in this case the challenge is magnified as this BGA component has 2600 balls at 1.0 mm pitch.
You're no quitter so you plow ahead. Removal of the component is delicate but routine for an experienced pro. You get that done with a little cautious foresight and plenty of monitoring thermocouples under and around the site.
Figure 1: BGA site vacuum system.
Next you find that clearing the site of excess solder is a little more delicate and you decide to use your BGA rework machine's installed vacuum solder removal system. (See Figure 1).
So far so good, but now you get hung up on the application of solder paste at this site. Even after you've chosen solder paste with the best possible slump characteristics you begin to confront some frustrating problems.
The metal stencil you've had fabricated for this site just isn't cutting it. You find that the old reliable metal stencil material is causing a depositional nightmare on a site of this size.
Proper application of solder paste, an acquired skill, even for low pin counts, is crucial for successful BGA rework, and the more pins there are, the more chances there are for uneven paste deposition.
With a metal stencil, you get one pass, if you try for a second or third, you risk pumping excess paste between the stencil apertures. This can cause paste bridging and, ultimately, a solder short, which is what you were trying to correct in the first place.
If, in the effort to prevent shorts, you under-paste several pads on this very large site, it's not inconceivable that you could induce opens under the component. Something as simple as whether or not there is even enough room to securely tape the stencil to the board has become an issue.
After doing a little research you finally decide to reduce your paste deposition risk factor by using a Flextac BGA Rework Stencil for the rework. Flextac Stencils are made of plastic polymer, which allows them to conform to the surface of your board, eliminating co-planarity issues.
Because they have a no-residue adhesive backing, if inadequate paste is deposited in stencil apertures on the first pass you can try another, or even one more.
The adhesive effectively isolates each pad, preventing paste bridging and dramatically reducing the risk of shorts and opens. Lastly, the adhesive backing means no tape to hold down the edge of the stencil, so the Flextac BGA Rework Stencils can help in a tight spot. (See Figure 2).
Figure 3: Edge balls shorting when BGA component "potato-chips" during rework.
There is no board available to complete a proper profile so you have to work your magic with an existing profile. On top of that you decide to be extra careful and place five digital thermocouples under and around the component (getting a few into bottom vias that connect directly to topside BGA pads) so you can properly track the critical temperatures during the process.
Additionally you place high temperature spacers under the four corners of the component to prevent shorting due to uneven column collapse or "potato-chipping" at the corners. (See Figure 3).
Well, you can't hold your breath for an entire BGA reflow cycle, but neither can you breathe easy until the component is properly reflowed and passes x-ray inspection. But your hard work, preparation and care pay off in a successfully reworked site.
Several members of the Circuit Technology Center team contributed to this feature story.
Customer Comments "We were certain the circuit board we sent to you were beyond repair, but we sent them anyway. They came back, we tested them, and they all functioned perfectly. Job well done!" A.Z. Guadalupe, Mexico