Re: bga's-with/without solderpaste????| 27 April, 1999
| Which is the preferred method for soldering a bga, adding solder paste or not? Solderpaste manufacturers say to add solder while the rework station manufacturer's say it's not necessary. | Paste is preferred by not necessary. Our studies, conducted over the last 5 years, show higher reliability with paste. However, if you use a paste flux, solder joints may easily be effected with eutectic ball device types.
BGA's and PCB's do not always like each other because of coplanarity issues. Paste flux and solder paste provide the compliance needed to allow all balls to reflow. If liquid flux is used, it doesn't provide the "interface" needed for wetting relatively evenly all around. This is why some balls on one side or another may not "completely" collapse and sufficiently wet.
Using solder paste will introduce voids in the solder joints. This brings up another hot issue. How much voiding is to much and where. My, and our, rule is voiding not exceeding 20% of a ball's volume. Using solder paste, obviously, provides additional paste volume that may or may not aid long term reliability but it does assure complete wetting.
Visual inspection will provide the first clue to sufficient solder wetting. If the part is aligned, as well wetted BGA's do, solder joints usually are acceptable provided the alignment criteria includes theta as well as all balls collapsing so the vertical part alignment is even all around the part's perimeter.
Lastly, we use X-Ray as an important tool for all our rework. It has programmed in algorithms such as that of excess, insufficient, or solder bridging. It also provides the ability to "step" down through solder joints from their tops to nearrer the PCB pads allowing an "internal" view of the joints and voiding.
All in all, BGA's are not a big deal unto themselves and original soldering techniques. However, this can change during rework if one doesn't do it right and know what causes acceptable solder joints and what is acceptable.