When double sided BGA's are used on an .062 FR4 10 layer board should via's be plugged and tented? The CACHE chips being used are PBGA's 1.27mm pitch. The pad size is .030 and the finished via dia. is .010. This is a dogbone type pad and via design. My concern is the CACHE chips are directly over one another on top and bottom. The current design has the via's tented on the secondary side of the board. No plugs are used. What is the best way for this type of design? Plug and tent primary side? Plug only? Any advise helpful. Thanks in advance, Ron
Regardless of the technique used for the mounting pad shape or definition, isolation of the plated through hole [PTH] from the mounting pad is important. If the PTH is contained within the mounting pad, solder will wick down the PTH. The amount of solder that wicks depends on many factors, including PTH finish and coating variations. Because of this, the results are somewhat unpredictable. Some solder connections may be unaffected, while others will be starved to the point of creating opens. The worst result is a partially starved connection with severely reduced cross section. This connection can have significantly lower fatigue life and result in early system failure. Because the quality of the solder connection is guaranteed by control rather than inspection, designs / processes that result in random distributions are generally considered unacceptable, and the PTH-in-pad design is not recommended. All via located between PBGA pads must be covered with solder mask. It is suggested that as a minimum, vias on the top-side be covered with solder mask. The bottom side can also be covered, but left the customer�s discretion. [Intel]
Certainly this applies to the primary side. I'd hesitate to apply this logic to tenting the secondary side in addition to the primary side, as you imply, because the air trapped between the top and bottom tents will expand and blow the socks off one or more of the tents when the board gets reflowed.
On the secondary side, consider masking upto the finished hole, maybe 0.012. It might be a bit cheaper than plugging.
It is difficult to give a complete recommendation on via tenting without an idea what process conditions are required (ex: wave pass, double reflow, rework etc.). Via Pads (as Test Points)
- On Bottom
The need to use the bottom side vias pads as test points is acceptable in all cases except wave. If using PTHs and not blind / buried vias, the via is connected from top to bottom and so the bottom side of the BGA via can be used for test. The bottom side vias do not need to be covered for std. SMT or double side reflow or even rework.
- Case of Wave Solder Process
In the case of wave, the vias would have to be taped off or use a peelable solder mask (these are in use for Au finger connectors on a regular basis). Via Pads (underneath BGA)
- On Top
The via pads on the top (underneath) the BGA should be covered to prevent the solder from wicking into the PTH. The via pads on the top side cannot be used anyway since they are covered by the device.