The use of BGA chips in prototyping ?

[walker1]

Up to now we have used large FPGAs placed in PLCC and PQFP houses. With up to 240 pins the later requires good eyes and a lot of patience if soldered by hand, while PLCC usually can be socketed into PGA footprints.

We do prototypes/very small series only, and we do not have any soldering machines.

The newer and larger chips, however, mostly come in BGA houses, it seems.

First of all, does anybody know how to mount a BGA ?

By a soldering machine or conducting glue, I guess, but how do you control that the chip is placed exactly on top of the pads ?

I guess it is OK to have vias through the BGA pads on the PCB, otherwise it will be very tricky to get the connections out, or .. ?

 

[melone]

An experienced technician can place a 17mmx17mm .65 pitch BGA by hand..... but it is tricky. They do make placement machines that allow you to place a BGA device, but they are fairly expensive. (NOTE: The placement machines use mirrors and 2 cameras to "project" the device onto the pads before you place the device. It is a pretty cool application of technology.)

With regards to vias through pads, yes, it is acceptable to have a via in a pad. One of the major headaches of BGA's, other than placing them on the PCB, is fanout / routing.

 

[felixc]

Many BGA parts manufacturers will have app notes or sections in their data sheets dedicated on how to properly lay out the signals for their parts. In many cases, you also need to spread heat out of the device to the power planes. In many cases, you do not need to have the vias on the ball pads. This allows the use of more standard holes, like .013. The smaller holes required for a hole on the ball pad may jack up the cost of your PC board.
As Melone said, a skilled person can place a BGA on a board. To do this, you need to place "copper corners" on the moiunted layer. They will be used as a visual guide for the alignment of the part. This can avoid paying for a pick-and-place setup charge.
To solder these parts, the easy way is to find a local PCB assembly shop, preferably a small one. They can manually place the BGAs on your board and pass the PCB to the reflow oven, with only the BGAs on. If you are lucky, the assembly house will have a BGA microscope, which can be used to confirm that the reflow was properly made. Then you can take back the boards and complete the job at your site.
If your parts support JTAG ports, please connect them. It may help to debug your designs, as shorted and open balls are pretty hard to find.