Layer stackup definiton

[eo1668]

Hi,
I am working on a 4-layer FR4 1.6mm, how do I get about defining the layer stackup for this board? I have differential pairs and impedance control at 75 ohms. My layers are :

Layer 1 : top routing
Layer 2 : gnd plane
Layer 3 : pwr plane
Layer 4 : bottom routing.

Also I have a BGA388, is 4-layer sufficient to route this BGA?

Best regards,
PL

[Tom]

Four layers (two routing layers) is not enough routing channels to route a 388 pin BGA unless 1/2 the pins are unused or go to a plane. You will need to use a 6 layer stack-up like this:

Layer 1: Top Routing
Layer 2: GND Plane
Layer 3: Signal
Layer 4: Signal
Layer 5: PWR Plane
Layer 6: Bottom Routing

It is much easier to control the transmission line impedance if the differential pairs are routed on internal layers. Use the impedance calculator (available on the "Site Index") to calculate Dual Stripline for 75 ohms. Let me know if you have any other questions regarding this issue. (I'll attach the calculator here to make your life easier. Note: This calculator is only good up to 1 Gig. Use the tabs in the lower left corner to toggle between the options)

[eo1668]

Hi Tom,
I am still trying to convince my manager to allow me to use 6-layer for the BGA388. He still insist I try for 4 layers. This BGA388 has got 352 balls for signals and 36 balls for Pwr and Gnd. I trying to see whether I can route 2 trace between pad. I worried about noise issue if using a 4 layer board.

Usually I use spilt planes for pwr/gnd planes. But for this case, it recommend to use slit plane for gnd. I have not using slit plane for quite some time, pls briefly describe to me how do u get about doing that.

Thanks for your precious time and hv a nice day.

Best regards,
PL