How to design & layout a 0.4mm BGA footprint right?


#1

Hi,

I am looking for the right way to design a 0.4mm pitch BGA [1] footprint (WLCSP34 package) on a 4-layer PCB.

For routing out the inner pads I need need the “Via-in-Pad (Filled & Capped Vias)” [2] technology. Because I think the “Dog-Bone” technology is not possible.

A: How to design & layout a 0.4mm BGA footprint “right”? Is my way the right way?

B: How do I specify a VIA as a “Via-in-Pad (Filled & Capped Vias)” in KiCad?

C: How to define them in the GERBER files?

D: Is there any other possibility to layout the inner pads of the package to the outside?

Best regards!

[1] https://www.multi-circuit-boards.eu/en/pcb-design-aid/bga-pcb-design-for-ball-grid-array.html
[2] https://www.multi-circuit-boards.eu/en/pcb-design-aid/surface/via-covering.html


#2

At these fine pitches you are looking at a limited choice in pcb fabs.
As it says in the first link:
“The PCB design for BGA with 0.4mm pitch is special production and requires the use of HDI-Technology and Via-in-Pad. Please always consult our technical department.”
Choose a suitable fab first and talk to them about their requirements.
We cannot give general answers here, but I can be confident that a 0.4mm pitch bga will be make your board expensive. Only four layers is ambitious at these densities.


#3

Recently I requested an offer for that technology at Multi-CB: 615€ for 5 pieces of a 4 Layer with 80x15mm. Yes, that is quite expensive.

Here you can find very nice posters from WE-direkt: https://www.we-online.de/web/de/leiterplatten/layout/design_guides/design_in_2.php

How you design that in KiCad, i’ve no clue, since we didn’t use that HDI technology.


#4

I have done 0.8mm devices on 8-layer HDI boards, but have not gone to 0.4mm yet. However, there are may of the same challenges.

A: How to design & layout a 0.4mm BGA footprint “right”? Is my way the right way?

As was mentioned, identify the board house that will be making the board and read their design guidelines. Also, find any application notes that you IC vendor may have. For instistance, Intel/Altera has a board design guidline App Note that cover 0.4mm BGA land pad, via, and escape routing on a 4-layer board:
Board Design Guidelines for Intel Programmable Device Packages

Note pages 14, 18, 27-29

B: How do I specify a VIA as a “Via-in-Pad (Filled & Capped Vias)” in KiCad?

I would add the vias in the pad in the PCBnew layout and not in part footprint, as not all pad will need vias. You will then have to communicate to the board house in extra fabrication instructions that something like
“Uxx, Uxx, Uxx contain vias-in-pads that shall be filled and capped”

C: How to define them in the GERBER files?

Per above, this should be done through the fabrication instructions, as Gerbers alone cannot communicate all the build requirements on complex boards. Make sure to verify that the board vendor confirmed reading the fabrication instructions.

D: Is there any other possibility to layout the inner pads of the package to the outside?

Check out he escape routing in the Design Guide link. At 0.4mm it does look like dog bones are out and via-in-pad is the way to go. Read the docs, follow your vendors guidelines and communicate clearly with the board house and you should be good-to-go. Don’t be afraid to ask the board house questions and let them review the board package before ordering.


#5

@davidsrsb @KarlZeilhofer @aaron

Thanks for your response,advices and sorry for my late response!

I will try to do it and maybe I can review my experience when I am done.

I think the big challenge is to reach the requirements of asian PCB manufactures and communicate the design rules. Does anyone have experience with thoose or knows some good one?


#6

I generally use a more local (US) based manufacture for HDI boards the first time around. That way I can make sure I didn’t screw anything up. Once you have good boards from one manufacturer, it becomes much easier to engage oversees manufacturers in my experience. If something goes wrong, you can point back to a known good PCB and fab with the same package from another vendor.