Trying to do multilayered boards


#1

Hi!

I am trying to create a motherboard with multiple layers. I will use a FBGA on a surface throughmount. I need multiple layers under the FBGA but it does not let me connect a bottom layer. This is understandable as an FBGA is a surface mount device but there is no way that you can connect a center pin to a different part on the board without crossing lines or going under layers. I tried making a dip, but it is double sided and not a throughmount. Any ideas?


#2

You will need to use vias to get from one layer to another. Standard vias go trough all layers and are available in every production process. More expensive processes for high layer count boards also allow for blind or buried vias that do not go through all layers. Which exact layers are available for this highly depends on your manufacturer and on the exact option you buy. Be aware that there will be a minimum hole (or drill) diameter and a minimum annular ring (defining the minimum overall size). This minimum will limit how fine pitched your bga can be.

And there are also micro vias which can only connect two layers right next to each other. These offer a very small “drill” size and can therefore fit much easier in the limited space between the pads of fine pitched bgas. (They are not drilled but most likely produced using a laiser) These are however seldom available in cheaper processes.

There is also the option of via in pad. But for BGA this really requires your via to be filled as you will otherwise run into trouble with soldering. (A very expensive option that is also not really supported by kicad. You would therefore need to communicate in some other way which vias to fill.)


I wonder if it is wise to try to create a design with a BGA if my assumption about you not knowing about vias is correct. Maybe start with something simpler to get experience.


#3

Laying a board for a BGA packaged parts often needs blind vias, filled vias and other exotica and also requires a detailed knowledge of both the PCB fabs capabilities and the board assembly house needs.
I have made 0.8mm pitch BGA board in the past, this had a few production issues, so the current 0.5mm packages scare me.


#4

Thank you for your input. After doing some research I have a better understanding about vias. Unfortunately for the project that I wish to develop it requires decent processing power and I want to use a comparable architecture for ease of development. Could you either recommend an ARM A-15 or A53 processor with multiple cores that is not on a BGA or a development book that could help me learn about BGA design? The university I attend does not offer courses such as this.


#5

Why not make a small project before starting the big one. A simple blinking light could already be enough to understand the basics of pcb design (invest a few weeks up front to learn the basics and your main project will go much more smoothly.)

For bga design i suggest you read manufacturer application notes about bga, research papers regarding reflow soldering in general and bga in particular, industry standards, your manufacturers guidelines and capabilities (compare multiple ones), …

Researching stuff on your own is after all the main thing you should take with you from university. This is what really differentiates a graduate of a university from somebody finishing a trade school.

And don’t be afraid to ask professors of your university. You will never again be able to get support by such highly qualified people for free. (Just ask one that could have knowledge in that field they at least can tell you who really knows about this stuff. If what you do is part of a course or thesis start with the people who are responsible for it. If it is for a student team then ask alumni of it.)


And if the pcb design side is not the main part of your work (If it is expected that you use the hardware you make for more than a basic software demo) then you might want to check if you can find a finished board that holds a processor with enough power. (designing a pcb for a complex thing like a processor of your listed power can easily take a few months of full time work with multiple iterations and therefore additional manufacturing waiting times. And then you need to somehow solder it as well which will add yet another skill to learn.)

Depending on your application either check a fpga supplier (XILINX has nice evaluation boards) or even things like nvidia jetson tk2 (or whatever the current version of it is) if you need a powerful graphics processor. If you already have hardware in mind check if the manufacturer does not offer an evaluation board or at least reference design for it or for something very similar.