How do you pick the right amount of PCB complexity? - Page 1
Forum section: electronics / general

author: aoreskovic
author_rank: Newbie
timestamp: October 10, 2025, 08:24:52 am
content: Hi everyone,I am a electronics designer, and I have been doing a lot of stuff over my last 7 years of work experience, from simpler stuff to my most complex project being a carrier for Nvidia AGX Xavier module, with all different peripheries such as camera connectors, PCIe memory, RGMII and so on. So far everything I have done was always done with only TH vias, no blind, no buried, no uVia, nothing.Now I got my first FPGA project - XC7S100-2FGGA676I Spartan 7. It is not the most dense thing to route - 1.0 mm pitch, but I do have a lot of lines for Camera, 2 DDR3 chips, some 0.5mm pitch ONFI memory and eMMC flash, with bunch of doo-dads.What I am wandering is how do you decide to increase the PCB "complexity" from only TH vias, and what are your conditions to do so? What is your next step up?The Spartan 7 SP701 Eval board is also routed with only TH vias on 14 layer stackup, but that requires going down to 3/3 mil spacing to route differential pair between all TH vias, which I don't really like. Also Eval is 150x150mm and my board is 100x100mm with more high speed stuff.But there are so many ways to go "up" in complexity, reverse buildups, X+N+X HDI uVia buildups, any layer interconnect, blind vias, buried vias, you can add more layers. I am not sure if I want to make my self life a bit easier, which of those do I pick? Time is here more of the essence then the price since it is a low volume product.TL;DR Designing a quite dense FPGA board for the first time, I am not quite sure to start with a complex HDI stackup from the get go, or start with simple stackup. What is your thought process when looking at a board, seeing something and deciding "okay now I need to go HDI / blind / buried / via in pad / I need more layers"

author: Rat_Patrol
author_rank: Frequent Contributor
timestamp: October 11, 2025, 12:33:40 am
content: I always go with the least complex (cheapest) board I can that meets the need. Check out fab specs to get the limitations for the different "grades" of boards.How to start the project is a bit of black magic, IMHO. Look at components and minimum pitch (compared against the PCB Fab's capabilities), look at the size of the PCB you have to play with, and go from there. Kinda the same way you look at a simple project and say "Yeah, 2 layer PCB no problem" vs "Going to need a 4 layer on this one, let's just setup that stackup right now"

author: aoreskovic
author_rank: Newbie
timestamp: October 11, 2025, 11:20:01 am
content: Yeah, this is what I normally do.I have never routed this full grid 676 pin or anything similar before so I feel like i need some kind of "higher" technology to handle that, but I am not sure which approach to take, one which is the lowest next step up, but helps

author: dmendesf
author_rank: Frequent Contributor
timestamp: October 11, 2025, 01:23:00 pm
content: Xilinx used to have a fan out guide for their chips... Can't find it now (i'm on mobile).

author: loki42
author_rank: Frequent Contributor
timestamp: October 11, 2025, 02:18:28 pm
content: Mainstream fabs will do pin in pad really cheaply. So if you can get away with that then it's an easy option.