Complex PCB stackups - Page 1
Forum section: electronics / projects

author: poorchava
author_rank: Super Contributor
timestamp: August 07, 2014, 09:25:30 am
content: I've recently been reading up on multilayer PCB technologies. As an example I took the iMX6 Rex SOM PCB developed by Robert Feranec (I guess most people know him), because it's quite well documented. Amongst other things I have found the PCB stackup documentation (attached a copy, downloaded from imx6rex.com).As you can see the PCB is 12 layer, including buried vias (both drilled and lasered) and lasered blind vias. I'm trying to understand 2 things: how's such pcb manufactured and how was such stackup devised.I'd guess that the manufacturing process goes like this:-manufacture conventional 8-layer board with drilled vias only-add prepregs-laser drill blind vias in added prepregs-deposit copper to make microvias-laminate copper foil and etch traces (not sure about the order of this and 2 previous steps)-add another prepreg-laser drill blind vias-deposit copper-laminate foil-mechanically drill through vias-etch-soldermaskWhat completly baffles me is how was such layer stack devised. The board contains 4 impedance controlled layers which I'm sure are used to route DDR memories. Blind vias had to be used in order to fan out signals from DDR chips which are on two sides of the board. Layers 2, 4, 9, 11 are presumably solid gnd to confine layer 3 and 10 signals as striplines. Center internal layers (5-8) are likely power and ground planes.Could anobidy with more experience with such somples stackups comment on this and explain piece by piece or just correct me where I'm wrong?

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: August 07, 2014, 11:17:31 pm
content: You're looking for "sequential build".http://www.cistelaier.com/eng/tecnologie/dettaglio.aspx?idtecnologia=11I don't know what you mean by "devised". It's just a bunch of rules to follow, either from the laminate manufacturer or the drilling/photo equipment manufacturer.

author: jakeypoo
author_rank: Regular Contributor
timestamp: August 08, 2014, 07:19:58 pm
content: What's the difference between 4 layers and 400? If you understand how 4 layer boards are made then it;s not difficult to figure out the rest.Why is another question. If you haven't, read Henry Otts articles about stack uphttp://lmgtfy.com/?q=henry+ott+stack+upI see 4 signal layers and 8 planes.Why so many planes? Maybe there are many supply layers. Also, adjacent Supply-Gnd planes make for good HF decoupling.Why did they use foil in the outermost layers on each side? Maybe the core was too thick and they used prepreg to keep spacing between layers tight, or overall thickness down.I would imagine Outermost planes (2, 4, 9, 11) are all ground planes, used to isolate digital + analog grounds or to shield one signal layer from the other. Then 5+6 and 7+8 are Supply + Gnd planes.So, 4 signal layers, 6 ground planes, 2 supply layers. What's so crazy about that?

author: poorchava
author_rank: Super Contributor
timestamp: August 08, 2014, 07:58:42 pm
content: I mean that most I have done is 6-layer once, 4-layer several times, but nothing more than that. And those were all conventional PTH-via-only boards. I've never had anything to do with microvias and HDI, that's why I'm curious.